Last year, members of Jabhat al-Nusra, al-Qaeda’s Syrian affiliate, dug a tunnel leading to the Syrian Air Force Intelligence building in Aleppo and detonated a massive bomb in an attempt to destroy the facility. Reported globally, this event was by no means a rarity in the ongoing Syrian civil war. As Benjamin Runkle warned last year, the United States and its allies must prepare for the subterranean future of warfare. His article was a broad and useful overview of the various threat actors using tunneling to negate the advantages that airpower and other technologies provide to Western militaries. As America increases its military involvement in Iraq and Syria, a more detailed look at the military significance of such structures is warranted. As of February of this year, there were nearly 4,500 U.S. troops in Iraq. Regardless of the merits of further intervening in the conflict, it is a fact that the United States and its allies are sending increasing amounts of troops to the region. Whatever the intentions of American leaders, this expanded presence is almost certain to result in greater contact with a variety of hostile forces. Al-Nusra, the Islamic State of Iraq and the Levant (ISIL), the Syrian government, and other factions in the war have all used tunnels to great effect throughout the conflict. U.S. and allied militaries must thus understand and prepare for subterranean warfare.

The Subterranean Landscape in Iraq and Syria

Both Syrian regime and rebel forces have burrowed a vast series of tunnels into the area around Damascus. Given the ever-shifting tide of battle, these structures have become neutral parts of the battlespace, rather than dedicated mobility corridors for one side or the other. Teams of up to 300 insurgents have labored with shovels and pick axes to dig these tunnels. The Free Syrian Army has reportedly even employed architects to design a tunnel, which it used to infiltrate a government military base near the town of Erbin. Outside of the Syrian capital, regime forces claimed they had destroyed a subterranean network near Harasta in June 2015. This massive underground structure included tunnels up to 200 meters in length equipped with lighting and ventilation ducts. Later that year, ISIL built an elaborate series of passageways in the border town of Sinjar during its battle with Kurdish forces. Likely constructed with jackhammers and hand tools, the network featured multiple exit points fortified with sandbags to protect against American-dropped ordnance. The Islamist group also smashed holes in walls between buildings to allow covered, aboveground transit in the face of withering American airstrikes. As part of ISIL’s tenacious defense of the area south of Mosul, the group has constructed an underground “city within a city” to protect its fighters against advancing Iraqi government troops.

Tunneling in the Offense

The various warring factions in Syria and Iraq have not only used burrowing below ground as a defensive or force protection measure, but they have also used tunnels to deliver troops and explosives against their enemies. When they threatened the Iraqi capital in the late summer of 2014, ISIL made use of Saddam-era subterranean routes to evade Iraqi Security Forces, hide from their aircraft, and deny them rear-area security. In the battle for Homs in November 2015, al-Nusra built passageways 15 meters underground, some of which stretched for 3 kilometers. According to Syrian government soldiers, the primary purpose of these structures was to allow insurgents to encroach on regime fortifications by maneuvering in a covered fashion.

In an almost herculean effort, another rebel group reportedly spent seven months building a tunnel under the Syrian Army’s Wadi Deif base. Instead of using the subterranean passageway to deploy troops, the rebels used it to detonate almost 60 metric tons of explosives in May 2014 and kill at least 20 soldiers. Possibly following the lead of other groups, ISIL detonated six metric tons of explosives under an Iraqi army headquarters in Ramadi in March of 2015. The insurgents had spent two months digging a 240 meter tunnel under the structure. Syrian regime forces have likewise maneuvered toward rebel checkpoints under the surface to detonate explosives below their unsuspecting targets.

Relevance to U.S. Forces

Although American troops have only participated in limited ground combat in Iraq and Syria, the vast subterranean networks throughout the region pose a potential threat. Both ISIL and al-Nusra are currently engaged in active hostilities with the United States, and the Syrian regime maintains an uneasy ceasefire with American forces. The standard U.S. anti-tunnel technique currently consists of using precision-guided munitions to seal visible entrances from the air, and this approach has some utility when ground forces are not present. As the United States steps up direct action operations such as hostage rescues and raids to capture ISIL leaders, simply destroying tunnels with airstrikes will not suffice. As the American-led air campaign continues to take its toll, ISIL and other groups will likely move their most valuable assets underground. Although U.S. forces have faced similar subterranean fortifications in other conflicts, such as in Vietnam and Afghanistan, few institutional memories of these experiences remain. Although some principles of tunnel clearing are timeless, operating below the surface in the modern Levant requires a variety of unique technical and tactical considerations.

In terms of intelligence measures, U.S. forces should ensure that combat outposts possess overlapping networks of unattended seismic ground sensors to identify offensive tunneling efforts. Once a tunnel is detected, “bunker buster” earth-penetrating bombs could halt any subterranean construction. Well-placed thermobaric explosives would prove highly effective when employed against entry/exit points. In preparing for offensive operations near tunnel openings, sensors such as Vehicle and Dismount Exploitation Radar (VADER) would prove vital. This technology allows for the remote identification of movement patterns — including those on foot — critical for finding high-traffic tunnel entrances, and thus likely enemy concentrations. Given ISIL’s frequent use of Improvised Explosive Devices (IED) and other booby traps, U.S. forces should also methodically avoid openings that do not show signs of life.

Once inside, tunnel-clearing units would face additional challenges. Navigation may appear to be a relatively simple matter in a tunnel, but without the ability to regularly surface and confirm one’s position, getting lost underground is a significant threat. Low-tech solutions — pull lines or using infrared chemical light sticks as a “trail of bread crumbs” — are effective, but they impose additional logistical requirements and reduce a unit’s flexibility. The Defense Advanced Research Projects Agency has wisely funded efforts to design inertial navigation systems, which can guide subterranean movement without access to Global Positioning System satellites. Such technologies are vital if U.S. forces are to maneuver effectively underground. Communicating is also a serious challenge, as several meters of earth will generally block any radio transmissions. Though considered a relic of past conflicts, reliable wired telephone systems are important for maintaining command and control when moving long distances. Finally, munitions such as chlorobenzylidene malonitrile (CS) riot control gas may prove critical for clearing tunnel systems effectively and humanely.

Tunneling Ahead

Even if the U.S. footprint in Iraq and Syria remains limited, the potential threat posed by the enemy’s use of subterranean networks is dire. All major factions in the ongoing hostilities have used such structures to great effect. Whether or not the United States has “boots on the ground” in the fight to retake ISIL-occupied cities such as Mosul, its Iraqi allies certainly do, and they have already faced enemy fighters burrowed into the ground “like rats.” The United States and other coalition forces have likewise begun identifying and striking ISIL tunnel systems near the city from the air. With increasing participation in the conflict, future efforts to capture enemy leaders or free hostages will likely require underground transit and combat by American troops. Prior preparation is a critical factor in determining success in the subterranean environment, so U.S. forces should properly equip themselves with the necessary intelligence, munitions, and training to maneuver and fight underground.

Walter Haydock served as Marine Corps Reconnaissance and Intelligence Officer. He cleared tunnels in Southwestern Afghanistan as Platoon Commander, and targeted Middle Eastern terrorist groups while serving at the National Counterterrorism Center. The views expressed in this article are those of the author and do not reflect the official policy or position of the Department of Defense, National Counterterrorism Center, or U.S. government.

Image: Marius Arnesen, CC