An Israeli military spokesperson's remark that "all of Gaza is an underground city" because of its extensive network of tunnels and bunkers is an exaggeration. But there is some truth to it.

The Israeli Defense Force says that Operation Protective Edge, its incursion into Gaza that began last week, is meant to prevent Hamas attacks on Israel. And those attacks, it appears, depend on tunnels.

Gaza citizens have been digging and using tunnels for years.

Until recently, though, the tunnels extended only into Egypt and were used mainly to smuggle in consumer goods. Because of the Israeli blockade of Gaza, introduced when Hamas won elections in 2007 and relaxed only recently, many items—foodstuffs, gas, clothing, cars—were unavailable through normal trade.

Now citizens