Before the war against Hamas in Gaza, the Israel Defense Forces were one of the most prepared militaries in the world for underground warfare. The IDF were the only army to have a full brigade-sized unit dedicated to training, manning, equipping, researching, developing new technologies and tactics, learning, and adapting solely for underground warfare. Still, the challenges they faced early in their campaign in Gaza, many of which they struggled initially to overcome, speaks to the incredible complexity of subterranean warfare. Their responses to these challenges signal a paradigm shift in modern approaches to underground warfare.
The Long List of Underground Challenges
One of the main reasons the IDF were unprepared for Gaza’s underground spaces was simply that no military had faced anything like it in the past—not even Israeli ground forces. The IDF faced a Hamas military organization that had spent over fifteen years engineering the infrastructure of an entire region—to include over twenty major cities—for war, with the group’s political-military strategy resting on a vast and expensively constructed subterranean network under Gaza’s population centers. The Hamas underground network, often called the “Gaza metro,” includes between 350 and 450 miles of tunnels and bunkers at depths ranging from just beneath apartment complexes, mosques, schools, hospitals, and other civilian structures to over two hundred feet underground. There are estimates of over five thousand separate shafts leading down into Hamas subsurface spaces. In past wars, where underground environments were used, the tunnel networks were subordinate to the surface and were not built solely under population centers mostly to be used as massive human shields.
IDF investigations and captured Hamas documents produced reports that it took Hamas a year to dig one kilometer of standard tunnel at a per-kilometer cost of $275,000. A number of factors—size, type, and function, for examples—can raise the costs well beyond that of a standard mobility tunnel. The variety of tunnels in Gaza makes it difficult to estimate the underground network’s overall cost, but Hamas reportedly spent $90 million to build just three dozen tunnels in 2014, and some analysts place the network’s total cost at over $1 billion.
On October 7, 2023, the IDF had a brigade of special operations forces engineers, the Yahalom unit, fully equipped with technologies and tactics to accomplish the full range of underground warfare tasks, from detecting, securing, and mapping tunnels and bunkers to exploiting, clearing, neutralizing, and destroying them. This unit has spent decades researching, developing, testing, and purchasing technologies to overcome the challenge of military operations underground. This work includes a decade-long antitunnel cooperation and exchange program between Israel and the United States to jointly develop technologies and tactics that address the challenges of underground warfare. The IDF also has a robust military working dog program, the Oketz unit, that includes dogs trained for operating in subterranean spaces.
IDF units like Yahalom had plenty of work to do to prepare for underground warfare. Soldiers need special equipment to breathe, see, communicate, navigate, breach obstacles, and even shoot underground. Almost every piece of their standard military equipment designed for the surface will not work once they enter the subsurface. Line-of-sight and satellite-enabled technologies—including navigation, communication, and drones—are rendered useless. Night-vision goggles that rely solely on ambient light will not work in an environment where there is none. A blast from a weapon or explosive detonated in enclosed underground spaces can cause harmful pressures and blast injuries making it dangerous to even fire a personal weapon if the soldier is not wearing the proper protective gear.
The Culture Component
A unique challenge that all militaries face in dealing with underground warfare is one of culture. Any military force’s culture is guided by its history, priorities, and warfighting concepts. Whether that culture acknowledges and prepares for the underground challenges described above is determined by an institutional belief about whether or not underground spaces will be prominent features in future warfare. For example, the US Army’s cultural views surrounding tunnels and subterranean spaces is that they are obstacles to be dealt with when encountered. The service’s doctrine recommends that US soldiers “should avoid entering and operating in subterranean environments when possible.” If entering cannot be avoided, the doctrine describes the primary tasks as clearing and securing the subterranean environment.
The IDF have their own long history of dealing with tunnels, especially cross-border tunnels. Hamas and Hezbollah have used cross-border tunnels in the past to conduct surprise attacks on IDF outposts or small patrols in a bid to kidnap Israeli soldiers. This led to the IDF to develop advanced detection, mapping, and navigating capabilities, as well as—in an emergency such as a soldier being taken back into a tunnel—the tactics to follow an enemy underground.
The IDF also developed advanced tunnel-striking capabilities with a wide variety of bunker-busting munitions. In the 2021 Operation Guardian of the Walls, the IDF believed they had destroyed sixty miles of Hamas tunnels in Gaza. Captured documents show that after this 2021 operation, the Hamas leadership authorized $225,000 to install more blast doors in tunnel segments to protect against IDF bunker-busting munitions collapsing more of the tunnel beyond the point where the bomb directly strikes. Hamas also increased production of handbooks showing their fighters how to survive and fight in tunnels.
In general, the IDF culture before 2023 was marked by the belief that tunnels should be dealt with by specially trained forces and that regular troops should only be sent underground as a last resort.
Adapting to Tunnels
At the beginning of the IDF operations against Hamas in Gaza after the October 7 attack, the IDF targeted many bunkers and tunnels with precision-guided bunker-busting munitions. These strikes were based on intelligence regarding the locations of tunnels, their purpose and value to the enemy, their contents, considerations about the presence of hostages or civilians, and other factors.
Once the ground campaign began, the IDF knew they would be encountering a lot of tunnels. They task-organized squad-sized elements of Yahalom to as many maneuvering units as possible. The force that entered Gaza rapidly learned how to identify visual indicators of tunnel shafts, such as markings on buildings, the presence of infrastructure needed in the tunnels for power or ventilation, and other identifying features.
Once a shaft was located, it was generally secured and then Yahalom was called forward to investigate it. Even identifying a shaft was dangerous and time-consuming. The IDF lost five soldiers in early November 2023 from a booby-trapped tunnel entrance. Hamas’s use of booby traps outside and inside their tunnels was pervasive. In some cases, Hamas tunnels were built with improvised explosive devices embedded into the walls. This allowed Hamas fighters to arm and then leave their booby-trapped tunnels quickly.
If a shaft was determined to be a tunnel it was carefully interrogated, mapped, and searched. Many advanced technologies were used in this process, including drones and robotic devices designed to work underground. In some cases, military working dogs with cameras mounted on their backs were deployed, but the risk of losing dogs to booby traps made this tactic rare. During this time, Israel continued to be reluctant to send troops underground and only did so after tunnels were searched for potential dangers.
In fighting Hamas defenders, the IDF immediately faced enemy brigades, battalions, and companies that each had tunnel networks supporting their operations. In northern Gaza, the IDF had weeklong battles over single neighborhoods because of Hamas’s ability to pop in and out these networks and avoid decisive engagement.
In one attempt to combat Hamas’s use of their tunnels, the IDF procured and deployed what is to reported to be at least five industrial pumps to push thousands of cubic meters of water per hour into the tunnels to literally flush Hamas fighters out of them. The flooding had minimal impact. In one case, as one IDF officer I spoke to during a research visit told me, it took two weeks for a small Hamas tunnel to fill before the IDF finally saw Hamas fighters on the surface where they could be targeted. Due to the tunnels’ porous concrete lining, the water simply drained out of them. Some tunnels were even built with drainage holes in them, while in others blast doors complicated the process. Flooding had little impact and was too time-consuming to use as a primary method to force Hamas fighters out of their tunnels. And ultimately, flooding would not destroy a tunnel.
The more the IDF engaged with the Hamas tunnel network, the more they adapted. Stopping for every suspected tunnel shaft and waiting for Yahalom to investigate severely slowed the momentum of maneuvering forces. Many of the suspected shafts were simply wells, civilian infrastructure, or other types of tunnels. The IDF quickly realized they had to push some of the specialized knowledge of Yahalom lower and to general-purpose soldiers. The regular IDF soldiers began to become proficient at dealing at least with shaft identification, site securing, and initial investigations.
The IDF began to realize that in many areas, the tunnels were a system of systems. Each Hamas company, battalion, and brigade had its own networks of tunnels that factored into how they would fight and move around. Some of these networks connected to each other while others were separate. Once the IDF were able to focus intelligence efforts on determining the classification and architecture of a tunnel system in a specific area or neighborhood, their success in finding and dealing with tunnels significantly increased.
The IDF also developed a typology of Hamas tunnels. Some Hamas tunnels were tactical, such as small-unit tunnels that ran from building to building giving Hamas fighters the ability to hold specific terrain. Some were more operational as they connected different battalions or brigades to each other or provided operational mobility—like the mile-long tunnels running underneath the river basin of central Gaza to connect the region’s northern and southern portions. What to do about a specific tunnel and the urgency of action could be determined by proper identification of the type of tunnel that had been encountered.
Despite the IDF adaptations, a challenge remained: that of Hamas forces using the tunnels for their defensive operations as long as they could and then simply lining the tunnels with booby traps as they fell back to different tunnels. The tunnels gave Hamas the ability to control the initiative of most battles in Gaza.
Transforming to Tunnels
One Israeli commander refused to allow Hamas fighters to control the initiative by using their tunnels. When the war in Gaza began, Brigadier General Dan Goldfus—a veteran of the Shayetet 13 unit, a naval special operations unit—commanded the elite 98th Paratroopers Division. In late November 2023, with some of the IDF’s best armor, artillery, and engineer units added to its paratroopers and commandos, the division was deployed into Hamas’s strongpoint city of Khan Yunis in southern Gaza. The city was considered to be the “center of gravity” of Hamas’s military forces.
General Goldfus learned from what the other IDF divisions had encountered in northern Gaza but also oversaw a rapid learning initiative to study the Hamas tunnels in Khan Yunis. He had his soldiers study the details of each tunnel, traveled into the tunnels with his soldiers, and had soldiers collect on how Hamas protected certain tunnels. He also used his unique connections in the IDF and Israel’s intelligence service to develop what might be called all-source intelligence about types and locations of tunnels as well as other trends in Khan Yunis. Eventually, the division built the confidence that it understood the enemy’s tunnel network.
General Goldfus developed a plan to enter Hamas’s tunnels without Hamas knowing his soldiers were there. This was unlike any IDF unit’s approach to tunnels in Gaza yet. His plan was briefed to his superiors for approval. He was given the approval to take the calculated risks that other units had not to that point. He then started sending his special operations forces, engineers, and others into uncleared tunnels at the exact same time he was maneuvering on enemy forces on the surface.
IDF special operations forces, commandos, and others were equipped with all the specialized equipment needed to breathe, navigate, see, communicate, and shoot underground. General Goldfus’s division headquarters refined the ability to control forces moving underground with the tempo of the surface forces. Incrementally, the division refined its tactics to the point its soldiers were conducting raids with separate brigades attacking on the surface while more than one subterranean force maneuvered on the same enemy underground.
For the first time in the modern history of urban warfare, General Goldfus and his soldiers were conducting maneuver warfare simultaneously incorporating the surface and subsurface in dense urban areas. They had turned tunnels from obstacles controlled by the defending enemy into maneuver corridors for the attacker.
More importantly, through General Goldfus’s leadership and his soldiers’ adaptations, innovations, and hard work, the division began to transform the IDF’s culture toward underground warfare. Its tactics were spread to other units, along with the understanding that the old culture of avoiding tunnels was no longer the IDF’s approach. The new culture of a deeper understanding of—and, in some cases, using—the enemy’s tunnels to facilitate maneuver warfare with simultaneous maneuver on the surface and subsurface is unlike that of any other military in modern history.
The Challenge That Remains
There is one subterranean challenge that even General Goldfus and his division could not overcome: destroying tunnels. Israel’s strategic goal in Gaza includes destroying Hamas’s major military capabilities. This logically requires destroying a certain percentage of the vast underground network the group has so heavily relied on.
Contrary to some reporting, removing Hamas’s ability to plan and conduct military operations does not require destroying all of Hamas’s tunnels. Not every tunnel is as important as others. The cross-border tunnels between Gaza and Egypt that served as Hamas’s strategic lines of communication—enabling vital weapons supplies—large tunnels linking northern and southern Gaza or connecting different cities or brigade areas of operations and allowing freedom of movement, and command-and-control tunnels like the data center found under a United Nations building in Gaza City do require destruction. The destruction of hundreds if not thousands of tactical tunnels that connect different buildings across the Gaza strip is not critical to achieving Israel’s military objective and may exceed any reasonable ability Israel has to remove those tunnels.
But there is a gap in military methods for destroying tunnels. Military history has generated far more knowledge about creating tunnels than destroying them. Modern bunker-busting bombs can penetrate the earth in a small area to strike at a target in a tunnel or bunker but cannot effectively destroy the full length of a tunnel.
From 2013 to 2019, Egypt used sewage and seawater to collapse primitive Hamas smuggling tunnels along its border with Gaza. As discussed earlier, however, this did not work on the more sophisticated tunnels found inside Gaza. In 2018, Israel pumped wet cement into cross-border Hezbollah tunnels along the northern border with Lebanon. The exact number of metric tons of wet cement required for a single tunnel, while not publicly reported, was substantial. This tactic may block tunnels under the right circumstances, but it is not practical where there are a lot of tunnels to address.
One of the few feasible methods to eliminate a tunnel, used historically and validated by modern case studies, is to place explosives through the full length of it. This is what Israeli forces have been doing in Gaza but they quickly ran into scaling and resourcing problems.
Israel has a couple of explosive options. One of these is injecting liquid TNT into tunnels. This involves drilling holes into the tunnel at 650-foot intervals—and requires twelve tons of explosives per kilometer. An alternative is methodically placing explosives along the inside of cleared tunnels. Reports show that to demolish just one kilometer of tunnel requires fifteen metric tons of TNT placed inside the tunnel. The amount of TNT needed for tunnels Israeli forces discovered in Gaza quickly exceeded their supplies of liquid or military-grade explosives like composition C-4. They therefore primarily relied on a field-expedient method of using both their own stockpile as well as captured Hamas explosives designed for other uses such as antitank mines to string together along tunnels.
The harsh reality is that there is likely not enough supply of explosives or enough time to destroy all the tunnels in Gaza. To find all the tunnels and then destroy them would potentially take years. The IDF seem to be focusing limited resources on destroying the tunnels that provided Hamas with the most military value to survive or conduct attacks against Israel.
The Future
The IDF have already taken their new understanding, culture, and approach to tunnels to another theater in southern Lebanon, where Hezbollah also built a vast tunnel network of hundreds of miles and which is referred to as the “Land of Tunnels.” The 98th Paratroopers Division, with its pioneering and advanced underground warfare skills, was one of the first units to conduct raids into the Hezbollah tunnels found along Israel’s northern border.
It is unlikely that any military will face a tunnel system like that in Gaza, where an enemy’s political-military strategy rests on the tunnels and they are deliberately placed under civilian areas. But militaries will continue to encounter subterranean environments in warfare. State actors like China, Iran, and North Korea continue to invest in thousands of miles of military tunnels and bunkers to protect everything from nuclear sites, radar installations, and runways to full military bases. It is also hard to separate urban warfare from underground warfare in major cities that have existing civil infrastructure underground for transportation, water, and other essential services.
The lessons from the IDF’s adaptations and, ultimately, transformation of culture toward underground warfare are deeply important for other militaries—especially those whose own cultures are characterized by the notion that tunnels are obstacles that should be avoided or only dealt with when required. The lessons learned by the IDF will save the lives of other soldiers in other battlefields. The IDF have also shown others that subterranean environments can be used for more than only defensive tactics. With the right culture, understanding, intelligence, technologies, and tactics, they can be used for simultaneous maneuvers on the surface and subsurface. That changes everything.
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 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: IDF Spokesperson’s Unit