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By all accounts, Hamas, which controls the Gaza Strip, is on a tunnel-building streak. Hamas has boasted of building fifty tunnels since the Israel–Gaza conflict of July, 2014, though Israel insists those numbers are exaggerated. But residents of kibbutzim that border Gaza are increasingly warning of clattering and drilling noises under their homes. “I’ve heard the sound of a hammer and chisel, and my neighbor says she can hear them digging under the cement,” one resident told Reuters. They are worried about a scenario in which Hamas militants emerge outside an Israeli kibbutz or village, and attack.

Two Hamas operatives died from a tunnel collapse earlier this month, according to reports coming out of Gaza. A week earlier, seven Hamas members were killed when another tunnel caved in, apparently because of heavy rains and flooding. “East of Gaza City, heroes are digging through rock and building tunnels, and to the west they are experimenting with rockets every day,” Ismail Haniyeh, the Hamas leader in Gaza, said while attending the funeral of the seven operatives.

In Israel, there is heated debate about the country’s preparedness for a “tunnel war.” Earlier this week, Defense Minister Moshe Ya’alon responded to the latest developments in Gaza. “We are not deluding ourselves,” he said. “We are prepared for the possibility that a southern front will open one of these days, and that we will have to defend ourselves.”

Israeli security forces believe that over the past eighteen months Hamas has been constructing three kinds of underground networks: weapons tunnels that operate within the Gaza Strip; smuggling tunnels into Egypt; and so-called offensive tunnels, or underpasses that snake all the way into Israel. In a rare sign of overt coöperation between Israel and Egypt since the ouster of Hosni Mubarak, Egypt in recent months has flooded and destroyed more than thirty Hamas tunnels under its Rafah border, apparently at Israel’s request.

Israel and Egypt’s continued blockade on practically all supplies going into the coastal strip, and a drying up of funds from Hamas sponsors in Iran and Qatar, mean that Gaza, home to 1.8 million Palestinians, is verging on economic collapse. Unemployment is at forty per cent, and Gazans, with no prospect of work or of ever leaving their besieged enclave, are increasingly driven to despair. As the reporter Avi Issacharoff wrote last week, thirty Gaza residents have attempted suicide this past month alone. While Haniyeh insists that “no new war on Gaza appears on the horizon,” there are indications that Hamas may be preparing to attack. Hamas is now said to employ a thousand tunnel diggers who work around the clock. And on Friday, the group’s military wing made a show of simulating an attack with assault rifles and a suicide bombing.

On July 8, 2014, Israel launched an offensive in Gaza with the declared purpose of halting Hamas rocket fire, which had intensified after the kidnapping and murder of three Israeli teen-agers and the subsequent burning of a Palestinian teen-ager. But that objective changed when, on July 17th, hours before a mediated ceasefire was to go into effect, thirteen Hamas militants came out of a tunnel into an open field outside an Israeli kibbutz, rattling the Israeli military, which had not understood the extent of Hamas’s underground network. Israel embarked on a ground offensive to destroy the tunnels, an operation that Ya’alon predicted would take “two to three days” but that lasted seven weeks and yielded only partial results.

Reports published after the war showed that the military had known of thirty-two tunnels before the offensive, but hadn’t realized their sophistication or that at least a third led straight into Israel. One engineering officer who helped identify three of those underpasses told Haaretz that while the Israel Defense Forces had seen “shallow” Hamas tunnels in the past, the ones found that July “made it clear that we were faced with something completely different: wide tunnels with internal communication systems. They had been dug deep underground and their walls were as thick as cement. You could walk standing upright without difficulty.” Once inside the tunnel, the officer added, “you realized that it wasn’t just meant to kidnap a soldier from the border, but that it could transport an entire enemy force in a short amount of time straight into the home front—and to attack us there.”

Hamas has relied on underground tactics since as early as 2006, when seven of its members surfaced from a tunnel some three hundred feet into Israeli territory, killed two soldiers, and abducted a third. That soldier, Gilad Shalit, was released in 2011 in exchange for a thousand and twenty-seven Palestinian prisoners. The defense minister at the time of Shalit’s abduction was Amir Peretz. I spoke to Peretz in his bureau a month after the Gaza war ended and asked him what he would have done about the tunnels had he still been defense minister. In an apparent dig at Ya’alon, who has held the job since 2013, Peretz said, “If I were the defense minister, I would have treated [the tunnels] as a strategic threat of the highest order, and the solution would have already existed by now.” He warned, “If we don’t solve this in Gaza, we’re going to encounter it in the north”—on Israel’s border with Syria or Lebanon.

Peretz told me that he had become aware of the gravity of the underground threat after the Shalit abduction, and that “back then we tried to solve it with vertical water tunnels and sensors.” But, he added after a pause, “It turned out to be more complicated than that.”

In recent weeks, there have been reports that Israel has spent more than seven hundred million dollars over the past year—and received an additional hundred and twenty million dollars from the United States—to field some four hundred different ideas for the detection and destruction of tunnels. There are signs that it may have adopted some of the suggested technologies: Hamas has complained of finding sensors and cameras at the sites of the collapsed tunnels, and a report published last year in the Israeli newspaper Globes stated that the military has started to implement a sensor-based system developed by an Israeli defense electronics company. That system reportedly cost upwards of a million dollars for each kilometer covered, while the building cost of an entire tunnel by Hamas is estimated to be no more than a hundred and twenty thousand dollars.

Israelis are expecting no less than an “underground Iron Dome,” as one resident put it, in a reference to the anti-missile system that had a whopping ninety per cent interception rate during the Gaza war, according to Israeli security sources. Long before Hamas launched rockets from Gaza, it reached deep into Israeli cities through the use of suicide bombings. But for Israelis, the fear of the tunnels lies in face-to-face encounters with enemy forces. The fear is basic, almost atavistic, in these days of advanced warfare.

In the wake of the war in Gaza, I sat down in Tel Aviv with Danny Gold, the retired general who is credited with having invented the Iron Dome. Gold took the tunnel threat seriously. “It’s not a simple ‘event,’ technologically speaking,” he told me. “Since the Vietnam War it hasn’t been solved. Between Mexico and the United States it isn’t solved. Sometimes it’s even harder than finding oil in the ground.” Still, he added with a smirk, “I think rockets are harder to defend against—and we did it.”