“Sir, what do you want us to do about the tunnel?” one of my squad leaders asked. We were conducting platoon attacks at the massive Range 220 complex at the Marine Air Ground Combat Center in Twentynine Palms, California as part of a training exercise in 2017. Nowhere in my training had tunnels come up or been addressed as a tactical problem.

“Seal it, post security, and we’ll deal with it later,” I replied, an answer that not only seemed best for my platoon at the time, but also sums up the US military’s response to tunnels and subterranean warfare over the last century. We were able to clear our lane successfully and I was proud of how the platoon preformed. But, the tunnel nagged at me. Was there a right answer, or even a good answer?

The Marine Corps has no clear or developed doctrine or training for dealing with underground facilities. The Army published its first doctrinal publication related to subterranean warfare in November 2017, Small Unit Training in Subterranean Environments. But one of the best descriptions of subterranean warfare comes from the world of fiction—the hypothetical scenario in Max Brooks’s World War Z, a novel set in a future where small paramilitary units are fighting a zombie infestation in the catacombs beneath Paris. The US Army Asymmetric Warfare Group has produced two good handbooks—the Subterranean Warfare Handbook and the Subterranean Operations Handbook—which start to address subterranean warfare, but both fall short of establishing either a subterranean doctrine or comprehensive set of tactics, techniques, and procedures. Aside from these publications, there are also a scattering of theses and dissertations from military researchers and case studies of underground warfare. Taken as a whole, though, the sum of work the military has put into studying subterranean warfare pales in comparison to what is needed. We, as a Marine Corps, need to develop doctrine and a method for fighting underground and train our Marines to win in subterranean warfare.

Underground facilities are a near constant in asymmetric warfare. As a 2007 Marine Corps Gazette article by William Birdzell, “Overcoming Enemy Counters to Combined Arms,” put it:

Tunnel systems and irregular urban warfare are essentially advanced methods of cover and concealment. Given that the mission of the infantry is to “locate, close with and destroy the enemy by fire and maneuver,” if the enemy can prevent us from locating him or closing with him, it will be nearly impossible for us to destroy him.

An Air Force officer put it even more succinctly: “Subterranean warfare may be the answer for the enemies of the United States.” For much of the era of manned flight, combatants have consistently gone underground in the face of enemy air superiority—either in strike capability or reconnaissance.

US forces have faced challenges posed by subterranean warfare before. Marines fought underground opponents on the islands of Tarawa, Peleliu, Iwo Jima, and Okinawa. Soldiers and Marines battled entrenched and underground Chinese troops in Korea.  In Vietnam, the 173rd Airborne Division encountered thousands of tunnels stretching hundreds of miles from the outskirts of Saigon to the Cambodian border. In Operation Desert Storm, the Air Force deemed underground Iraqi bunkers the most secure places in the country. In Afghanistan the last hideout of al-Qaeda was a cave-and-tunnel complex in Tora Bora. But those experiences have mostly come far from cities. We’ve been spared from inclusion in most major historical cases of subterranean warfare in urban environments—in places like Stalingrad, Hue, or the Warsaw Ghetto. We’ve never had to deal with a subterranean layer in a major city like the subways under New York City or London, the catacombs and quarries under Paris and Rome, or the PATHS network under Toronto, all daunting threat environments. Clearing Mosul, with a prewar population of only seven hundred thousand and no significant prewar subterranean networks, took nearly nine months. Adding an underground layer onto that operation would have made it substantially more complex and time-consuming.

An approach aimed at addressing our skills gap should include four lines of effort. We need to (1) define and codify subterranean warfare in an official publication, (2) develop a school or course with a curriculum dedicated to training our soldiers and Marines in the skills they need to win underground, (3) incorporate subterranean warfare into existing schools, and (4) develop gear for subterranean warfare. Doing so jointly, with multiple services involved, has the advantages of leveraging more resources and experience, but may be slower to get going.

Codifying Subterranean Warfare

We need to change the way we think about subterranean warfare. While underground environments need not necessarily be labeled a separate “domain” of warfare alongside sea, air, land, space, and cyber, they should be understood as more than simply an additional dimension of other existing environments as it is currently (typically, urban terrain). We need to understand that any type of terrain can have a subterranean component. It is not just an urban problem; in fact, historically, our military has faced far more tunnel networks in jungles, mountains and rural areas than urban environments. However, if the fastest way to make subterranean training available to the operating forces is by piggybacking on existing urban training, then that offers a starting point. Isolated case studies and examples should give way to a comprehensive understanding; we need a single, robust framework within which to understand both Vietcong tunnel networks and the London Tube. Our enemies can and do leverage underground facilities across all six of the warfighting functions, using tunnels and underground storage to protect their forces and serve as logistics hubs. Tunnel systems have been used to maneuver forces to attack above-ground objectives from the tactical level and the strategic level. North Korea constructed tunnels across the DMZ that would have been able to move ten thousand troops an hour across the border in to South Korea. At the siege of Petersburg in the Civil War and in the Battle of Messines Ridge in World War I, units used tunnels to undermine enemy positions and detonate massive amounts of ordnance underneath them, effectively applying subterranean fires on surface targets. To fully appreciate the value that tunnels and underground facilities can provide our enemies we need only to look at history.

An Underground Warfare School

After redefining subterranean warfare, we need to develop a school that can train Marines, soldiers or both how to fight underground and offers courses built around a comprehensive subterranean-focused doctrinal publication. Leading voices in the Army have argued for the creation of a “megacities combat unit,” trained at a special school. The Marine Corps has neither the money nor the manpower for that level of specialization; what we need are skills that we can distribute throughout the force. We should start by creating a “Subterranean Leader” designation, given to those who successfully complete training at the subterranean warfare school.

The school should not be limited to individuals; it should also be capable of training whole units as well. Together, the individual and unit training will establish a breadth and depth of knowledge across the force. A similar “Leader”-centric model works well at the Marine Corps Mountain Warfare Training Center. Another option is to incorporate a significant subterranean portion into the curriculum of a Joint Close Combat School that has been proposed and discussed elsewhere.

The Corps also already has a fifteen-day Urban Leader Course and an Advanced Urban Combat Course, run by the 1st Marine Division in Camp Pendleton and Security Forces Battalion in Chesapeake, Virginia, respectively, which could serve as introductory courses or primers for a longer, weeks-long course. Army schools—like the 10th Mountain Division’s Urban Combat Leaders Course—use similar models.

The course would start with a thorough definition of the subterranean domain and a framing of the problem for our forces.  Next, the course would provide an overview of the history of subterranean warfare, from the siege of the Roman fortress of Dura Europos to karez clearing in Afghanistan—carefully considering all of the case studies in the same analytical framework. It is vital that we look at subterranean warfare as a comprehensive concept, not isolated and disparate cases.

The subterranean warfare course also needs a practical application portion, where students are shown best practices for underground warfare—from communication techniques and interior movement to mapping and breaching. The tactics, techniques, and procedures must start with tunnel detection, since the first step in addressing the subterranean threat on the battlefield is uncovering it. This is best done with a combination of ground search techniques and aerial survey with drones. Next, students need to be taught a framework for answering the critical question: Do we need to enter the tunnel? Depending on the tactical situation above ground, commanders may not want to devote resources to a lengthy and exhaustive subterranean fight. On the other hand, it might be necessary to achieve military objectives. Commanders need to be prepared to make this decision, and Marines or soldiers who have the Subterranean Leader designation need to be prepared to advise their commanders. Then students need to learn how to systematically clear and map underground facilities. They need to master any special equipment designed for underground use, like air filters or wire-based communications. After students have mastered these skills, they should be tested on them again in complete darkness and with MOPP gear.

The course should work with the widest possible range of partners to provide the best possible product for soldiers and Marines. The Army’s Asymmetric Warfare Group is currently at the forefront of DoD thinking about urban and subterranean tactics and environments. Many civilian organizations and groups work with and in underground facilities daily and have deeply relevant experience. The Drug Enforcement Agency and Border Patrol routinely deal with smuggling tunnels. A range of civilian organizations and companies work deep in mines, build and maintain subway tunnels, and emplace underground pipelines. Geologists, archaeologists, and spelunkers spend years studying and mapping complex cave systems—each with a unique perspective. We should leverage their expertise. We would also be well served by reaching out to allies who have developed and maintained their subterranean expertise, like Israel.

The course should also incorporate a field portion in which students visit different types of underground facilities. The extent and scope of this portion of the course would be determined by the available resources, but students would benefit by visiting subways, mines, natural cave networks, underground missile silos, or complex manmade infrastructure like the Cheyenne Mountain Complex in Colorado.

The culminating event of the course should challenge the students to use subterranean warfare and underground facilities for their advantage in a small-unit force-on-force exercise. Students need to leave the schoolhouse not only with skills and understanding that will help them counter enemy advantages underground, but also with the skills to exploit our own advantages. We will succeed underground when we cannot only deny subterranean spaces to the enemy but, use them to maneuver and advance our own objectives.

In addition to teaching, the schoolhouse should also be responsible for the further refinement of doctrine and development of subterranean tactics, techniques, and procedures. Because these are only in their adolescence (where they exist at all), it is essential to continue refinement. This merely reinforces what then-Commandant of the Marine Corps Gen. Charles Krulak wrote in the foreword of MCDP 1, Warfighting: “Military doctrine cannot be allowed to stagnate, especially an adaptive doctrine like maneuver warfare. Doctrine must continue to evolve based on growing experience, advancements in theory, and the changing face of war itself.”

Even though the Army published a subterranean manual in 2017, the publication itself recognizes that it is incomplete, acknowledging that it was only vetted through an “urgent” process and soliciting additions and refinements in the preface. The immediate need to aggregate a baseline degree of subterranean warfare knowledge across America’s close-combat forces has thus far precluded a full and exhaustive research and writing period of several years. We should be willing to accept a 70-percent solution with regard to subterranean doctrine and tactics, techniques, and procedures in the short term, but we must remain committed to their further development simultaneously with their implementation.  A team from a subterranean warfare schoolhouse responsible for further development would travel widely, gathering and compiling all available information on subterranean warfare or subterranean operations from allies, external organizations, and history. Subterranean warfare is not new, and the information that our soldiers and Marines need is out there.

The school also needs to offer specialized courses in communications, engineering, canine handling, explosive ordnance disposal, and countering biological, radiological, and nuclear threats. These community-specific courses would complement the training aimed at infantry forces. If the enemy can operate across all six warfighting functions underground, we should be able to, as well.

Up-Gunning Current Courses with Subterranean Components

The third line of effort toward improving our underground capabilities is incorporating basic subterranean skills into existing schoolhouses and courses. This is the fastest way that we can get critical information and skills out to operating forces. It will take time (and resources and consequent budgetary negotiations) to stand up a school and develop an entirely new course. However, incorporating days of underground training into current courses in the Marine Corps—Marine Combat Training, the Basic School, the School of Infantry, and the Infantry Officer Course—and the Army—Basic Combat Training and certain Advanced Individual Training and Basic Officer Leaders Courses—can happen almost immediately. Soldiers and Marines who benefit from that training can have an impact on operating forces in less than a year. Students at those courses already receive dedicated urban training (though often limited to entering and clearing rooms) and other environment-specific training. We can add subterranean training to those schools and build mock tunnels or facilities to facilitate training. The Marine Corps has been making progress at building and incorporating subterranean elements into training at the Marine Air Ground Combat Center, but there is more—like developing a subterranean live-fire complex, for example—that can be done.

A Unique Environment Requires Unique Equipment

Finally, we need to either develop a new kit or modify current equipment to meet the particular requirements of underground environments. It is simply a fact that fighting underground involves unique equipment needs that are not met by current gear. The “Tunnel Rats” in Vietnam developed their own kit for tunnel operations and the Army developed limited specialized gear that made it into the hands of a few operational units.  At the most basic level, the kit needs to improve the ability of Marines or soldiers to see, shoot, and communicate underground.

We have to be able to see underground in complete darkness. Our current night-vision optics will be rendered largely ineffective in an environment with no ambient light and little to no temperature variation. Our PEQ-16s and headlamps work, but they need to incorporate more redundancy and longer battery life. Rifles have no place in the cramped confines of the underground. Units forced to fight underground from Okinawa to Vietnam and Afghanistan ditched rifles in favor of pistols when clearing tunnels. Compact submachine guns equipped with lights and suppressors are potentially an ideal solution, offering automatic fire and compact size. Operating underground also requires wire-based communications; VHF radios, which rely on line of sight, will fail in labyrinthine underground environments. Unfortunately, the wire communications that were an essential part of tunnel clearing gear in Vietnam are currently in the process of being phased out of the Marine Corps. That’s simultaneously a problem and an opportunity to look to the future and replace existing radios with new ones that would offer underground capability through networking, like the TSM-X Waveform radios marketed by TrellisWare. Marines and soldiers also need adequate hearing protection, preferably over-ear, active protection, because noises like gunfire and explosions are amplified significantly underground, but forces also need to be able to hear to communicate effectively.

Other items that should be considered are devices to measure air quality, helmets designed to protect hazards unique to subterranean environments, and air tanks, and mapping kits for facilities deep underground where compasses and GPS will not work. If soldiers or Marines are forced to clear tunnels with flashlights and pistols like their Tunnel Rat forebears in Vietnam, we have failed.

Opponents of this proposal will argue that we lack the resources it would require and that subterranean warfare should be treated as a part of existing (but woefully insufficient) urban training. Subterranean warfare training is worth every penny, even at the cost of cuts elsewhere. It is a skill set that we need both for peer threats and asymmetric threats. Underground facilities and defenses have cost the US military dearly across our history. During some of Marine Corps’s bloodiest battles, like Iwo Jima and Okinawa, Marines were unprepared to face dug-in Japanese defenders who used underground facilities to maximum effect and were impervious to our airpower and naval gunfire. The greatest threat on the Korean Peninsula is a North Korean military prepared to fight for every mile of the country like the Japanese did on the islands in the Pacific (and whereas the Japanese had months to prepare their defenses, the North Koreans have had decades). Even in low-intensity conflict and stability operations—the so-called small wars—observers need only look to the subterranean networks from Vietnam to Afghanistan to see the challenges they pose. Arguing that subterranean warfare should simply be a subset of urban training simply doesn’t take the threat seriously enough. While, subterranean warfare is certainly part of the urban-terrain problem set, it is also unique. And we would do well to remember that underground facilities are often in non-urban environments.—the deadliest underground defenses we’ve faced were far from urban centers.

Both as independent services and as a joint effort, the Marine Corps and the Army have to be able to fight and win anywhere, in “any clime and place.” We are not currently prepared to fight underground. Time and time again Marines and soldiers have had to relearn old lessons. Subterranean warfare is nothing new, and it is not going anywhere. We can no longer afford to ignore subterranean warfare or expect to adapt to it after hostilities begin. We must develop new and better doctrine, train our soldiers and Marines to win underground with a new school of subterranean warfare and the addition of subterranean warfare to existing schools, and equip them properly. Our goal should not be to deny the enemy use of underground spaces; it should be nothing short of dominating them.


1st Lt Walker D. Mills is a student at the Defense Language Institute in Monterey, CA. He wrote this paper while serving as a Rifle Platoon Commander with 2nd Battalion, 1st Marine Regiment.


Image credit: Sgt. Jessica DuVernay, US Army