Tunnels built by the militant group Hamas are likely to be one of the biggest challenges for the Israeli military should it decide to launch a ground invasion of the Gaza Strip.

Outside of North Korea's underground facilities, Hamas, which is classified as a terrorist organization by the US, European Union and others, likely runs the largest tunnel network in the world.

"The scale of the challenge in Gaza, where hundreds of miles of tunnels crisscross below ground in the enclave, is entirely unique," John Spencer, chair of urban warfare studies at the Modern War Institute, part of the United States Military Academy West Point, wrote in an article this week. "This expansive underground complex is the wicked problem, one for which no perfect solution exists, awaiting Israeli ground forces."

The network of an estimated 1,300 tunnels is thought to be around 500 kilometers (310 miles) long altogether, with some tunnels as deep as 70 meters (230 feet) underground. Reports suggest that most of the tunnels are just two meters high and two meters wide.

Experts say it's likely this is where the 200 or so hostages Hamas took after its October 7 terror attacks on Israel are being kept. Also in the tunnels will be stockpiles of weapons, food, water, generators, fuel and other equipment. Researchers who have studied the tunnels believe Hamas' leaders are also most likely underground, too. 

Experts say the tunnels are going to further complicate an already complex and difficult fighting scenario.

"The tunnels allow fighters to move between a series of fighting positions safely and freely," Spencer explained. "In short, the tunnels are a great equalizer, neutralizing Israel's advantages in weaponry, tactics, technology and organization."

"Alongside the dangers of not being able to discriminate between military and civilian targets, which you have to do under international law, they [the Israeli army] have the problems of urban terrain, which essentially can be described as fighting in three dimensions," Mike Martin, a specialist in the psychology of warfare at Kings College in London, added.

"So you'll have people firing down on you from above, from tower blocks; and then, of course, from underground … And of course, if you destroy a building, that will then become a pile of rubble, which becomes a very easy place for someone to defend and fire back at you from," he told DW. "So, really, urban terrain is the most difficult terrain that an army can face."

Finding Hamas' tunnels

Originally, underground tunnels in this area were used to smuggle goods between Gaza and Egyptand then Gaza and Israel. Over time, due to increased Israeli overhead surveillance with drones and other electronic spy equipment in Gaza, Hamas began to invest manpower and money into extending the tunnel network. 

But it was not until a 2014 military operation in Gaza that the Israeli army discovered the true extent of Hamas' tunnels. After that, Israel's government began to develop a barrier along the Gaza border that extended underground to prevent tunnelers from accessing the Israeli side. 

It's not easy to locate the tunnels, which may be under buildings of all kinds. Still, there are various ways to do so, including using radar and other detection techniques that measure thermal patterns, magnetic signatures and acoustics.

Most often, though, the underground passages tend to be found by human detective work, as the Rand think tank reported in a 2017 research briefingon the topic. That is, by soldiers on patrol or when, for example, a tracked Hamas operative's phone signal suddenly disappears when he heads underground.

Underground combat

In the past, tear gas or chemical agents have been used to clear tunnels, one of the world's leading experts in this area, Daphne Richemond-Barak, explained in her book, "Underground Warfare." But these "would likely be regarded as unlawful today," she writes.

It is also possible to bomb tunnels, and Israel has what are known as "bunker buster" bombs that can penetrate deep underground. However, at just around 40 kilometers long by between 6 and 14 kilometers wide, and with a population of 2.2 million people that has been blockaded there by Israel since 2007, Gaza is one of the most densely populated areas in the world. So even if the Israeli military knew where the tunnels were, the situation on the ground would make that kind of bombing extremely difficult, if not impossible.

Israel has also used so-called precision bombs on tunnels to close them, the researchers at Rand reported. But just bombing tunnels has only met with mixed success.  

Additionally, it is also far from easy to fight in tunnels. It is darker and far colder underground, sounds, such as that of weapon fire, are amplified, and the use of guns also kicks up dust. Tunnels can easily be booby-trapped, too. In fact, in the past, Israeli soldiers were not allowed to go into tunnels until they had been secured by specialist teams.

Since 2014, the Israeli military has deployed special units for fighting in tunnels. Such units often train in simulated physical or virtual reality environments inside Israel. The special units include soldiers trained to use dedicated sensors for working out what's happening in tunnels, as well as others who fight underground. These units are also aided by robots as well as trained dogs when accessing tunnels.

Chances of success?

West Point's John Spencer, one of the founders of the International Working Group on Subterranean Warfare, pointed out that he's never seen any other military force do as much preparatory work on tunnel warfare as the Israeli army.

However, Richemond-Barak, another founder of the International Working Group on Subterranean Warfare, has expressed concerns. "Israel would need to engage in a prolonged and extensive air and ground operation to degrade this underground infrastructure," she wrote in an article for the British daily newspaper, the Financial Times, this month.

The Israeli army could collapse, flood or otherwise destroy and seal off the tunnels, but this would be very difficult, especially while under fire in the urban landscape, and it could take months. "Even in such a scenario — which would come at an unthinkable human cost — it is unlikely that the entirety of Gaza's tunnel network would be destroyed," the underground warfare expert noted. 

Kings College expert Mike Martin also thinks the tunnels will prove a huge challenge.

"There are all sorts of clever things with radar and seismic intelligence that you can use [to detect the tunnels]," he told DW. "But I will just caution one thing. There's clearly a gap in the Israeli intelligence-gathering apparatus. They totally missed an attack of such a size," he said, referring to Hamas' October 7 incursion, in which over 1,200 people were killed.

"And that really tells us that they have some blind spots ...  and it seems to me that if Israel does have that gap in human intelligence, it's not really going to know what Hamas intends to do, how it intends to defend Gaza or what its further plans are. So it seems to me that there are some big question marks over what Israel does and doesn't know," Martin concluded.

With additional reporting by Kevin Lynch

Edited by: Jon Shelton and Davis VanOpdorp