Not only was the burrow remarkable in depth and length — 1.5 miles long and 66 feet underground — it was equipped with electricity and contained enough cookies, yogurt and other provisions to last its occupants several months. Israeli forces estimated that Hamas had dumped $10 million and 800 tons of concrete into the two-year project.

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Such “terror tunnels,” the Israeli military said in a statement Friday, are “complex and advanced.” And their use, Israel said, is “to carry out attacks such as abductions of Israeli civilians and soldiers alike; infiltrations into Israeli communities, mass murders and hostage-taking scenarios.”

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Describing this emerging “tunnel war,” a Palestinian militia document obtained by the news Web site al-Monitor said the objective of the underground network was “to surprise the enemy and strike it a deadly blow that doesn’t allow a chance for survival or escape or allow him a chance to confront and defend itself.”

On Saturday, in what The Washington Post called an “audacious attack,” Hamas fighters wearing Israeli army uniforms slipped from central Gaza into Israel through a tunnel and attacked an Israeli army patrol, killing two soldiers. In a second attempt Saturday to enter Israel through their tunnels, The Post reported, the Israeli military discovered Hamas operatives carrying handcuffs and tranquilizers in an apparent attempt to kidnap soldiers; the militants were killed. Also Saturday, a militant climbed out of a concealed tunnel in southern Gaza and started firing at soldiers.

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Then early Monday, a spokesman for the Israeli military said two “terrorist squads” had infiltrated Israel through a tunnel from northern Gaza. Israeli aircraft hit one group; the second fired an antitank missile at an army vehicle before 10 of the operatives were killed by return fire. And now, as the full extent of the Hamas tunnel network becomes clear, the IDF says dismantling the burrows is a seminal priority.

“We’ve expanded the forces on the ground in order to accomplish that mission,” said Capt. Eytan Buchman, an Israeli military spokesman. “All of Gaza is an underground city, and the amount of infrastructure Hamas built up over the years is immense. There are tunnels, extended bunkers, weapons storage facilities, even within urban areas.”

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Analysts said the tunnels are a major prong of Hamas’s military strategy against Israel. The IDF has sussed out 36 of what it calls “terror tunnels,” but there are probably more. While the Gaza Strip remains mired in poverty — the 2011 per capita income was $1,165 — Hamas is thought to have sunk more than $1 million into the excavation and maintenance of every tunnel. “Much to the misfortune of the people of Gaza, Hamas has invested far more resources in ‘underground Gaza’ than in ‘upper Gaza,'” wrote al-Monitor’s Shlomi Eldar. “The ‘change and reform’ that Hamas offered its voters was invested in its tunnels at the expense of the people of Gaza.”

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If the tunnels are a result of Hamas’s 2006 election, the group got down to business almost immediately. In June of that year, Hamas used a tunnel to capture Israeli soldier Gilad Shalit. That day, according to Haaretz, militants crossed underneath the Gaza border and attacked Shalit’s guard tower at 5:13 a.m. Within six minutes, the attackers had secreted Shalit back across the border in a tunnel — and he wasn’t returned to Israel for five years.

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In the years since, Hamas’s tunnels have come to possibly rival those used by the Viet Cong during the Vietnam War. Communist guerrilla fighters were said to have constructed thousands of miles of tunnels in the region around Saigon, then the capital of South Vietnam.

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But Hamas’s tunnels materialized in a different context. One reason they were built, Eldar wrote in a separate article, was to keep soldiers occupied during a period of relative peace. “What do you do with thousands of motivated, armed men with the urge to fight?” he said. “You come up with some operational occupation. Digging an underground tunnel.”

One tunnel digger, who says workers stay underground for long periods, explained how they’re constructed. “The drilling is done via a mechanical device, not an electric one, to avoid making noise,” he told al-Monitor. “It uses a [pedal-powered] chain, similar to a bicycle chain. [The chain] moves metal pieces that dig through the dirt. During the digging, the digger lies on his back and pedals with his feet.” The tunnels are high enough to walk through standing up and are reinforced by concrete.

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The tunnels take a human toll, according to an unusually detailed article by James Verini in the August issue of National Geographic, which describes this incident:

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It was about 9 p.m., and the brothers were on a night shift doing maintenance on the tunnel, which, like many of its kind — and there are hundreds stretching between Gaza and Egypt’s Sinai Peninsula — was lethally shoddy in its construction. Nearly a hundred feet below Rafah, Gaza’s southernmost city, Samir was working close to the entrance, while Yussef and two co-workers, Kareem and Khamis, were near the middle of the tunnel. They were trying to wedge a piece of plywood into the wall to shore it up when it began collapsing. Kareem pulled Khamis out of the way, as Yussef leaped in the other direction. For a moment the surge of soil and rocks stopped, and seeing that his friends were safe, Yussef yelled out to them, ‘Alhamdulillah! — Thank Allah!’ Then the tunnel gave way again, and Yussef disappeared.

There are three types of tunnel, experts say. The first are economic: hundreds of tunnels burrowing into Egypt, which allowed Hamas to funnel in resources, guns and rockets until the Egyptians sealed off many of them.

Another set of tunnels reportedly services the Hamas high command. “Every single leader of Hamas, from its lowest ranking bureaucrats to its most senior leaders, is intimately familiar with the route to the security tunnel assigned to him and his family,” al-Monitor reported. “The most senior leadership has its own specific tunnel.”