Just after midnight on Tuesday, the Israel Defense Forces announced that they had struck a tunnel that allowed Hamas to “infiltrate Israel through the sea.”

The sea tunnel was unusual, a sign that Hamas has designed deadly new ways to attack Israel. The armed group has miles of tunnels under the Gaza Strip — a U.S. official likened them to “miniature cities” — but the exit to this one was in the sea near a beach in Israel.

Among the possible reasons Israel delayed sending troops into Gaza after the Hamas attack on Oct. 7, one stands out, military experts say: the tunnels.

Underneath the tiny coastal strip and its more than 2 million people is a vast network of subterranean pathways, rooms, cells and even roads for vehicles. Hamas, which oversees Gaza, is believed to hide weapons, fighters and even command centers in the warren of underground chambers.