On May 8, 2014, about three years into Syria’s civil war, anti-government forces prepared explosives—reportedly about 20 tons—in a freshly dug tunnel under the city of Aleppo. Above ground, Syrian soldiers in a hotel being used by the Syrian army were oblivious. The massive blast that followed killed approximately 40 soldiers. It took only 33 days to dig the 100-meter-long tunnel using hand tools.

Just five days later, rebels detonated another explosive-laden tunnel, this one 850 meters long and packed with 60 tons of explosives, killing 20 soldiers at a checkpoint on a Syrian military base. This tunnel took only 50 days to dig.

Two months later and about 500 kilometers away, Israel launched a major combat operation into Gaza. Among Operation Protective Edge’s stated objectives was destroying an extensive network of tunnels. Hamas had spent years planning and preparing a broad underground warfare strategy with tactical to strategic objectives. Its members used tunnels to infiltrate Israel—in some cases wearing Israeli military uniforms—to conduct kidnappings and ambushes, move rockets and other weapons, and hide ammunition. Its tunnels were found under schools, mosques, hospitals and private housing in dense urban areas.

Subterranean warfare is not new. From Assyrian forces’ tunneling in the 9th century B.C. to sieges of the medieval era to the Vietnam War, underground terrain has been part of the battlefield. But it’s seeing a resurgence, as the Aleppo tunnel bombs, the Israel-Gaza conflict and even more recent examples from the battle for Mosul, Iraq, make clear. Yet, despite the increasing threat posed by subterranean warfare, the U.S. Army remains largely unprepared—in terms of doctrine, training and equipment—for this environment.

A Dark Threat

The Islamic State group learned from Hamas’ experience in Gaza, and heavily invested in a broad tunnel warfare strategy. The group used tunnels to help hold the major cities it captured in 2014 and 2015. It dug tunnels under and between buildings to mitigate coalition intelligence, surveillance and reconnaissance and aerial-strike capabilities; house fighters; store weapons and equipment; and enable covered and concealed maneuver from building to building as the battles moved throughout the city.

Daphne Richemond-Barak, author of the 2018 book Underground Warfare, reveals how the use of tunnels in warfare has transitioned from a practice of the weak to one routinely used by both state and nonstate actors across the spectrum of conflict. Tunnels have been dug to cross borders in Gaza, Egypt and North Korea. Military sites around the world are buried deep underground for protection. Vast urban subterranean infrastructure networks alter how a city will take shape as a battlefield. And tunnels have been used extensively for defense, evasion, maneuver and offensive underground strikes in almost every battle over the past 10 years in Syria and Iraq.

Doctrine of Avoidance

According to the Army’s latest doctrine on subterranean techniques, Army Techniques Publication 3-21.51: Subterranean Operations, “Entering and fighting in a subterranean environment is extremely high risk and should be avoided whenever possible.” Yet it is both folly and fantasy to believe soldiers will be able to avoid warfare’s reach into the underground.

Whether an ancient underground waterway, modern subterranean infrastructure such as subway or sewer, or enemy-dug pathway to circumvent our vast technological advantages, underground warfare is an unavoidable aspect of today’s battlefield that must be confronted. With a trend toward greater urbanization, it will become even more difficult to avoid the subterranean environment.

Fortunately, there are efforts underway by U.S. military and supporting organizations to prepare for underground warfare. The Army’s Asymmetric Warfare Group and Maneuver Center of Excellence, for example, provide short training sessions on subterranean warfare fundamentals. The Defense Advanced Research Projects Agency (DARPA), the U.S. Army Combat Capabilities Development Command and the Maneuver Center of Excellence have recently pursued technologies optimized for the underground environment—from autonomous flying drone scouts to robots that will conduct underground tasks that would otherwise be left to soldiers.

One of the biggest efforts, the multiyear DARPA Subterranean Challenge, looks to achieve new approaches to rapidly map, navigate and search underground environments. The challenge allows the scientific and engineering communities and public companies to compete for prizes for developing new technologies and concepts to make subterranean environments less challenging to warfighters.

More Can Be Done

Despite these efforts, the Army remains largely unprepared to face an adversary that is either building or utilizing existing tunnels. More can be done to ensure our force is prepared. Without a greater investment in technology and training, our adversaries will use tunnels to neutralize our advantages.

Here are some recommendations:

• Continue investing in technology. All the new technologies being pursued by defense capabilities developers will be needed to give U.S. soldiers a decisive advantage in underground warfare. The ability to send a robot or drone into a subterranean area to map, navigate and accurately identify its contents will be a game changer. New lightweight ground sensors, ground-penetrating radar and advanced aerial terrain analysis will enhance our tunnel-detection capabilities. New night-vision devices, light sources, air monitors, breaching tools, ballistic shields, and communication and navigation tools designed for underground operations will provide warfighters with tools they never had before.
• Study the past. Technology only supplements individual soldier proficiency. Soldiers will confront problems they are not trained for and do not have the tools to address, and they will be required to adapt. While the Army waits for autonomous robots to reduce the risk of entering and fighting in tunnels, it needs to be ready today. History provides solutions that were developed by soldiers in the past. These should be discussed, trained and war-gamed, and the underground tactics, techniques and procedures used in Vietnam, Afghanistan and Gaza should inform warfighters’ approaches to tunnels today.
• Increase training. A few of the most basic situations encountered in past battles will significantly challenge a U.S. Army formation deployed to combat today. These include how to rapidly identify, map, neutralize, clear or destroy a tunnel. The Army should make a greater investment in subterranean facilities at training sites, and units, from the squad to the battalion level, should increase training for this environment. With frequent exposure to underground warfare, units will be forced to identify solutions using equipment they have on hand.
• Emphasize intelligence. History shows human intelligence is still vital for detecting tunneling activities. One of the basic requirements of tunneling is finding a place for the excavated dirt, and fresh residue can be a telltale sign of recent digging. The Islamic State group was aware of this in 2016 and even filled entire rooms of houses with loose dirt. Frequent patrolling, searching and visiting structures around a patrol base or outpost in an urban area is vital to identifying tunnels, and looking for these environmental cues should be incorporated into training.
• Consider nonlethal measures. In Vietnam, soldiers relied heavily on tear gas to render tunnels unusable. They dumped grenades and packets of CS powder into tunnel entrances and even pumped gases into the tunnels. Tear gas has since been banned in warfare, but its use demonstrates that research, development and training on nonlethal methods can still reduce the risk to soldiers in tunnels.
• Expand doctrine and train to destroy tunnels. To destroy a tunnel requires special equipment and considerable information about the tunnel’s construction, size and depth. But sometimes, simply rendering a tunnel temporarily unusable is sufficient. Soldiers in Vietnam often attempted to just close a tunnel’s opening so a mission’s momentum would not be slowed or hindered. Their field-expedient methods included dropping multiple fragmentary grenades down the opening at the same time or using a Bangalore torpedo, which consists of pipes filled with explosives that can be fitted end to end to extend the reach of the explosives. Any methods to close tunnel openings would require advance planning for soldiers’ load and logistics, and therefore soldiers must be trained.

Sending robots underground in lieu of people might be in the near future. But soldiers need to be prepared to confront the nightmares of underground warfare today. It cannot be avoided. Soldiers should be encouraged to think—aided by doctrine and history—about what tactics and equipment are most useful in underground environments. And we must continue to develop, test, field and train tools and concepts that will give them overmatch when, not if, they enter the subterranean.