The Rise of Tunnel Warfare as a Tactical, Operational, and Strategic Issue

Abstract

The presence – or suspected presence – of tunnels on a battlefield affects all aspects of a mission. It took time for militaries to grasp the impact of tunnels at the tactical level, from equipment to small unit engagement. The operational and strategic ramifications of underground warfare, for their part, have remained under-explored. Building operational and strategic capabilities for subterranean warfare requires minimal investment but would go a long way in enhancing preparedness for subterranean threats of all types. This article explores three key steps in the much-needed shift toward a more operational and strategic approach to subterranean threats: more accurate risk assessments of underground threats, an understanding of how tunnels bear on the broader mission, and a genuine effort at anticipating innovative uses of tunnel warfare at the hands of states and non-states alike.

Underground tactics have long presented a challenge to conventional armies. British and German armies saw entire regiments decimated by tunnel tactics.¹ The U.S. lost countless men in the Pacific as it fought entrenched Japanese forces during World War II, and again in Vietnam nearly three decades later.²French forces were dumbfounded in 2011 by the difficulty of finding and clearing caves in Mali’s Valley of the Ametetai, where AQIM had established its stronghold.³ Tunnels expose the limits of modern precision warfare, often driving states to resort to highly destructive measures: B-52 bombers in Vietnam, GBU-43/B Massive Ordnance Air Blast (MOAB or Mother of All Bombs) in Afghanistan, mass flooding with sewage water in Egypt, heavy artillery bombardment in Mali, flamethrowers in Syria, and airstrikes in Israel.⁴ These types of weapons and anti-tunneling measures, however, are at times wielded without a clear endgame in mind, and their impact, while enormously destructive, tends to be short-lived. Yet the subterranean domain remains under-researched, poorly understood, and often underestimated.

Even states that have experienced subterranean threats have been hesitant to develop doctrine, strategy, and tactical guidelines in part because subterranean threats take many forms and evolve over time. Different actors have used the underground differently – from smuggling to hiding ammunition, protecting civilians and fighters, ambushing enemy forces, kidnapping, launching weapons, taking control of territory, and retaining command-and-control while under attack. This versatility has complicated the development of military doctrine on the use of the underground domain in war. It has also slowed down the technology to counter tunnel threats – and apparently deterred scholars from engaging in its study. The dearth of scholarly work on subterranean warfare is striking and creates significant methodological obstacles for those who wish to examine the tactic academically. Nevertheless, academia has much to contribute to the study of underground warfare, as growing academic and military cooperation suggests.⁵

Underground warfare has also undergone significant changes due to its appropriation by terrorist actors post-9/11, who have adopted and adapted the tactic to their own objectives and modus operandi. The strategic advantage violent non-state actors draw from urban warfare, the ease with which they interact with one another and exchange information, their shared goals and common enemies, and the appeal and versatility of the underground, have all contributed to the evolution of underground warfare into a global security threat.⁶ This is unlikely to change dramatically, even as new technology continues to improve tunnel detection and mapping, and states make renewed use of the underground in a context of great power competition.

This article advocates for a second-generation shift in how tunnels are perceived, which would move past the tactical to the operational and strategic levels. The presence of tunnels, or the threat thereof, deeply affects the conduct of operations at all three levels.⁷ While the tactical aspects are key, they only partially account for the impact of tunnels on military operations. Subterranean threats have a direct and substantial impact on the mission itself and magnify many of the difficulties encountered at the operational and strategic levels, particularly in urban operations. In addition, many aspects of subterranean warfare that have so far been considered as tactical belong instead to the strategic realm. But overcoming navigation, communication, rescue, and intelligence gathering hurdles is not only a matter for the tactical level. Unless the operational and strategic aspects of subterranean warfare are acknowledged and understood, tunnels will slow down major operations and unsettle even the most competitive force.

Academically, this amounts to identifying the unique features of subterranean warfare and how it interacts with other military domains (such as naval warfare) and terrains (such as urban warfare). Military- and policy-wise, a shift in the perception of underground threats from tactical to strategic – without, in most cases, being existential – will minimize the “unknown factor” and element of surprise inherent to the subterranean, produce more calibrated and efficient responses, and enhance preparedness.

Three steps would help in realizing this shift: first, a more accurate conceptualization of tunnels to improve risk assessments; second, a greater understanding of how tunnels affect the entire mission; and third, a more targeted effort at anticipating future trends and studying innovation and diffusion patterns of tunnel tactics.

Conceptualizing Tunnels

Despite its constant use in warfare over centuries, subterranean warfare has traditionally and surprisingly attracted little attention. The lack of a systematic treatment of underground warfare in history, military theory, the law of armed conflict, and security studies has left this topic underexplored in academic scholarship and underestimated by militaries and security experts. Historically, valuable accounts have been made of the use of the tactic in key conflicts.⁸ What is missing, however, is an attempt at understanding the evolution and diffusion of the tactic, and its operational and strategic ramifications.

With the re-emergence of underground tactics on the contemporary battlefield, particularly at the hands of terrorist organizations, more resources have been allocated to this subset of land warfare. The use of the tactic in urban settings has triggered a reflection among military and security scholars, more academic interest, and increased information sharing.⁹ Among militaries most exposed to subterranean threats, this heightened awareness for subterranean warfare has materialized primarily through R&D, training, and at times military guidance. In the United States, it has taken the form of major investments in training and supplying brigades,¹⁰ the publication of ATP 3-21.51 entitled Subterranean Operations,¹¹ and the launch of at least two important DARPA projects.¹² The British army has been actively training forces in subterranean terrain,¹³ and Israel has created a mobile unit that combines young talent and performant algorithms to improve tunnel detection.¹⁴

As noted above, the slow and hesitant involvement of scholars and practitioners in subterranean warfare can be attributed to the difficulty of classifying tunnels for purposes of tunnel detection, threat assessment, and neutralization. Tunnels not only take multiple shapes and forms; they also serve a range of functions across regions, conflicts, and centuries.¹⁵ At times, the underground enables the population to save water or other resources.¹⁶ At others, it facilitates the undetected movements of fighters.¹⁷ It helps weaken fortifications,¹⁸ blow up enemy positions,¹⁹ and survive conflicts.²⁰ In contemporary warfare, tunnels have become a tool of choice to carry out kidnappings²¹ and infiltrate enemy territory.²² Some tunnels are dug for short-term purposes (take, for example, rudimentary tunnels used for tunnel mining)²³ others serve as living quarters and ammunition caches and require ventilation or electricity.²⁴

Yet conceptualizing subterranean threats is a first and critical step towards a more strategic approach. A consistent treatment of subterranean threats limits the uncertainty associated with the discovery of any type of underground structure. A more accurate and systematic classification shortens response times and saves resources, particularly with regard to identifying the authorities in charge and the type of response needed. It can also assist in the identification of new trends over time if records of discoveries are maintained.

Various attempts at conceptualization have been essayed but, in our view, those need to be further refined. They do not, at present, account for some important features of contemporary subterranean threats and hinder a shift towards a more strategic and operational approach to underground warfare for the reasons we outline below.

The U.S. officially classifies tunnels based on their functions: tunnels, natural cavities, and caves; urban subsurface systems such as parking garages, subways and sewers; and underground facilities (military purposed) that are deeply buried and mostly serve command and control functions.²⁵ Research presented by the Defense Analysis Department at the Naval Postgraduate School and the Asymmetric Warfare Group had previously (and more appropriately) classified tunnels based on a combination of five attributes: function, infrastructure, mobility, threat, and accessibility.²⁶ It identified three main categories of tunnels: underground facilities (bunkers and deeply buried facilities), tunnels (rudimentary and sophisticated), and urban and natural cavities (civil works, sewers, basements, and caves).²⁷

The UK similarly identifies three categories of underground structures based primarily on their usage or purpose: caves, natural cavities and tunnels formed by natural processes but developed for human use (rudimentary or sophisticated), urban subterranean systems built to support the civilian population (divided into substructures (basement, shelters and parking lots), civil works such as sewers, and underground facilities (sophisticated complex structures specifically designed and built to provide maximum concealment and protection for the people and equipment within them)).²⁸

Focusing instead on offensive and defensive tunnels, Israel distinguishes among smuggling, defensive tunnels (used mainly as hideouts), and offensive tunnels (used to launch attacks).²⁹

Classifications based on the purpose, use, or the defensive or offensive nature of tunnels have clear limitations: they fail to capture the versatility of underground structures and can mislead commanders in charge of an operation. While the impact of such a classification may not be felt at the tactical level, it will be felt at the strategic level. Tunnels dug for smuggling goods can be used to carry out acts of terror – and vice versa – as Hamas’s underground tactics at the Gaza-Egypt border have shown.³⁰ Ultimately a tunnel is a tunnel is a tunnel, and with every cross-border tunnel inevitably comes a security threat regardless of how the tunnel is (presently) being used.³¹ A given tunnel will likely serve a wide range of functions over time; acknowledging this upfront as part of their classification will enhance the ability to anticipate future threats at the strategic level.

In addition to being highly adaptable and versatile, tunnels can be expanded or repurposed. Unless a tunnel is fully eliminated, it can be used, inherited, or expanded by any actor for any purpose even decades after its initial discovery. Take, for example, the Cù Chi underground complex which was inherited by the Vietcong from the Indochina War,³² and the Afghani Karez tunnel complex, which formed naturally but was ultimately used both as a shelter by civilians and as a bunker by Mujahideen forces.³³ Tunnels and cave networks built by the Mujahideen were then used against the Soviets in Afghanistan, and later inherited by al-Qaeda and the Taliban. Saddam Hussein’s tunnels were used by ISIS fighters as a means to acquire territory, move around freely, and hide from aerial attacks.³⁴ ISIS also used an abandoned train tunnel as a training site near Mosul, Iraq.³⁵ Whereas at the tactical level the threat would seem contained and short-term, an operational and strategic take would see a long-term, ongoing threat that needs to be eliminated or carefully monitored. This would be the difference between seeing the U.S.-Mexico tunnels as smuggling tunnels posing merely an economic threat – and acknowledging that they could at any time be used to undermine U.S. national security.

A more strategically focused conceptualization of tunnels – and a more accurate assessment of subterranean threats – would incorporate two critically important parameters: (1) the burrowing of tunnels under a recognized border, and (2) the proximity of tunnels to civilian populated areas.

First, the discovery of a cross-border tunnel raises an array of security-related risks, ranging from violations of sovereignty and border incidents to infiltrations leading to full-blown attacks, the introduction of underground warfare in an existing theater of operations, and even the start of a new conflict. In peacetime, cross-border tunnels affect the security of the victim state and, in war, the course and nature of the conflict. The victim state will need to establish whether a newly discovered cross-border tunnel can be attributed to the neighboring state itself – or to a group within that state, with or without the state’s involvement. These questions remain unresolved in the context of cross-border tunnels discovered at the Indo-Pakistani border.³⁶ Importantly, cross-border tunnel-digging in peacetime could also constitute an act of transnational terrorism.³⁷ Therefore a cross-border tunnel will raise very different risks at the strategic level than a tunnel dug on the territory of a single state.

The second most important parameter to consider in assessing the level of threat posed by an underground structure is its proximity to civilian populated areas. The proximity of tunnels to civilian populated areas heightens the threat posed by tunnels and limits the methods available to neutralize them. If a tunnel is discovered near populated areas, authorities should assume that civilians could be targeted. All urban tunnels, cross-border or not, directly and intentionally endanger civilians from the very moment the digging commences: they can lead to collapse,³⁸ water contamination, and explosions along the tunnel’s route and beyond.³⁹ These tunnels raise very different risks at the strategic level than those dug or excavated in mountainous areas, as in Mali or Afghanistan.

A lack of attention to these parameters can have dramatic consequences. Israel’s failure to anticipate that a smuggling cross-border tunnel could be used to meet other strategic needs of Hamas led to the kidnapping of IDF soldier Gilad Shalit in 2006 and a controversial prisoner exchange five years later.⁴⁰ Even following the kidnapping, cross-border tunnels emanating from Gaza continued to be seen as a means to carry out sporadic terror attacks rather than as a strategic threat requiring unique expertise, doctrine, and solutions. It took a major confrontation with Hamas in the summer of 2014 for Israel to start moving away from treating Hamas tunnels mainly as a tactical threat. Following Operation Protective Edge in 2014, Israel stepped up its effort by boosting training for soldiers and special forces alike, building training sites, developing tailor-made equipment, and significantly improving tunnel detection and neutralization techniques.⁴¹ Operation Northern Shield, an unprecedented effort to expose Hezbollah’s tunnels at the Lebanon-Israel border in 2018, confirmed that Israel had shifted to a more strategic approach.⁴²

The UK has also advocated an approach that goes beyond the tactical level. The UK Doctrine declares that it is “written for the tactical level”, while noting that “concurrent operational level” considerations might also apply.⁴³ Beyond the planning and execution of battles and engagements in a subterranean environment, militaries must contemplate how the campaigns and major operations in such environments might be better planned and executed, using operational art to achieve military objectives. Strategically, resources must be allocated, and greater cooperation and transparency is required at the multi-national level.⁴⁴ There is still a long way to go for the view that tunnels are also relevant to operational and strategic levels to take hold. Until then, a lack of preparedness at the strategic and operational levels seems unavoidable.⁴⁵

This is not to say that tunnels are the most significant or complex threat – let alone an existential threat – facing states.⁴⁶ When it comes to underground threats, even a limited investment in operational and strategic aspects can significantly improve preparedness. Moreover, despite the complexities inherent to this unique terrain and their deep impact on the conduct of hostilities, tunnels rarely shift the overall balance of power among belligerents. Tunnels can change the course of a conflict, but rarely its outcome. Tunnels do provide their users with an advantage, typically by re-establishing a level of symmetry between belligerents of vastly unequal capabilities. Underground warfare also triggers a deep fear of the unknown – due to a lack of understanding of underground threats, the invisible nature of tunnels, and the unpredictability of their use. Tunnels draw significant resources, require re-thinking at the tactical, operational and strategic levels, sap energy, slow down operations, and complicate the protection of forces – but they rarely win wars.

At the outset, the classification of tunnels must be done with an eye to the strategic dimension. A conceptualization of tunnels based solely on the type of underground structures (man-made or formed naturally), their purpose (smuggling or urban), or defensive or offensive nature, does not account for many of the aspects that make subterranean warfare a strategic threat: infringements of sovereignty and the likelihood of aggression in the case of cross-border tunnels, and a looming risk to civilians and the dearth of neutralization methods when tunnels are dug in urban areas.

Ramifications of Subterranean Threats on the Broader Military Mission

A second step in realizing a shift towards a more strategic and operational approach to subterranean warfare lies in taking stock of the ramifications tunnels have on the entire mission. The unknown factor permeates all aspects of underground operations. The ability of a belligerent to know and maintain control decreases when the enemy makes use of the subterranean. Tunnels curtail intelligence gathering on the enemy’s modus operandi, intentions, and even an assessment of enemy capabilities when ammunition is stored in underground facilities. They take forces by surprise⁴⁷ and increase the need for force protection, due to limited intelligence, heightened risk of kidnapping, claustrophobia, and booby-traps.⁴⁸ Underground, the fog of war is thicker than aboveground.

This is true when tunnels form part of the original mission; it is even more striking when tunnels are unexpectedly discovered as part of an unrelated mission. In both situations, securing the surface surrounding the tunnel present challenges that even well-trained soldiers might find difficult to overcome. Aboveground, tunnels transform the battlefield into a sphere. It is no longer sufficient for soldiers to secure the rear, as tunnels can emerge from anywhere; the battlefield becomes multi-dimensional.⁴⁹ Having to maintain situational awareness while securing the area and moving to neutralize, deny, or enter the tunnel can significantly hinder the completion of the original mission.

The unexpected discovery of a tunnel in an unrelated mission will add uncertainty and have an unsettling effect on forces. It may delay the mission or completely transform it. Making the decision to bypass, enter, deny, or clear a tunnel in the midst of an operation calls for unique leadership and decision-making skills among non-specialized forces and special forces alike. These aspects affect even those for whom “entering and fighting in a subterranean environment” should be avoided whenever possible.⁵⁰

Performance will depend, in large part, on training and equipment. Because underground structures can be encountered in a wide variety of contexts and environments, from urban to jungle and mountainous terrain, basic underground training must be extended to all soldiers.⁵¹ For special units, who may have to end up entering tunnels, both training and vetting must be carefully done.⁵² Underground systems, with their “maze-like, non-geometrical aspect[s]” require “unique cognitive ability.”⁵³ Forces must be “trained and equipped to maneuver and combat at short distances and poor visibility conditions.”⁵⁴ Testimonies of U.S. soldiers who fought in Vietnam point to the moral and psychological effects of fighting in tunnels.⁵⁵ Officials at the U.S.-Mexico border similarly speak of “the claustrophobia and darkness and all the unknowns involved. We try to adopt as many people as we can onto the team but there are some people that freeze. It’s just not for them.”⁵⁶

The difficulty of communicating, navigating, and operating underground “can drastically slow the pace of an operation”: “Strained communications, degraded global positioning systems, confined space in unknown terrain, and other difficult environmental factors make navigation, command and control, and even fratricide prevention measures extremely difficult.”⁵⁷ Though the impact is felt first and foremost on the ground, monitoring, anticipating threats, and intelligence sharing are equally indispensable to preparedness. The immediate need for relay radios and vertical lift skills should not detract from operational and strategic needs.

Soldiers will find it hard to communicate inside tunnels and keep in contact with the rest of their unit above ground: “you can’t go more than one floor deep underground without losing comms with everybody who is up on the surface … our capabilities need some work.”⁵⁸ Unlike on land, in the air, or at sea, common means of communication are either ineffective or not suitable underground. Soldiers often advance and operate alone inside tunnels; without appropriate means of communication, they can easily be disconnected from their command and the rest of their units. These communication hurdles also affect tunnel users who must learn to communicate among themselves inside underground structures, and with members of their forces at the surface.

Alternative methods must be found to enable team coordination and ensure the completion of the mission. The mining industry has shown that trailing a cable inside an underground environment or quickly installing a leaky feeder (a special type of coax cable designed to radiate signals) may be sufficient at times.⁵⁹ The earth radio, used regularly in mine rescue, or the medium frequency radio also offer valuable solutions.⁶⁰ Wave relay radio technology has been used successfully in both military and civilian settings because it does not require any infrastructure to operate nor any repeater to enable communication.⁶¹ Repeater stations or radio repeaters may be used if they are available.⁶² Though options range from low tech messengers to pricey handheld MPU-5 smart radios,⁶³ a solution available across the board has yet to be found.

Militaries have sought to overcome these communication hurdles by investing in robots and other unmanned systems. Despite their appeal, robots come with certain limitations: the communication link between a tactical robot and its control station is referred to as the “Achilles heel of any ground robotic operation in the field.”⁶⁴ This is because structures that impede frequencies, like buildings or tunnels, also impede the ability of the robot to function properly.⁶⁵ The DARPA Subterranean Challenge played an important role in understanding such limitations and spearheading the development of robots able to operate in complex, rugged, wet, dark, and convoluted underground structures.⁶⁶ Robots can be used to map tunnels, detect IEDs and chemical substances, and provide key information on the physical features of the tunnel and the presence of fighters.⁶⁷

In terms of equipment, soldiers entering tunnels should carry air supply and protective masks, special gear to monitor air quality and oxygen levels,⁶⁸ highly performant night vision goggles, and, when the size of the tunnel allows, ballistic shields to provide cover from enemy fire.⁶⁹ The confined space of tunnels also makes it necessary to protect soldiers from loud noises. Weapon suppressors minimize the noise of fire and earplugs limit the risk of hearing loss.⁷⁰ Soldiers should know to request additional personnel and equipment whenever necessary, from engineering support needed for rescue or remediation to explosive ordnance disposal and the handling of biological or hazardous substances.⁷¹ To enable surrender, or if the presence of civilians is suspected, soldiers should be given loudspeakers or a bullhorn, as well as infrared ropes for rescue operations.⁷² Inside tunnels, forces may be exposed to biological and chemical hazards, smoke inhalation, blast injury, booby-traps, infectious diseases, as well as brain injury and hearing loss.⁷³ Explosions in confined environments are also more destructive compared to open-air explosions.⁷⁴ The difficulty in accessing the site and the need “for specialized training and extrication skills” further enhances the risk posed to soldiers.⁷⁵

As underground warfare becomes a constant feature of contemporary conflicts, procedures for the rescue and evacuation of wounded soldiers must be updated to avoid placing forces in harm’s way.⁷⁶ Civilian products used in fire departments, commercial mining, industrial safety, or recreational climbing may be adapted for military use in the subterranean.⁷⁷ Take, for example, seismic sensors developed to locate miners in search-and-rescue operations or detect sound frequencies in the ruins of destroyed buildings.⁷⁸ Planning for subterranean warfare should therefore come with a view to integrating and harmonizing technologies developed across the civilian and military sectors. In addition, soldiers should have access to harnesses and rappel seats and be familiar with vertical evacuation.⁷⁹ Collapsed or narrow tunnels, or obstacles such as gates and fortified doorways, can delay operations and require additional skills and equipment. During the British Army’s subterranean training exercise in Leeds, the 21 Engineer Regiment came up with “innovative solutions” to reduce the difficulties soldiers faced, such as welding large wheels onto stretchers.⁸⁰

Finally, as traditional GPS-operated systems do not function below ground, navigation raises its own share of hurdles.⁸¹ Border patrols working inside the cross-border tunnels used by drug traffickers between the U.S. and Mexico report the difficulty in establishing whether, or where precisely, a tunnel has crossed the border. These challenges prompted the U.S. military as early as 2010 to research the potential of a system known as the Sferics-Based Underground Geolocation (S-BUG) that would permit soldiers to navigate in enemy tunnels thanks to a system of underground receivers.⁸² A few years later, the U.S. Army published a Request for Information on products that facilitate tunnel mapping, possibly in combination with ground-penetrating radar and unmanned robots.⁸³ Improving situational awareness is key to the success of any military campaign and yet it is severely hampered in dark, confined environments where signals are not available.⁸⁴ A return to basic solutions – hand-drawn maps, pace counts, and wall markings – has clear limitations and may not offer the most suitable alternative.⁸⁵

In sum, and as the tunnel rats understood decades ago in Vietnam, the use of the subterranean inevitably calls for a rethinking of basic assumptions, from the tactical to the operational and the strategic levels. Underground structures magnify the difficulty inherent to any military operation. Clearing a tunnel is similar to clearing a building – “[the] principles are exactly the same, but now do it without light, now do it in a confined space…, now try to breach a door using a thermal cutting torch when you don’t have air.”⁸⁶ Overcoming navigation, communication, rescue, and intelligence gathering hurdles is not only a matter for the tactical level as it can jeopardize the mission itself. Though they may seem contained to small unit actions and low-level engagements, underground encounters have the potential of slowing down major operations and unsettling even the most competitive force. The presence of tunnels must be factored in at the operational level as part of the planning and execution of a campaign. National strategy must include multilateral engagements to learn from previous engagements, and national efforts to improve visual intelligence capabilities, dedicate large R&D investments, and anticipate future trends must be prioritized.

Anticipating Future Trends

Anticipating future trends is a third key element in successfully realizing a shift towards a more comprehensive approach to underground threats. Understanding and preparing for underground threats means, among other things, anticipating the evolution and diffusion of the tactic among states and non-states.

Since 2001, underground warfare has diffused through both individual and group channels via the use of the internet, formal and informal links between terrorist groups, and among relatively young/flexible actors who are not set on a particular method of attack and have the organizational capacity to adapt.⁸⁷ Terrorist groups have adapted the tactic to their own needs and integrated it within their broader strategy. The Syrian War and the use of tunnel mining (also known as tunnel bombs) demonstrated that violent actors are constantly looking for new ways to reap benefits from the subterranean.⁸⁸ As the tactic continues to diffuse, it provides fertile ground for adaptation and innovation for terrorist groups but also for states that might wish to reestablish an advantage over this strategic terrain.

Four important future trends are discussed here: states’ growing attempts at reestablishing military superiority over the subterranean, the convergence between subterranean warfare and urban warfare, the intersection between subterranean warfare and naval warfare, and the use of tunnels in law-enforcement contexts.

Reestablishing State Dominance over the Subterranean

In contemporary warfare, violent non-state armed groups have used the underground primarily to counter the ever-growing Intelligence, Surveillance and Reconnaissance (ISR) capabilities of their technology-savvy enemies. By reestablishing a level of symmetry between adversaries with vastly asymmetric military capabilities, the underground serves as the great equalizer. Willingly or not, states have largely relinquished this strategic terrain to their enemies (with the notable exception of Syria and North Korea).

A more strategic take on the tactic has gradually led some states to shift gears, suggesting that this could shape the evolution of underground warfare going forward. Armenia reportedly invested in the development of its tunnel warfare capabilities, with the aid of Iran, as part of a strategic shift designed to increase offensive capabilities against Azerbaijan.⁸⁹

In a bold move and building on know-how from the oil and gas industries, the United States has innovated by considering how the rapid construction of tunnel networks of a tactical nature could enable safe and efficient resupply in deployment.⁹⁰ This project signals an inclination to ‘re-own’ the subterranean domain.

Ukraine’s own use of underground structures in the war against Russia similarly suggests that states might become more willing to resort to a tactic that has, in recent years, been stigmatized by its violent use at the hands of terrorist groups.

Ukrainian forces have used tunnel warfare with the aim of getting a tactical and strategic advantage over Russia and defending against a land invasion. The experience of Ukrainian soldiers wishing to exploit a massive steelworks complex in Mariupol shows the importance of anticipating the use of the tactic by state (and non-state) actors. It also underscores the need for long-term strategic thinking in underground warfare. Going underground is not an end in itself; it requires careful planning: How will the use of the underground impact the broader military operation, is it sustainable, does it further strategic objectives, and what alternatives exist for extraction? Food supplies, drinking water, sanitary and medical arrangements, internet connection, and electricity are key to a sustained stay underground. In addition, entering an urban and subterranean complex full of civilians places those civilians in danger.⁹¹ Evacuating Ukrainian civilians who had sought shelter in Azovstal once Ukrainian forces entrenched in the complex proved particularly difficult.⁹² The Azovstal steel plant, referred to as “Mariupol’s last stronghold,”⁹³ demonstrates why strategy matters. The decision to entrench in Azovstal was a tactical one, made without a long-term strategy in place: “Ultimately, Azovstal became a trap. The presence of civilians hampered the soldiers’ ability to defend themselves. The presence of soldiers meant the civilians had to endure a vicious siege as food and clean water ran out.”⁹⁴ Notwithstanding any operational impact Azovstal may have had, it remains that the lack of long-term, strategic thinking ultimately endangered Ukrainian civilians and soldiers. It also provides a glimpse of how tunnel warfare might be used in future conventional wars.

There are good reasons to believe that tunnels and other types of underground structures will be exploited in the context of great power confrontations. An increasing number of states will come to see underground warfare not merely as a liability, but also as an opportunity. A more strategic and forward-looking approach to underground warfare should take stock of this growing trend.

The Convergence between Subterranean Warfare and Urban Warfare

Subterranean warfare will continue to merge with urban warfare. From Mosul to Marawi, tunnel warfare has come closer to cities and civilians as tunnels burrow under civilian dwellings, civilians find refuge in underground shelters, and fighters and civilians cohabit in underground networks. Gaza and Aleppo are both densely populated, traversed by a warren of underground passages invisible to most conventional surveillance technologies, and a dangerous place for soldiers and civilians alike. They also happen to have become a place of choice for tunnel users.

It is important to recall the natural connection between the urban and underground terrains. The critical infrastructure that many urban centers rely on possesses a subterranean component, including sewage systems, commercial centers, factories, underground transportation, and more. Military doctrine also traditionally addresses the subterranean as part of urban; examples include the MCWP 3-35.3 Military Operations on Urbanized Terrain (1998),⁹⁵ the Joint Publication 3-06 (2013),⁹⁶ and the UK Doctrine Note 20/05 Subterranean Operations in the Urban Environment (2020).⁹⁷

Operationally, urban tunnels – tunnels dug near, by, or against civilians – magnify the challenges of both urban and subterranean warfare. Tunnels dug in proximity to civilian populated areas pose a direct and often intentional danger to civilians at all stages of their construction and use. Equally challenging for the protection of civilian populations are the conditions in which these tunnels (or those inhabited by civilians) may be eliminated. The destruction of a tunnel may result in the destruction of civilian structures (such as electricity, gas, and water-supply infrastructure) and civilian casualties. An anti-tunnel airstrike that killed 22 civilians in 2021 was reportedly caused by the collapse of a nearby building not directly located on the route of the tunnel targeted by the Israeli Defense Forces.⁹⁸ Tunnels used by civilians raise similar issues, this time for civilians inside rather than above the tunnels. Heightened constraints in urban terrain call for a rethinking of methods designed to detect and destroy tunnels, as many of the traditional methods may be devastating to civilians.

The Intersection between Subterranean Warfare and the Naval Domain

Perhaps less intuitively, the strategic appeal of the underground will be increasingly exploited in connection with the naval domain. Like tunnels, this is not a new phenomenon: during World War II, Japanese naval forces in Okinawa built “elaborate underground headquarters”,⁹⁹ and in 1957, the Soviets built a submarine base in the Crimean Peninsula under almost 400 feet of rock. The naval base was “constructed to be ready for nuclear war at any moment” and “be virtually indestructible.” It included repair shops, weapon storehouses, command posts, anti-nuclear shelters, administrative offices, and a deep-water canal almost 2,000 feet long connecting between the bases’ various sections.¹⁰⁰ It was meant to “withstand a direct hit by a nuclear bomb of up to 100 kilotons” and enable survival for 30 days in case of a nuclear attack.

That the naval domain provides fertile ground for the diffusion and adaptation of underground tactics was further demonstrated when Israel uncovered a Hamas underwater tunnel off the Gaza Strip. The land-to-sea tunnel enabled Hamas naval commandos to exit Gaza by diving in the sea and infiltrating Israeli territory without being detected.¹⁰¹ It was 6.5-10 feet deep, resembled a sewage tunnel, and extended from a land-based Hamas-affiliated military facility to the Mediterranean Sea. IDF forces referred to it as a “blue tunnel”, reflecting the connection between the naval domain and subterranean warfare.¹⁰² Since 2014, the Israeli Navy has placed sensors along the naval border to detect any suspicious activity in the water.¹⁰³

Naval-underground convergence seems to have caught the interest of states in recent years. As of 2019, the Swedish naval headquarters have returned to the Muskö Naval Base, a Cold War-era underground fortress equipped with “cavernous underground docks” and capable of hosting several warships. This facility also features a vast network of tunnels, underground offices and medical facilities.¹⁰⁴ Similarly, Norway has been renovating a former naval base at Olasvern to host U.S. submarines. The 32,000 square feet docking space is equipped with barracks, storage and maintenance facilities.¹⁰⁵ Satellite imagery has shown a Chinese nuclear submarine entering a submarine cave system at the Yulin Naval Base, in the South China Sea.¹⁰⁶ China possesses another naval-underground facility at the Jianggezhuang Naval Base which provides a hiding place for ballistic missile submarines.¹⁰⁷ It has also been reported that the U.S. Navy built an underground nuclear weapons storage complex at the Strategic Weapons Facility Pacific, a base that maintains Trident II ballistic missiles and their warheads for a fleet of submarines in the Pacific Ocean.¹⁰⁸ Taiwan, North Korea, and Iran have also contemplated the use of underground maritime spaces for strategic purposes.¹⁰⁹

Source: Sentinel (@stratsentinel), “Yulin Naval Base, Hainan, #China is home to a large underground facility with covered tram from magazine to loading areas,” Twitter post, February 19, 2017, https://twitter.com/StratSentinel/status/833093316862435328.

The underground/naval intersection may be less intuitive and visible than the convergence witnessed between underground warfare and urban warfare. But this growing phenomenon – relevant for states and non-states alike – has strong historical roots and should be a focus of strategic thinking.

The Use of Tunnels in the Law Enforcement Context

Lastly, underground warfare will likely play a more prominent role in law-enforcement operations. Tunnels will be used in a wider array of settings, not necessarily limited to the traditional battlefield. Engineers conceive of tunnels as “critical structures that each carry thousands of passengers each day, cost millions of dollars to build, and in some cases, stand as valuable cultural icons.”¹¹⁰ Tunnels have been used to carry out attacks against civilians in law enforcement contexts before. Successful examples include a series of suicide attacks committed in the London Underground in 2005 and the Maelbeek metro attack in Brussels in 2016. A failed chemical plot against the U.S. embassy in Rome¹¹¹ and an unsuccessful attack in the Times Square subway tunnel in 2017¹¹² further demonstrate what tunnel-related attacks could look like outside active theaters of hostilities. Underground spaces will likely continue to be used to facilitate or carry out violent attacks against governmental, military, and civilian targets outside or on the sidelines of armed conflict.

Pre-existing underground civilian infrastructure enables tunnel-users to reap the strategic benefit of tunnels without incurring any of the financial and human cost typically associated with tunnel-digging. Even mapping is likely to be much easier. While in some corners of the world, “it will be necessary to acquire, in advance, detailed, up to date town plans and maps of underground passageways and photomosaics”;¹¹³ in others, it will not be the case.

Tunnels have therefore reemerged as a vulnerable and dangerous environment, in wartime as in peacetime. At the strategic level, there is a need to raise awareness about the nature and scope of the threat, enhance preparedness (including amongst law enforcement and first responders), monitor and secure urban underground spaces, and ensure that critical civilian infrastructure and military bases are secured from below the ground up. The repurposing of preexisting underground civilian infrastructure such as sewage systems, home basements, parking spaces, commercial spaces, university maintenance tunnels, transportation tunnels, underground pedestrian passageways, and tunnels passing near or under critical infrastructure for violent purpose, presents a serious risk to civilians. These preexisting underground complexes can even be combined with man-made tunnels to create complex, layered subterranean networks. Importantly, the adaptation of existing underground civilian infrastructure for hostile purposes requires little time, resources, or training as compared to the digging of the tunnels of Hamas, al-Qaeda, and the like. The unique features of these underground structures, and the threat they can pose in peacetime, call for tailor-made strategies.

Conclusion

Urban warfare has brought tunnel warfare back into the spotlight. Although tunnels have been a part of warfare for time immemorial, they have traditionally received scant attention from military theory, the law of armed conflict, and security studies. This newfound spotlight is welcome and has already contributed to the development of new doctrine, significant investments in R&D, and more focused discussions – particularly across the civilian-military divide. However, efforts have concentrated for the most part on the tactical level since tunnels do significantly transform the experience of individual soldiers and small units on the battlefield.

In this piece we argue for a second generation shift as regards subterranean warfare which would move past the tactical to the operational and strategic levels. Three key steps would help realize this shift: first, a more accurate conceptualization and classification of tunnels to improve risk assessments and strategic outlooks; second, a greater understanding of how tunnels affect the entire mission even when the general policy is one of avoidance; and third, a more targeted effort at anticipating future trends and studying innovation and diffusion patterns of tunnel tactics.

We identify specific trends worthy of attention. Though the tactic has been stigmatized by its use by terrorist actors since 2001, there are reasons to believe that states will attempt to reestablish dominance over the subterranean in future conflicts. This will not only take the form of deeply buried facilities, which have been consistently popular, but also that of integrated tunnel tactics as part of great power competition dynamics and classic combat engagement in the subterranean. We also argue that urban warfare and underground warfare will continue to merge and influence each other. This, however, should not detract from developing operational and strategic thinking for the other forms underground warfare will take going forward – in particular the convergence between the naval and the subterranean domains and the use of pre-existing underground civilian infrastructure in the law enforcement context.

Acknowledgement

The authors would like to thank Shai Sisso, Physics Research Department, DDR&D; and Maj. (ret.) John Spencer, Chair of Urban Warfare Studies at the Modern War Institute (West Point) for their constructive comments on an earlier draft; and two anonymous reviewers for providing valuable advice ahead of publication.

Disclosure Statement

No potential conflict of interest was reported by the author(s).

Notes

1 Simon Jones, Underground Warfare 1914–1918 (Barnsley: Pen & Sword, 2010), 164.

2 C. Peter Chen, “Battle of Iwo Jima,” World War II Database, February 20, 2017, http://ww2db.com/battle_spec.php?battle_id=12; and Tom Mangold, and John Penycate, The Tunnels of Cù Chi: A Harrowing Account of America’s ‘Tunnel Rats’ in the Underground Battlefields of Vietnam (London: Pan Books, 2005).

3 Rukmini Callimachi, “Al Qaeda Fighters Carve Out Own Country in Mali,” Washington Times, January 16, 2013, http://www.washingtontimes.com/news/2013/jan/16/al-qaeda-fighters-carve-out-their-own-country/?page=all; and Jean-Philippe Rémy, “Voilà, On a Cassé le Donjon d’AQMI,” Le Monde, March 7, 2013, http://www.lemonde.fr/afrique/article/2013/03/08/voila-on-a-casse-le-donjon-d-aqmi_1844350_3212.html (in French).

4 US Army Corps of Engineers, “Historical Vignette 062—How Army Engineers Cleared Viet Cong Tunnels,” US Army Corps of Engineers, January 2003, https://www.usace.army.mil/About/History/Historical-Vignettes/Military-Construction-Combat/062-Viet-Cong-Tunnels/; James Griffiths, Barbara Starr, and Angela Dewan, “Afghanistan: US Military Defends Dropping ‘Mother of All Bombs’ on ISIS in Afghanistan,” CNN, April 14, 2017, http://edition.cnn.com/2017/04/14/asia/afghanistan-isis-moab-bomb/; Michael Schmitt and Peter Barker, “‘The Mother of All Bombs’: Understanding the Massive Ordnance Air Blast Weapon,” JustSecurity, April 15, 2017, https://www.justsecurity.org/40022/the-mother-bombs-understanding-massive-ordnance-air-blast-weapon/; Fares Akram and David Kirkpatrick, “To Block Gaza Tunnels, Egypt Lets Sewage Flow,” New York Times, February 20, 2013, http://www.nytimes.com/2013/02/21/world/middleeast/egypts-floods-smuggling-tunnels-to-gaza-with-sewage.html; Thomas Fessy, “French Fight in Mali’s Hostile Desert,” BBC News, March 25, 2013, http://www.bbc.com/news/world-africa-21919769; Tom Parfitt, “Putin’s Deadliest Flamethrower Wipes Out Evil Islamic State Militants in Syria,” Express, January 27, 2016, https://www.express.co.uk/news/world/615448/Vladimir-Putin-Islamic-State-ISIS-Syria-Iraq-Latakia; and CNBC, “Israeli Strikes Hit Gaza Tunnels as Diplomats Work for Truce,” CNBC, May 17, 2021, https://www.cnbc.com/2021/05/17/israel-says-gaza-tunnels-destroyed-in-heavy-airstrikes.html.

5 For example, the academic conference held at Reichman University in December 2019 (https://www.runi.ac.il/en/research-institutes/government/subtwg/conference/) which brought together academic and military experts, produced material on the subject and raised awareness about subterranean threats.

6 On the diffusion of innovation in the social and military contexts, Everett Rogers, Diffusion of Innovations (New York: Free Press, 2003); and Michael Horowitz, The Diffusion of Military Power: Causes and Consequences for International Politics (Princeton: Princeton University Press, 2010).

7 Joint Chiefs of Staff, “Joint Publication 3-0,” Joint Operations, October 22, 2018, https://irp.fas.org/doddir/dod/jp3_0.pdf.

8 Jones, Underground Warfare 1914–1918; Alexander Barrie, War Underground: The Tunnellers of the Great War (New York: Ballantine Books, 1961); and Mangold and Penycate, “The Tunnels of Cù Chi”.

9 International Working Group on Subterranean Warfare, https://www.runi.ac.il/en/research-institutes/government/subtwg/.

10 Jeremiah Rozman, “The Army Is Preparing to Go Underground,” RealClearDefense, July 3, 2019, https://www.realcleardefense.com/articles/2019/07/03/the_army_is_preparing_to_go_underground_114555.html.

11 U.S. Army, “Subterranean Operations - ATP 3-21.51,” Headquarters, Washington, DC: Department of the Army, November 2019, https://irp.fas.org/doddir/army/atp3-21-51.pdf.

12 DARPA, “DARPA Subterranean (SubT) Challenge (Archived),” DARPA RSS, https://www.darpa.mil/program/darpa-subterranean-challenge; and DARPA, “Underminer (Archived),” DARPA RSS, https://www.darpa.mil/program/underminer.

13 The British Army, “Household Cavalry Sharpen Urban Ops Skills on the Rock of Gibraltar,” The British Army, April 20, 2021, https://www.army.mod.uk/news-and-events/news/2021/04/exercise-daring-rock/; and The British Army. “Going Underground: Royal Engineers Advance through the Yorkshire Dark,” The British Army, July 12, 2022, https://www.army.mod.uk/news-and-events/news/2022/07/exercise-hypogeal-bear-in-leeds/.

14 Yossi Yehoshua and Reuven Weiss, “We spend days and nights in the field until we find the tunnel,” Ynet News, October 14, 2018, https://www.ynetnews.com/articles/0,7340,L-5370114,00.html; and Yoav Zitun, “IAF Bombed Land-to-Sea Gaza Tunnel Used by Hamas Naval Commandos,” Ynet News, October 6, 2018, https://www.ynetnews.com/articles/0,7340,L-5283568,00.html.

15 The task is further complicated by the fact that underground facilities “may have segments from each of the different subcategories.” U.S. Army, “ATP 3-21.51”, para 1-48.

16 Atlas Obscura, “Siloam Tunnel – East Jerusalem, Israel,” Atlas Obscura, July 26, 2016, https://www.atlasobscura.com/places/siloam-tunnel.

17 Ali Ahmad Jalali and Lester W. Grau, The Other Side of the Mountain: Mujahideen Tactics in the Soviet-Afghan War (Fort Leavenworth, Kansas: United States Marine Corps Studies and Analysis Division, 1998), http://www.tribalanalysiscenter.com/TAUDOC/Other%20Side%20of%20Mountain.pdf.

18 Richard Cavendish, “Kazan Falls to Ivan the Terrible,” History Today 52, no. 10 (2002), 54–5.

19 Iain Banks, “Digging in the Dark: The Underground War on the Western Front in World War I,” Journal of Conflict Archeology 9, no. 3 (2014): 156–76.

20 Emma Graham-Harrison, “With Cats, Ferrets and Handcarts, Life Goes On Underground in Kharkiv,” The Guardian, March 29, 2022, https://www.theguardian.com/world/2022/mar/29/kharkiv-ukraine-underground-refuge-life.

21 Amos Harel, “How Were Palestinian Militants Able to Abduct Gilad Shalit?” Haaretz, October 18, 2015, http://www.haaretz.com/how-were-palestinian-militants-able-to-abduct-gilad-shalit-1.390573.

22 Times of India, “Underground Tunnel Unearthed in J&K’s Samba,” Times of India, November 22, 2020, https://timesofindia.indiatimes.com/india/massive-anti-tunneling-operation-on-along-ib-in-jks-samba/articleshow/79352066.cms.

23 Mariam Karouny, “Massive Tunnel Bomb Hits Syrian Army Base,” Reuters, May 15, 2014, https://www.reuters.com/article/us-syria-crisis-airstrikes-idUSBREA4E09520140515.

24 Chen, “Battle of Iwo Jima”.

25 U.S. Army, “ATP 3-21.51”, para 1–48.

26 Joshua S. Bowes et al., “The Enemy Below: Preparing Ground Forces for Subterranean Warfare,” Defense Technical Information Center. Naval Postgraduate School, December 2013, https://apps.dtic.mil/sti/pdfs/ADA620416.pdf.

27 Bowes et al., “The Enemy Below”.

28 The British Army, “UK Doctrine Note 20/05: Subterranean Operations in the Urban Environment,” The British Army, August 2020.

29 Israel Defense Editorial Team, “Maneuver in the Underground,” Israel Defense, July 15, 2014, http://www.israeldefense.co.il/he/content/%D7%AA%D7%9E%D7%A8%D7%95%D7%9F-%D7%91%D7%AA%D7%AA-%D7%94%D7%A7%D7%A8%D7%A7%D7%A2 (in Hebrew).

30 James Verini, “Gaza’s Tunnels, Now Used to Attack Israel, Began as Economic Lifelines,” National Geographic, July 21, 2014, https://www.nationalgeographic.com/history/article/140721-gaza-strip-tunnels-israel-hamas-palestinians.

31 Daphné Richemond-Barak, “From Gaza to Ukraine: Three Principles of Underground Warfare,” Jerusalem Post, May 29, 2022, https://www.jpost.com/opinion/article-708002.

32 Mangold and Penycate, “The Tunnels of Cù Chi”, 32; and Arthur Herman, “Notes from the Underground: The Long History of Tunnel Warfare,” Foreign Affairs, August 26, 2014, https://www.foreignaffairs.com/articles/middle-east/2014-08-26/notes-underground.

33 Paul Kelso, “Taliban Secret Weapon: Ancient Irrigation Trenches,” National Geographic, November 5, 2001, https://web.archive.org/web/20011116033143/https://news.nationalgeographic.com/news/2001/11/1105_wirekarez.html.

34 Michael Georgy and Ahmed Rasheed, “Tunneling Through Triangle of Death, Islamic State Aims at Baghdad from South,” Reuters, August 4, 2014, http://www.reuters.com/article/us-iraq-security-south-insight-%20idUSKBN0G41CO20140804.

35 Isabel Coles and Jamie Bullen, “Abandoned Train Tunnel in Mosul Used by ISIS as Underground Training Camp for its Elite Fighters,” The Mirror, March 6, 2017, http://www.mirror.co.uk/news/world-news/abandoned-train-tunnel-mosul-used-9975543.

36 Deeptiman Tiwary, “Six Tunnels on Jammu Border in Four Years: Walkway, Air Pipes,” The Indian Express, November 29, 2020. https://web.archive.org/web/20201203070549/https://indianexpress.com/article/india/jammu-kashmir-tunnels-walkway-air-pipes-7072302/lite/.

37 Interlocutory Decision on the Applicable Law: Terrorism, Conspiracy, Homicide, Perpetration, Cumulative Charging, Case No. STL-11-01/I, ¶ 85 (Special Trib. for Lebanon Feb. 16, 2011) (defining the crime of terrorism in times of peace as including the following three elements: (1) the perpetration of a criminal act (such as murder, kidnapping, hostage-taking, arson, and so on), or threatening such an act; (2) the intent to spread fear among the population (which would generally entail the creation of a public danger) or to directly or indirectly coerce a national or international authority to take some action, or to refrain from taking it; and (3) a transnational element).

38 See, e.g. AFP and Times of Israel Staff, “Hamas Operative Killed in Gaza Tunnel Collapse”, Times of Israel, January 20, 2017, http://www.timesofisrael.com/hamas-operative-killed-in-gaza-tunnel-collapse/.

39 Maan News, “Hamas Fighter, Civilians Killed in Gaza Tunnels Near Rafah,” Maan News, February 19, 2015, https://web.archive.org/web/20150731211446/http://www.maannews.com/Content.aspx?id=759515; and Saed Bannoura, “Palestinian Killed in Tunnel Accident in Gaza”, IMEMC, June 14, 2012, http://imemc.org/article/63724/.

40 Yoram Schweitzer, “A Mixed Blessing: Hamas, Israel, and the Recent Prisoner Exchange,” Strategic Assessment 14, no. 4, (January 2012).

41 Daphné Richemond-Barak, “Is Israel Winning the Underground Fight?,” Small Wars Journal, December 19, 2018, https://smallwarsjournal.com/jrnl/art/israel-winning-underground-fight.

42 Loveday Morris and Ruth Eglash, “Israel Announces Operation to Destroy Hezbollah Tunnels under its Northern Border,” The Washington Post, December 4, 2018, https://www.washingtonpost.com/world/middle_east/israel-announces-operation-to-destroy-hezbollah-tunnels-in-its-territory/2018/12/04/f9e74058-f7cb-11e8-8642-c9718a256cbd_story.html  

43 The British Army, “UK Doctrine Note 20/05”.

44 Andrew Harvey, “The Levels of War as Levels of Analysis,” Army University Press, November-December 2021, https://www.armyupress.army.mil/Journals/Military-Review/English-Edition-Archives/November-December-2021/Harvey-Levels-of-War/.

45 Haley Mercer, “Assessment of the Operational Implications of 21st Century Subterranean Conflict,” Divergent Options, May 4, 2019, https://divergentoptions.org/2019/05/08/assessment-of-the-operational-implications-of-the-twenty-first-century-subterranean-conflict/.

46 Yonah Jeremy Bob, “IDF Chief: Hamas Tunnels Are Not a Strategic or Existential Threat,” Jerusalem Post, March 23, 2017, https://www.jpost.com/Arab-Israeli-Conflict/IDF-chief-Hamas-tunnels-are-not-a-strategic-or-existential-threat-484853.

47 The Syrian authorities had no knowledge of a 351-feet-long tunnel being dug under their noses in the early stages of the Syrian War; the Israeli military nearly missed a cross-border tunnel dug by Hamas between the Gaza Strip and the Israeli town of Ein Hashlosha; and French and Chadian forces were ambushed by AQIM forces emerging from caves and tunnels in Northern Mali after failing to detect communication for weeks. Chris Hughes, “Mali Menace: Why We Will End Up Fighting Al-Qaeda’s Army in Africa,” Mirror, January 16, 2013, http://www.mirror.co.uk/news/uk-news/mali-menace-why-we-will-end-up-fighting-1536512; and Rukmini Callimachi, “Al Qaeda Fighters Carve Out Own Country in Mali,” Washington Post, January 16, 2013, http://www.washingtontimes.com/news/2013/jan/16/al-qaeda-fighters-carve-out-their-own-country/.

48 Mangold and Penycate, “The Tunnels of Cù Chi”; US Training Circular para 3-1: “The Subterranean Environment Puts Soldiers at a Disadvantage.”

49 British Army Review, “BAR Special Report,” British Army Review, 2018, https://www.army.mod.uk/umbraco/Surface/Download/Get/11771, 108.

50 U.S. Army, “ATP 3-21.51”, para. 4–12.

51 Ellen Ioanes, “DARPA Is Asking Universities for Access to Their Tunnels ASAP, and It’s Because the US Military Thinks Its Next War Will Go Underground,” Business Insider, August 31, 2019, https://www.businessinsider.com/darpa-underground-spaces-prep-urban-warfare-2019-8.

52 Mike Rennie and Michael Lindenthal, “Psychological Challenges in Urban Operations,” presented at Urban Operations Expert Talks, 2022.

53 Corrine Berger et al., “Psychophysiological Predictors of Soldier Performance in Tunnel Warfare: A Field Study on the Correlates of Optimal Performance in a Simulation of Subterranean Combat”, Military Medicine 188, no. 3–4 (2023): 711–717, https://doi.org/10.1093/milmed/usab312.

54 José Paulo Silva Bartolomeu, “Operations in Subterranean Systems: Terrain and Weather Variable”, Security and Defence Quarterly 29, no. 2 (25 June 2020): 39–60, https://doi.org/10.35467/sdq/119946.

55 Peter Gorner, “Life of a Tunnel Rat: Fighting Fear in ‘Nam’”, Chicago Tribune, June 28, 1985, http://articles.chicagotribune.com/1985-06-28/features/8502110841_1_cu-chi-american-tunnel-rats-john-penycate; and Gordon L. Rottman and Brian Delf, Tunnel Rat in Vietnam (Botley, Oxford, UK: Osprey Publ., 2012), 60.

56 Jason McCammack, “What Lies Beneath”. U.S. Customs and Border Protection, February 20, 2017. https://www.cbp.gov/frontline/what-lies-beneath.

57 U.S. Army, “ATP 3-21.51”, paras. 1–32 and 1–18.

58 Matthew Cox, “Army Is Spending Half a Billion to Train Soldiers to Fight Underground,” Military.com, June 25, 2018, https://www.military.com/daily-news/2018/06/24/army-spending-half-billion-train-troops-fight-underground.html.

59 Think Defence, “Communications in the Subterranean Environment,” Think Defence, September 23, 2021, https://www.thinkdefence.co.uk/2021/08/communications-in-the-subterranean-environment/.

60 Think Defence, “Communications”.

61 Kris Osborn, “WiFi Technology Connects Soldiers for Underground Combat,” Defense Systems, May 10, 2017, https://www.realcleardefense.com/2017/05/10/wifi_technology_connects_soldiers_for_underground_combat_292935.html; CNN, “How Radios Are Helping Thailand Cave Rescue,” CNN, July 4, 2018, https://edition.cnn.com/videos/world/2018/07/04/radios-thailand-cave-rescue-liebermann-pkg-vpx.cnn; and Amit Katwala, “The Thai Cave Rescue is Relying on Some Totally Ingenious Tech Hacks,” Wired, July 6, 2018, https://www.wired.co.uk/article/thai-cave-rescue-boys-news-tech.

62 U.S. Army, “ATP 3-21.51”, para. 4–49.

63 Cox, “Army Is Spending Half a Billion to Train Soldiers to Fight Underground”.

64 Hoa G. Nguyen et al., “Maintaining Communication Link for Tactical Ground Robots,” Space and Naval Warfare Systems Center, San Diego CA, 2004, https://apps.dtic.mil/sti/citations/ADA426296.

65 Nguyen et al., “Tactical Ground Robots”.

66 Marco Tranzatto et al., “Cerberus in the DARPA Subterranean Challenge,” Science Robotics 7, no. 66 (2022), https://doi.org/10.1126/scirobotics.abp9742; and Greg Nichols, “Darpa Challenge: Underground War Robots,” ZDNet, May 31, 2019, https://www.zdnet.com/article/darpa-challenge-underground-war-robots/.

67 Jon Harper, “Going Underground: The U.S. Government’s Hunt for Enemy Tunnels,” National Defense, Volume 102, no. 770 (January 2018): 28–32, http://www.jstor.org/stable/10..2307/27022062; and Aaron Morris et al., ‘Recent Developments in Subterranean Robotics’, Journal of Field Robotics 23 (1 January 2006): 35–57, https://doi.org/10.1002/rob.20106.

68 U.S. Army, “ATP 3-21.51”, para. A-8.

69 U.S. Army, “ATP 3-21.51”, para. 4–2.

70 U.S. Army, “ATP 3-21.51”, para. 3–38.

71 U.S. Army, “ATP 3-21.51”, para 3–33.

72 John Spencer, “My Underground Warfare Wish List,” Modern War Institute, February 18, 2019, https://mwi.usma.edu/underground-warfare-wish-list.

73 Spencer, “Underground Warfare”.

74 Daphné Richemond-Barak, Underground Warfare (New York: Oxford University Press, 2018), 195–6.

75 Alex Sorkin et al., “Medical Challenges in Underground Warfare”, Military Medicine 186, no. Supplement_1 (1 January 2021): 839–44, https://doi.org/10.1093/milmed/usaa447.

76 Richemond-Barak, Underground Warfare, 87.

77 Bowes et al., “The Enemy Below”.

78 Richemond-Barak, Underground Warfare, 95.

79 U.S. Army, “ATP 3-21.51”, para. 3–35 and A-20.

80 The British Army, “Going Underground: Royal Engineers Advance through the Yorkshire Dark,” The British Army, July 12, 2022, https://www.army.mod.uk/news-and-events/news/2022/07/exercise-hypogeal-bear-in-leeds/.

81 Karin Mascher et al., “NIKE BLUETRACK: Blue Force Tracking in GNSS-Denied Environments Based on the Fusion of UWB, IMUs and 3D Models”. Sensors 22, no. 8 (2022): 2982, https://doi.org/10.3390/s22082982.

82 Lin Edwards, “US Military Developing Geolocation System for Underground,” Phys.org, March 11, 2010, https://phys.org/news/2010-03-military-geolocation-underground.html.

83 Jeff Reinke, “Army Digging into Subterranean Warfare Options”, Thomasnet, May 24, 2019, https://www.thomasnet.com/insights/army-digging-into-subterranean-warfare-options/.

84 U.S. Department of the Army, “Advance Situational Awareness,” Army Publishing Directorate, April 2021, https://armypubs.army.mil/epubs/DR_pubs/DR_a/ARN34875-TC_3-22.69-000-WEB-1.pdf.

85 U.S. Army, “ATP 3-21.51”, paras. 6-89 and 6-90.

86 Cox, “Army Is Spending Half a Billion to Train Soldiers to Fight Underground”.

87 Michael C. Horowitz, “Nonstate Actors and the Diffusion of Innovations: The Case of Suicide Terrorism,” International Organization 64, no. 1 (December 2010): 33–64, https://doi.org/10.1017/s0020818309990233.

88 John Spencer, “The Return of Tunnel Bombs: A Medieval Tactic on the Modern Battlefield,” Modern War Institute, December 30, 2019, https://mwi.usma.edu/return-tunnel-bomb-medieval-tactic-modern-battlefield/.

89 Avinoam Idan and Brenda Shaffer, “Israel’s Role in the Second Armenian-Azerbaijan War” in The Karabakh Gambit: Responsibility for the Future, ed. Turan Gafarli and Michael Arnold (TRT World Research Centre, 2021), 197–8.

90 DARPA, “DARPA Completes Underminer Program, New Tactical Tunnelling Technologies Will Support National Security Objectives,” DARPA, March 31, 2022, https://www.darpa.mil/news-events/2020-04-01.

91 Erin Burnett and Rhea Mogul, “We’ll Eat Birds If We Need to: Ukrainians Trapped in Azovstal Steel Plant Consider Desperate Measures as Russians Close In,” CNN, May 3, 2022, https://edition.cnn.com/2022/05/03/europe/azovstal-steel-plant-ukraine-desperation-intl-hnk/index.html.

92 Michael Schwirtz, “Last Stand at Azovstal: Inside the Siege That Shaped the Ukraine War,” The New York Times, July 24, 2022, https://www.nytimes.com/2022/07/24/world/europe/ukraine-war-mariupol-azovstal.html.

93 Benoît Vitkine, “Visiting the Azovstal Tunnels, Mariupol’s Last Stronghold Transformed into a Labyrinth of Death and Destruction,” Le Monde, June 15, 2022, https://www.lemonde.fr/en/international/article/2022/06/15/visiting-the-azovstal-tunnels-mariupol-s-last-stronghold-transformed-into-a-labyrinth-of-death-and-destruction_5986828_4.html.

94 Vitkine, “Azovstal Tunnels”.

95 U.S. Marine Corps, “Military Operations on Urbanized Terrain (MOUT) – MCWP 3-35.3,” U.S. Marine Corps, 1998, https://www.marines.mil/Portals/1/MCWP%203-35.3.pdf

96 Joint Chiefs of Staff, “Joint Urban Operation - JP 3-06,” Joint Chiefs of Staff, November 20, 2013, https://www.jcs.mil/Portals/36/Documents/Doctrine/pubs/jp3_06.pdf.

97 The British Army, “UK Doctrine Note 20/05”.

98 Patrick Kingsley, Iyad Abuheweila, and Evan Hill, “Dreams in Rubble: An Israeli Airstrike and the 22 Lives Lost,” New York Times, June 17, 2021, https://www.nytimes.com/2021/06/17/world/middleeast/gaza-israel-airstrike-tunnel.html.

99 Atlas Obscura, “Naval Underground Headquarters,” Atlas Obscura, August 24, 2010, https://www.atlasobscura.com/places/naval-underground-headquarters.

100 Erin McDowell, “A top-secret, abandoned Soviet submarine base that was hidden from the public for decades is now a museum. Take a look inside,” Insider.com, July 26, 2021, https://www.insider.com/inside-an-abandoned-secret-soviet-submarine-base. The base itself was decommissioned in the 1990s after the collapse of the Soviet Union.

101 Entsar Abu Jahal, “Israel announces destruction of underwater Hamas tunnel,” Al Monitor, June 19, 2018, http://www.al-monitor.com/pulse/originals/2018/06/israel-destroy-underwater-tunnel-hamas-gaza.html#ixzz5LJbGmgpy.

102 Zitun, “IAF bombed land-to-sea Gaza tunnel”.

103 Ibid.

104 David Crouch, “Swedish navy returns to vast underground HQ amid Russia fears,” The Guardian, September 30, 2019, https://www.theguardian.com/world/2019/sep/30/swedish-navy-returns-to-vast-underground-hq-amid-russia-fears.

105 Joseph Trevithick, “This Is the Cave Facility in Norway That U.S. Navy Submarines Could Soon Operate From,” TheDrive.com, October 13, 2020, https://www.thedrive.com/the-war-zone/37043/this-is-the-cave-facility-in-norway-that-u-s-navy-submarines-could-soon-operate-from.

106 H. I. Sutton, “Chinese Navy Submarines Are Protected By Underground Tunnels,” Forbes.com, May 5, 2020, https://www.forbes.com/sites/hisutton/2020/05/05/chinese-navy-submarines-are-protected-by-underground-tunnels/?sh=3cd960076312; and Tyler Rogoway, “Image Shows Chinese Submarine Entering Mysterious Cave Facility At South China Sea Base,” TheDrive.com, August 19, 2020, https://www.thedrive.com/the-war-zone/35837/image-shows-chinese-submarine-entering-mysterious-cave-facility-at-south-china-sea-base.

107 Felix K. Chang, “China’s Nuclear Interest in the South China Sea,” Foreign Policy Research Institute, April 27, 2017, https://www.fpri.org/2017/04/chinas-nuclear-interest-south-china-sea/.

108 Hans Kristensen, “Navy Builds Underground Nuclear Weapons Storage Facility; Seattle Busses Carry Warning,” Federation of American Scientists, June 27, 2016, https://fas.org/blogs/security/2016/06/pacific-ssbn-base/.

109 Dennis V. Hickey, “China’s Military Modernization and Taiwan’s Defense Reforms: Programs, Problems, and Prospects,” in Taiwan’s Defense Reform, ed. Martin Edmonds and Michael M. Tsai (New York: Routledge, 2006) 41–60; James Dennis, “DPRK Briefing Book: HARTS in North Korea,” Nautilus Institute for Security and Sustainability, 1986, https://nautilus.org/publications/books/dprkbb/military/dprk-briefing-book-harts-in-north-korea/; and H. I. Sutton, “Iran Deploys Missiles Covering the Strait of Hormuz,” Forbes.com, April 8, 2020, https://www.forbes.com/sites/hisutton/2020/04/07/iran-deploys-missiles-covering-the-strait-of-hormuz/.

110 Laurie A. Shuster, “Bridge and Tunnel Security,” Civil Engineering 74, no. 9 (September 2004): 40–9.

111 Rory Carroll, “US Goes Underground to Check Rome Plot,” The Guardian, February 27, 2002, https://www.theguardian.com/world/2002/feb/27/rorycarroll1.

112 Michael Wilson, “‘The Tunnel.’ Depressing, Claustrophobic and Now a Terror Target,” The New York Times, December 12, 2017, https://www.nytimes.com/2017/12/12/nyregion/subway-tunnel-passageway-bombing.html.

113 British Army Review, “BAR Special Report,” British Army Review, 2018, https://www.army.mod.uk/umbraco/Surface/Download/Get/11771.