As an Israeli ground offensive in the Gaza Strip looms in its most devastating war yet with Hamas, one of the greatest threats to both its troops and the 2.3 million Palestinians trapped inside the seaside enclave is buried deep underground.
An extensive labyrinth of tunnels built by the Hamas militant group stretches across the densely populated strip, hiding fighters, their rocket arsenal and more than 200 hostages they now hold after an unprecedented October 7 attack on Israel.
Clearing and collapsing those tunnels will be crucial if Israel seeks to dismantle Hamas.
But fighting in densely populated urban areas and moving underground could strip the Israeli military of some of its technological advantages, while giving an edge to Hamas both above and below ground.
John Spencer, chairman of Urban Warfare Studies at the Modern War Institute, said urban defenders have “time to think about where they are going to be and there’s millions of hidden locations they can be in”.
He warned: “They get to choose the time of the engagement – you can’t see them but they can see you.”
Overnight on Saturday, the Israeli military said its warplanes struck 150 underground Hamas targets in northern Gaza, describing them as tunnels, combat spaces and other underground infrastructure.
Tunnel warfare has been a feature of history, from the Roman siege of the ancient Greek city of Ambracia to Ukrainian fighters holding off Russian forces in 15 miles of Soviet-era tunnels beneath Mariupol’s Azovstal Iron and Steel Works for some 80 days in 2022.
The reason is simple: tunnel battles are considered some of the most difficult for armies to fight. A determined enemy in a tunnel or cave system can pick where the fight will start – and often determine how it will end.
That is especially true in the Gaza Strip, home to Hamas’s tunnel system that Israel has named the “Metro”.
When Israel and Egypt imposed a punishing blockade on Gaza after Hamas seized control of the territory in 2007, the militant group expanded construction of its tunnel network to smuggle in weapons and other contraband from Egypt.
While Egypt later shut down most of those cross-border tunnels, Hamas is now believed to have a massive underground network stretching throughout Gaza, allowing it to transport weapons, supplies and fighters out of the sight of Israeli drones.
Yehia Sinwar, Hamas’s political leader, claimed in 2021 that the militant group had 310 miles of tunnels. The Gaza Strip itself is only some 140 square miles – roughly twice the size of Washington DC.
The Israeli military has known of the threat since at least 2001, when Hamas used a tunnel to detonate explosives under an Israeli border post.
Since 2004, the Israeli military’s Samur, or “Weasels”, detachment has focused on locating and destroying tunnels, sometimes with remote-controlled robots. Those going inside carry oxygen, masks and other gear.
Israel has bombed from the air and used explosives on the ground to destroy tunnels in the past. But fully dislodging Hamas will require clearing those tunnels, where militants can pop up behind advancing Israeli troops.
During a 2014 war, Hamas militants killed at least 11 Israeli soldiers after infiltrating into Israel through tunnels. In another incident, an Israeli officer, Lt Hadar Goldin, was dragged into a tunnel inside Gaza and killed. Hamas has been holding his remains since then.
Ariel Bernstein, a former Israeli soldier who fought in that war, described urban combat in northern Gaza as a mix of “ambushes, traps, hideouts, snipers”.
He recalled the tunnels as having a disorienting, surreal effect, creating blind spots as Hamas fighters popped up out of nowhere to attack.
“It was like I was fighting ghosts,” he said. “You don’t see them.”
Israeli defence minister Yoav Gallant on Friday said he expects a difficult ground offensive, warning it “will take a long time” to dismantle Hamas’s vast network of tunnels.
The Israeli military also said on Friday it had carried out “very meaningful” air strikes on underground targets.
Typically, modern militaries have relied on punishing air strikes to collapse tunnels. Israeli strikes in Gaza so far in this war have killed more than 7,300 people, according Gaza’s Health Ministry. But those strikes can inflict only limited damage on the subterranean network.
US forces fighting the Vietnam War struggled to clear the 75-mile network known as the Cu Chi tunnels, in which American soldiers faced tight corners, booby traps and sometimes pitch-dark conditions in the outskirts of what was then Saigon, South Vietnam. Even relentless B-52 bombing never destroyed the tunnels. Nor did Russian strikes on the Ukrainian steel mill in 2022.
Underlining how tough tunnels can be to destroy, America used a massive explosive against an Islamic State group tunnel system in Afghanistan in 2017 called “the mother of all bombs”, the largest non-nuclear weapon ever used in combat by the US military.
Yet in all those cases, advancing militaries did not face the challenge that Israel does now with Hamas’s tunnel system – the militant group holds some 200 hostages there.
Hamas’s release on Monday of 85-year-old Yocheved Lifshitz confirmed suspicions that the militants had put hostages in the tunnels. She described Hamas militants spiriting her into a tunnel system that she said “looked like a spider web”.
Clearing the tunnels with hostages trapped inside likely will be a “slow, methodical process”, with the Israelis relying on robots and other intelligence to map tunnels and their potential traps, according to the Soufan Centre think tank.
“Given the methodical planning involved in the attack, it seems likely that Hamas will have devoted significant time planning for the next phase, conducting extensive preparation of the battlefield in Gaza,” the Soufan Centre wrote in a briefing.
“The use of hostages as human shields will add an additional layer of complexity to the fight.”
Daphne Richemond-Barak, a professor at Israel’s Reichman University, warned the underground battlefield could force the Israeli military into firefights in which hostages may be accidentally killed.
Explosive traps could also detonate, burying alive both soldiers and the hostages.
But even with those risks, she said the tunnels must be destroyed for Israel to achieve its military objectives.
“There’s a job that needs to get done and it will be done now,″ she said.