It was an attack by 13 Hamas gunmen who emerged from one underground passage near a kibbutz in southern Israel on July 17 that prompted Israel to begin its ground assault after 10 days of aerial bombardment. Though that effort was thwarted, another surprise attack two days later killed two Israeli soldiers and one militant when Palestinian fighters slipped into Israel through another tunnel and attacked a border patrol before militants retreated underground into Gaza.

Tunnels from Gaza to Israel have had a powerful hold on the Israeli psyche since 2006, when Hamas militants used one to capture an Israeli soldier, Gilad Shalit, who was held for five years before being released in a prisoner exchange.

The tunnels can be quite elaborate. The tunnel toured by journalists on Friday was reinforced with concrete and had a rack on the wall for electrical wiring. It also featured a metal track along the floor, used by carts that removed dirt during the tunnel’s construction, that could be used to ferry equipment and weapons, the Israeli military said.