The Israel Defense Forces released a video on Thursday that they claim shows a mosque in the Gaza Strip where two tunnel shafts, digging tools and several weapons were allegedly being protected by Hamas militants. (Israel Defense Forces)

The Israel Defense Forces released a video on Thursday that they claim shows a mosque in the Gaza Strip where two tunnel shafts, digging tools and several weapons were allegedly being protected by Hamas militants. (Israel Defense Forces)

Of all the threats posed by Islamist militants in the Gaza Strip, nothing — not Hamas drones, not rockets that reach Tel Aviv — can match the fear Israelis feel about the dozens of newly discovered “terror tunnels” burrowed into their back yards.

Internet videos posted by the Israeli military and its antagonist Hamas, which show masked militants armed with rocket-propelled grenades crawling out of holes in the ground, have shaken Israel. So have stories, almost certainly untrue but widely promoted by social media, that children in Israel can hear shoveling beneath their beds.

On Friday morning, a squad of Hamas fighters emerged from an underground shaft in the no-man’s land where Israeli troops were preparing to detonate a tunnel. The militants captured one Israeli officer -- it is not known yet whether he is alive or dead -- and killed two soldiers.

Scenes like this are stoking unprecedented domestic support for Israel’s military offensive in Gaza, despite international condemnation over soaring Palestinian civilian casualties. In repeated opinion polls, a majority of Israelis have said they want the offensive to go on as long as it takes to destroy the subterranean threat.

Prime Minister Benjamin Netanyahu said Thursday that “with or without a cease-fire,” Israel will not withdraw its ground forces from Gaza until all tunnels into Israel are destroyed. When a 72-hour humanitarian pause was announced early Friday by the United Nations, it allowed for Israeli troops to remain in place inside Gaza, where tunnel demolition is likely to continue.

An Israeli army officer gives explanations to journalists during an army organised tour in a tunnel said to be used by Palestinian militants for cross-border attacks. (Jack Guez/AFP/Getty Images)

“I won’t agree to any proposal that will not enable the Israeli military to complete this important task for the sake of Israel’s security,” Netanyahu said, adding that the tunnels allow Hamas to “abduct and murder civilians and soldiers.”

The Israeli military says it has discovered 32 “offensive tunnels” that begin in Gaza and point east toward Israel. On Thursday, an Israeli intelligence officer said in an interview that only a third of those tunnels reach all the way into Israeli territory. Hamas has used tunnels to carry out at least three attacks against Israeli soldiers during the three-week-old conflict.

Mushir al-Masri, a Hamas politician and spokesman, appeared on al-Aqsa television this week, boasting that the Israelis had destroyed only a fraction of the tunnels dug by fighters.

“And we will dig many more, God willing,” he said.

Tunnels used in 2008 war

Gaza is a narrow-waisted coastal enclave bordered by Israel, Egypt and the Mediterranean Sea. Surrounding the area are no-man’s lands, high fences built by Israel and topped with machine guns, and watchtowers. To get at the Israelis, Hamas chose to dig.

In 2006, the group used a tunnel to capture Israeli soldier Gilad Shalit, who was held captive for five years. Military officials say tunnels also were widely used during a three-week war between Israel and Hamas that began at the end of 2008.

These tunnels are quite different from the hundreds of smuggling tunnels that until recently connected Gaza and Egypt. Those for-profit passageways were used to ferry food, fuel and even automobiles into the Palestinian territory to bypass Israeli and Egyptian blockades. Hamas, the militant group that controls Gaza, demanded a tax on every item. Israel alleges that those tunnels were used to import weapons and rocket-making materials, as well.

1 of 63 Full Screen Autoplay Close Aug. 1, 2014 Friday July 31, 2014 Thursday July 30, 2014 Wednesday July 29, 2014 Tuesday Skip Ad × Israeli airstrikes of Gaza continue View Photos The strikes continued even after condemnation for the Israeli shelling of a U.N.-run school sheltering evacuees from the Gaza conflict that killed at least 16 people and wounded dozens of others as they slept, according to Palestinian health officials and the U.N. agency in charge of the school. Caption The strikes continued even after condemnation for the Israeli shelling of a U.N.-run school sheltering evacuees from the Gaza conflict that killed at least 16 people and wounded dozens of others as they slept, according to Palestinian health officials and the U.N. agency in charge of the school. Aug. 2, 2014 A picture taken from inside a car in the village of Umm Al-Nasser, in the northern Gaza Strip, shows Palestinians in a car loaded with their belongings as some families return home. The Israeli army informed residents of Beit Lahiya, in northern, Gaza that it was “safe” to return to their homes, and witnesses said troops were seen leaving the area. Marco Longari/AFP/Getty Images Buy Photo Wait 1 second to continue.

There are also “hundreds, maybe thousands” of other underground structures in Gaza, said Atai Shelach, a former commander of an elite Israeli military engineering unit, describing militant bunkers, stockrooms, command centers and refuges for Hamas leaders and fighters.

The underground passageways are reinforced with concrete, electric lights, trolley tracks and weapons-storage caches and — in at least one case, the Israeli military said — supplied with a “kidnap kit” that included vials of tranquilizers and plastic handcuffs. The tunnels extend for two kilometers (about 11 / 4 miles) or more, starting in greenhouses, basements, mosques and clinics in Gaza and emerging in fields in Israel.

The tunnels were constructed — quietly, methodically, over months and years — by burrowers who used pneumatic drills and were guided by a knowledge of Gaza geology. To foil Israeli surveillance, Hamas hid sand and soil in sacks that United Nations organizations used to distribute bulk food, according to Israeli military officers.

The underground passageways are an average of 60 to 80 feet deep and can have a dozen exits or entrances — like a fork with many tines. Some contain booby traps, which have killed or injured several Israeli soldiers in recent weeks. The Israeli military sends in sniffer dogs and robots with video cameras first; bulldozers and dynamite follow.

“The biggest challenge is to locate the tunnels. Destroying them is simple,” Shelach said. “We won’t find them all. And the minute we leave, they will start digging again.”

Costly investments

Hamas fighters have emerged from tunnels during the past 24 days of fighting to ambush and strike Israeli forces. One video released by Hamas this week shows a group of masked militants, carrying automatic weapons and grenade launchers, crawl from a hole in the ground. They run across a field in daylight and then attack a group of Israeli soldiers manning one of the observation towers along the Gaza border fence. Five soldiers were killed.

Israeli military officials estimate that the tunnels cost $500,000 to $2 million to build and maintain and that one tunnel might take 800 tons of concrete — large investments in a place where Israel has restricted construction materials and Hamas has struggled to pay public workers.

“The tunnels cost Hamas time and money,” said Reuven Erlich, a former Israeli intelligence officer who is now director of the Meir Amit Intelligence and Terrorism Information Center.

Avi Melamed, a fellow at the Eisenhower Institute in Washington and a former senior Israeli intelligence officer in Israel, said that Mohammed Deif, who heads Hamas’s military wing, was charged with developing the tunnels to “move its war into Israeli territory.”

Retired Col. Yossi Langotsky, a geologist who was hired by the Israel Defense Ministry a decade ago to develop ways to shut down tunnels, said he repeatedly warned military leaders then that Hamas was building the passageways.

“This is a big mess,” he said. “I am just praying now that there will not be a committee of investigation into the matter and I will be asked to show the tens of warnings that were sent to them.”

A senior military official, who could not be named according to military protocol, said it “takes a long time to shut down hundreds of tunnels.”

“We have learned a lot over the past few weeks on how to find tunnels,” he said, estimating that the military would need just days — not weeks — to destroy all of them.

“Every day, you realize how dangerous and crazy it would be if suddenly Hamas militants would come out of two or three or four tunnels at a time,” said Sarit Ariely, a resident of the southern coastal town of Ashkelon, who said Israel had no choice but to destroy every tunnel.

“I live only 10 minutes from the tunnels, and only once we demolish them will we feel safe again,” she said.