Thirty-one men working in a massive underground tunnel in Los Angeles were rescued Wednesday night after a portion of the structure collapsed, officials said.

Fire officials said the trapped workers managed to escape by scrambling over a roughly 12- to 15-foot mound of loose soil to reach four co-workers who had gone in to help.

Workers were 400 feet (121 meters) underground and as much as 6 miles (9.6 kilometers) inside from the only entrance when the cave-in Wednesday evening threatened to trap them on the far side of the tunnel’s boring machine,