The small Israeli army drone let loose inside an arched concrete corridor kept flying for several minutes, along a 300m (980ft) tunnel large enough for a tall man to walk through unbowed.
To the left and right were rooms with air-conditioning units, functioning toilets and kitchens with running water as well as electrical and communication cables and a now-demolished blast-proof door that Hamas fighters could shoot through.
The tunnel that the Israel Defence Forces (IDF) said they filmed last month below Gaza’s al-Shifa hospital was, by any measure, a substantial military capability. But it is also only a small part of Hamas’ vast subterranean domain that officials and analysts have said will define the strategic outcome of Israel’s campaign against the militant group.
The drive to root out fighters and weaponry in Hamas’ tunnels and demolish the network itself is one reason why the Israeli military is pressing on with its punishing offensive after a week-long truce, despite growing international pressure over the bloodiest Israeli-Palestinian conflict in decades.
“Destroying Hamas’ tunnels is the most difficult aspect of the Israeli military’s mission ... and among its most important,” said Daphné Richemond-Barak, a professor at Israel’s Reichman University and author of a book on underground combat. “We have to be patient. It will take time.”
The web of tunnels, estimated to be bigger than the London Underground train network, enables Hamas’s most senior leaders and fighters to take shelter. Most are thought to have survived almost eight weeks of relentless Israeli attack below ground.
The tunnels – immune from drone surveillance and many of Israel’s other capabilities including air strikes – are also where Hamas is thought to keep its arsenal of rockets, as well as more than 130 hostages it still holds after seizing them from Israel in its devastating October 7th attack.
One former senior Israeli security official said the word “tunnels” did not do justice to what Hamas has created under Gaza, calling them “underground cities”.
Yocheved Lifshitz, an 85-year-old hostage released by Hamas in October, described the tunnel network as an elaborate “spider web” that was “kilometres long” with “large hall[s]” big enough to hold 25 people.
Tunnels are an ancient war-fighting technique. Jewish rebels used them in a famed revolt against Roman rule 2,000 years ago, as did the Viet Cong fighters who ultimately beat US forces in the Vietnam War.
But after burrowing through Gaza’s soft sandstone geology since taking control of the enclave 16 years ago, Hamas has taken the concept to a new level.
“The modern battlefield is seeing a fusion of ancient and digital capabilities,” said Anthony King, an urban war expert at the University of Exeter. “And sometimes it is the ancient techniques [such as tunnels] that can win out and beat the rest.”
The IDF has made the tunnels’ destruction a priority, but has not fully spelt out how it plans to achieve it. So far it has located more than 800 shafts, destroyed 500 of them and collapsed what Israel’s military has described as “many miles” of tunnels.
“On a tactical level, wherever our soldiers [on the ground] manoeuvre we have a high success rate destroying tunnels,” said one person familiar with Israeli military planning.
But the network is estimated to be more than 500km long, and many of the shafts emerge in civilian buildings such as hospitals, mosques and schools, according to the IDF.
The IDF said on Sunday that its fighter jets and helicopters had “struck terror targets in the Gaza Strip, including terror tunnel shafts”, after the breakdown of the truce that had enabled the exchange of dozens of Israeli hostages for more than 200 Palestinian prisoners.
Israeli forces now control much of northern Gaza, at least above ground. Yet even after taking control, IDF soldiers still faced attacks by Hamas fighters popping up from tunnels behind them and then retreating “like mice”, one officer told local media.
Such resistance has helped to prolong the fighting, increase the death toll of Israeli fighters and erode the Jewish state’s international support as Palestinian civilian casualties mount.
The tunnels are also a threat in themselves. The IDF said four soldiers were killed on November 10th at one tunnel entrance in Gaza’s north-eastern corner. More than 70 Israeli soldiers have died since the IDF launched its ground attack on October 27th.
“The tunnels are a massive challenge,” one Israeli official said. “They [Hamas] have also placed things inside – booby traps, obstacles to our movement inside the tunnels – that increase the risk [to our forces].”
The IDF last week blew up the tunnel it had found under the al-Shifa hospital, amid concerns that the rest of the network was primed with explosives.
Hamas had learned from previous attacks, Israeli officials said. That includes the 5,000lb GBU-28 laser-guided “bunker buster” bombs that Israel reportedly used during a 2021 offensive against the militant group aimed at destroying “the Gaza Metro”, as its tunnels are known. That operation achieved only limited success.
“The lesson Hamas likely learned from the 2021 air strikes ... was to dig deeper and to encase the tunnel system with reinforced concrete,” said Yehuda Kfir, an Israeli civil engineer and captain in the IDF reserves who is also an expert in underground warfare.
“Hamas [has] likely built different layers of tunnels,” Kfir added. “An upper ‘defensive’ level with booby-traps, very narrow [tunnels] and the blast-proof doors we’ve already seen, and a lower ‘offensive’ level that is deeper and wider and hold things like logistics centres, living quarters and weapons stores.”
The militant group has also built smuggling tunnels into Egypt, on which Cairo has sought to crack down.
Israel has received $320 million of US military aid since 2016 to develop anti-tunnel techniques, although none has provided a silver bullet.
The country also has a dedicated corps of anti-tunnel engineers and underground commandos equipped to probe tunnels and try to collapse them. But to preserve soldiers’ lives, the IDF has relied more on tunnel dogs, robots and drones.
“The [Israeli] government is opening bureaucratic bottlenecks and pouring more resources into finding a solution,” said an Israeli official.
The first step is to locate the tunnels. Ground-penetrating radar and acoustic sensors can work, although Gaza’s dense urban environment and the rubble left by Israel’s aerial bombardment limit their usefulness.
A simpler tactic, known as “purple hair”, involves throwing a smoke grenade into a tunnel entrance, which is then sealed with expanding foam to see if smoke emerges elsewhere.
The next step is to destroy the tunnels. Localised explosions cause only limited fall-ins, which can be cleared away or bypassed by surviving fighters. To fully demolish a tunnel, engineers and military experts said, required explosives set down along long portions of the underground passageways.
Kfir said one method was to use liquid explosives that fill the tunnel space and then detonate. Another possibility, he said, was thermobaric weapons, which suck in oxygen to generate a high-temperature explosion that flows around obstacles. But these are controversial because of the broader impact of the explosions, especially in populated areas.
Pumping in seawater from the Mediterranean at high pressure is a third option, and one that Israel has reportedly already started to use. Richemond-Barak said this technique had the advantage of already being used in the oil and gas industry. But, she added, the problem with flooding is “that you don’t know how much you have achieved”.
The amount of water required depends on the size of the tunnels and ground absorption, she said: “In the past, using water has not produced a ‘hard kill’.”
Another possibility, which would pose less risk to the hostages than flooding or explosions, is for the IDF to dig tunnels that intercept Hamas’ network and burst into its control nodes.
“Israel should ... get to the heart of the Hamas system not from above, but from below,” Kfir said. “You would need something like automated excavating machines ... that would dig towards the target.”
Such science fiction-like approaches highlight the difficulties and time needed to destroy Hamas’ underground realm. They also explain why some officials regret that Israel did not complete the task years earlier.
“We should have destroyed it all when it [Hamas’ tunnel network] was smaller. We had all the intelligence,” said the former senior security official.
– Copyright The Financial Times Limited 2023
