A view of a tunnel reportedly dug by Palestinians beneath the border between the Gaza Strip and Israel and recently uncovered by Israeli troops, on October 13, 2013.(DAVID BUIMOVITCH/AFP/Getty Images)

The Israel Defense Forces’ ground incursion into Gaza which began last week had a very specific aim: to target and destroy tunnels built by Hamas that stretched underground from Gaza into Israel. But the tunnel system uncovered by IDF forces in the past few days is a network far more extensive than previously thought—with four of the 25 IDF casualties occurring during confrontations with Hamas gunmen exiting the tunnels—suggesting that the military offensive, in which nearly 600 Palestinians have been killed, could yet expand in response.

IDF spokesman Lt. Col. Peter Lerner compared the network to a subway system. “It’s like the Underground, the Metro or the subway,” he said. Lerner has also described the tunnel system as “an entire underground Gaza.”

The tunnel system, which reportedly began as a means of smuggling goods into Gaza following the blockade in 2007, has grown significantly. According to the New York Times, the ground operation has exposed “23 underground tunnels in Gaza and 66 access points.”

Some of the tunnels have been used by Hamas militants to carry out incursions into Israeli territory and there have been several deadly clashes between militants emerging from tunnels on the Israeli side of the border and Israeli soldiers. Israeli officials say the tunnels were also designed for attacks on Israeli civilian communities.

Gilad Erdan, Israel’s minister of communications, has said, “Israel must not agree to any proposal for a cease-fire until the tunnels are eliminated.”

The tunnels are more sophisticated than simple crawl spaces; in October 2013, Israeli forces discovered a 1.5-mile tunnel stocked with explosives leading from Gaza to Kibbutz Ein Hashlosha, and in March a nearby tunnel exposed with as much as seven feet of clearance inside, as well as an electric generator, tools, and fresh footprints.

Hamas used tunnels to kidnap IDF soldier Gilad Shalit in 2006.

Here’s a video of the IDF detonating a tunnel in Gaza.

Tunnel systems have long been a militaristic strategy, and are particularly useful for insurgent forces. Underground networks were a major part of the Vietnam War—the most famous were the Cu Chi tunnels dug by Viet Cong forces, which spanned tens of thousands of miles. The Cu Chi tunnels are now part of a major tourist attraction in Ho Chi Minh City.

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