The Israeli military’s rescue of a hostage from an underground tunnel in the Gaza Strip on Tuesday highlighted one of the biggest remaining impediments to Prime Minister Benjamin Netanyahu’s goal of eradicating Hamas: the enclave’s vast and complicated subterranean network that shelters many of the militant group’s remaining leaders.

Israel’s military said on Tuesday that it had rescued Farhan al-Qadi, a member of Israel’s Bedouin Arab minority who was abducted on Oct. 7, from an underground Hamas tunnel in southern Gaza. According to two senior officials who spoke on the condition of anonymity because they were not authorized to talk to reporters, Israeli forces appear to have found Mr. al-Qadi by chance as they were combing through a tunnel network for Hamas fighters.

It was the second time in two weeks that Hamas’s network of tunnels featured prominently in Israel’s accounts of hostage recovery efforts, shining some light on a mostly unseen aspect of the war that looms large for the country’s military and government officials. Last week, Israeli troops said they had recovered the bodies of six hostages hidden behind concrete lining in an underground route connected to a 10-meter-deep tunnel shaft.

These underground discoveries after nearly 11 months of war show just how elaborate and extensive Hamas’s tunnel network has turned out to be, experts say. Some of the tunnels are hundreds of miles long, according to Israeli, Hamas and U.S. officials.

“The tunnels are massive,” said Dan Byman, a senior fellow with the Warfare, Irregular Threats, and Terrorism Program at the Center for Strategic and International Studies in Washington. The war in Gaza has revealed two surprises about the subterranean system that Hamas built, he added: There are more tunnels, and they are more serpentine, than previously believed.