If Israel launches a ground campaign into Gaza to clear the dense areas of Hamas military capability—and all indications are that it intends to—Israeli forces can expect to face a wide range of challenges. Some of these are common features of modern urban warfare history, while others stem from the unique characteristics of Gaza’s urban terrain. But one challenge spans both categories: tunnels. Subterranean spaces have featured in other urban battles—not only recently but stretching back to ancient history. But the scale of the challenge in Gaza, where hundreds of miles of tunnels crisscross below ground in the enclave, is entirely unique. This expansive underground complex is the wicked problem—one for which no perfect solution exists—awaiting Israeli ground forces.

Tunnel warfare is not new. From medieval mining and countermining, its long history extends through the subterranean component of the World War I battles of the Somme, Vimy Ridge, and Messines to the deep natural and manmade tunnels used at Mariupol, Bakhmut, and Soledar during the ongoing war in Ukraine. The US military’s experience with tunnels includes the Civil War sieges of Vicksburg, Mississippi in 1863 and Petersburg, Virginia in 1864, the massive underground complexes of the Vietnam War, and both to al-Qaeda and ISIS tunnels in Afghanistan, Syria, and Iraq. In Vietnam, where North Vietnamese and Viet Cong forces used miles of tunnel networks to protect their supply routes, military forces, and bases in places like Cu Chi, the problem became so severe that it forced the development of new tactics such as sending soldiers, called tunnel rats, into tunnels armed with only a pistol and flashlight.

The Israel Defense Forces (IDF) are very aware of the presence of Hamas tunnels in Gaza. They often refer to the group’s tunnel systems as the “metro.” The destruction of these tunnel complexes was one of the objectives of previous ground campaigns into Gaza, in 2008 and 2014. In total, there were believed to be over three hundred miles of tunnels in 2021, when Israel claimed to have destroyed sixty miles of tunnels during an eleven-day bombing campaign. Even if those tunnels have not been rebuilt or replaced, that means that it is likely that there remain hundreds of miles of intricate, complex, and deep tunnel infrastructure in Gaza. It is a veritable city underneath the cities on Gaza’s surface.

In the event of a ground campaign launched by Israel, Hamas would use its tunnels both defensively and offensively. The way it employed these spaces against the IDF during Israel’s 2014 Operation Protective Edge offers clues about what to expect in the days and weeks ahead.

Defensively, Hamas will use tunnels to escape IDF observation and attack. Any Hamas military capability that survives Israel’s current air campaign will mostly be deep underground. Hamas will have already placed its leadership, fighters, headquarters, communication, weapons, and supplies like water, food, ammunition in its tunnel complexes to prepare for the ground assault by Israeli forces. The tunnels will allow fighters to move between a series of fighting positions safely and freely under massive buildings, even after the IDF drop thousand-pound bombs on them. Hamas tunnels often have generator power, air ventilation, water pipes, and stockpiles of food that will allow the group’s fighters to better withstand the most basic challenges, like normal exhaustion, that result from urban siege and isolation. Hamas leaders and fighters will use the tunnels to remain mobile to escape entire sections of the combat area when they feel they are about to be decisively attacked or surrounded. Importantly, Hamas has also dug a large portion of its tunnels under, and connected to, civilian sites like school, hospitals, and mosques in dense urban areas. Among other reasons for doing so, this is part of its defensive lawfare strategy.

Offensively, Hamas tunnels allow the group’s forces to conduct protected and surprise attacks. They will use the tunnels to infiltrate behind IDF positions to surprise Israeli forces that might not be as well prepared or equipped for combat as those spearheading the campaign, like those in logistical areas. Interconnected tunnels under urban areas will allow Hamas to move quickly between prepared attack positions with caches of sniper rifles, antitank munitions, rifle-propelled grenades, and other weapons and ammunition. Tunnels will be the vital element of Hamas’s guerrilla warfare strategy. Its fighters will form small hunter-killer teams that move underground, pop up, strike, and pop quickly back into a tunnel. Hamas also uses the tunnels to hide and move rockets. These rockets can be remotely detonated or transported to hidden launch sites at the last minute. Hamas will also have many tunnels rigged with hundreds of pounds of explosive to function as tunnel bombs under main roads and buildings that the IDF might be lured into.

Entering tunnels presents unique tactical challenges, many of which cannot be addressed without specialized equipment. In some cases, it can be impossible to breathe without oxygen tanks in tunnels, depending on their depth and air ventilation. It can also be impossible simply to see. Most military night-vision goggles rely on some ambient light and cannot function when it is entirely absent. Any military navigation and communication equipment that relies on satellite or line-of-sight signals will not work underground. A weapon fired in compact spaces of tunnels, even a rifle, can produce a concussive effect that can physically harm the firer. A single defender can hold a narrow tunnel against a much superior force.

Of course, not all military tunnels and bunkers are the same. I have seen firsthand the wide variety, having been in North Korean invasion tunnels discovered in South Korea, Iraqi military bunkers, defensive bunkers and tunnels in Nagorno-Karabakh, and Hezbollah tunnels along the Israeli-Lebanese border. Two features typical of Hamas tunnels are important to note. First, Hamas tunnels are almost all very narrow, largely because of the prefabricated concrete sides and tops Hamas favors to build them. The average Hamas tunnel is just two meters high and a meter wide, making entering, moving through, fighting in them extremely hard. Second, because of Israel’s advancements tunnel detection and destruction, Hamas has dug its tunnels deeper and deeper. In 2020, Israel found a Hamas tunnel that descended 230 feet below the surface, the deepest found up to that point.

On the other side of the equation, there are a number of particular capabilities available to the IDF to deal with the challenge of Hamas tunnels. I have studied the phenomenon of subterranean warfare in conflicts around the world, have worked with leading scholars and experienced military practitioners to better understand its role in urban warfare, and am a founding member of first and only International Working Group on Subterranean Warfare. Of all the forces I have studied, the IDF has done the most work to prepare for dealing with tunnels in war.

The IDF has the capability to find, recover hostages from, clear, neutralize, and destroy tunnels. There are specialized units like the IDF Combat Engineering Corps’s Yahalom Unit, an elite commando unit whose soldiers specializes in finding, clearing, and destroying tunnels. The large force includes subordinate units like Sayfan, which trains to handle the threat of unconventional weapons, Samur, which specializes in entering, clearing, and destroying tunnels. The Yahalom is one of the largest units in the world that trains, mans, equips, experiments, and develops new ways to deal with underground warfare. The IDF’s canine unit, Oketz, has dogs trained for operating underground. And the IDF, police, and intelligence services all have special units—like Sayeret Matkal, the Yamam, and others—who share best practices for dealing with terrorists and combatants underground.

The Yahalom and other IDF units also have special equipment specifically developed for tunnels. Tunnel reconnaissance units, for example, use ground and aerial sensors, ground-penetrating radar, drilling equipment, and other systems to find tunnels. There are radios and navigation technologies that to work underground, night-vision goggles that use thermal and other technologies to see in complete darkness, and a suite of remote or wire-controlled flying or crawling robots that can look into and map tunnels without risking soldiers. The IDF also uses virtual reality training simulators that allow soldiers to train for underground warfare even when they aren’t at the physical training sites that include subterranean environments.

Israel has also developed special tactics for dealing with tunnels once they are found. It has a wide range of ground-penetrating munitions like the GBU-28, which can penetrate one hundred feet into the earth or through twenty feet of concrete. IDF ground forces also have multiple types of explosives to collapse or seal tunnels. They also have plenty of bulldozers they can use to seal tunnels—a tactic the US Marines employed when it sealed Japanese defenders in their caves and tunnels during the last part of the 1945 Battle of Iwo Jima. When multiple Hezbollah tunnels were discovered along Israel’s norther border during Operation Northern Shield in 2017, the IDF poured truckloads of wet cement into the tunnels to close them. Egypt is known for trying to neutralize Hamas cross-border smuggling tunnels along its border with Gaza by flooding them with seawater and sewage.

But the hard truth is that the depth and scale of Hamas tunnels in Gaza will surpass Israel’s specialized capabilities. It may come down to IDF infantry and engineers dealing with tunnels as they discover them.

It will also not be a simple matter of finding and destroying Hamas tunnels for several reasons. Gaza is not the mountainous and sparsely populated terrain of Afghanistan, for instance, where in 2017 the US military dropped America’s most powerful nonnuclear bomb—the 21,600-pound GBU-43/B Massive Ordnance Air Blast bomb—on an ISIS cave and tunnel complex. Moreover, Hamas will likely put weapons and explosives in tunnels that can trigger unintended explosions elsewhere or travel through to other parts of the tunnel network, causing damage where the IDF didn’t foresee. Finally, Hamas will likely put civilians and hostages in their tunnels as human shields. All of this means that Israel will have to take a deliberate approach to each of the tunnels they will discover.

There is no uniform solution to the problem that tunnels will present Israeli ground force, undoubtedly one of the biggest they will face in Gaza. Dealing with each tunnel will require a situation-dependent mix of capabilities. But above all, given the scale and complexity of the underground infrastructure in Gaza, one thing is certain: overcoming the challenges posed by tunnels will require a lot of time.

John Spencer is chair of urban warfare studies at the Modern War Institute, codirector of MWI’s Urban Warfare Project, and host of the Urban Warfare Project Podcast. He is also a founding member of the International Working Group on Subterranean Warfare. He served twenty-five years as an infantry soldier, which included two combat tours in Iraq. He is the author of the book Connected Soldiers: Life, Leadership, and Social Connection in Modern War and coauthor of Understanding Urban Warfare.

The views expressed are those of the author and do not reflect the official position of the United States Military Academy, Department of the Army, or Department of Defense.

Image credit: Israel Defense Forces