NEAR THE ISRAEL-GAZA BORDER — The curved concrete top of the tunnel grazes the dark-brown buzz cut of Lt. Col. Oshik Azulai, putting it 5 feet and 7 inches above the sand floor. The walls are about 30 inches apart — wide enough for two people to squeeze past each other, unless both are in body armor. It is cool in the tunnel, 46 feet under, and dark, of course. Cellphones do not work.

Colonel Azulai, deputy commander of the Israeli military’s Southern Gaza Division, said this tunnel stretched eight-tenths of a mile into Israel, next to a field filled with watermelon, ripe but unpicked because of the war. It ended about 600 yards from Kibbutz Ein Hashlosha, a rural enclave of 325, but was unfinished: Unlike the tunnels used to infiltrate Israel from Gaza in recent days, this one still had electric lines along the wall and carriage tracks used to ferry out dirt.

Destroying such tunnels was the stated goal of Israel’s ground invasion of Gaza, which began July 17. But 11 days into the mission, and after Israeli officials say they have found 31 tunnels and destroyed 15, Palestinian militants again penetrated underground into Israel on Monday evening and confronted soldiers in a staging area. Multiple soldiers were killed, a senior military official said, as was at least one of the men from Gaza.

“We will not complete the operation without neutralizing the tunnels, the sole purpose of which is the destruction of our civilians and the killing of our children,” Prime Minister Benjamin Netanyahu of Israel declared in a televised address afterward. “It cannot be that the citizens of the state of Israel will live under the deadly threats of missiles and infiltration through tunnels — death from above and death from below.”