First responders rescued 31 trapped workers from a collapsed tunnel in Los Angeles on July 9, officials said.

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,

An investigation is underway to determine what led to a tunnel, under construction for the Los Angeles County Sanitation Districts, to partially collapsed in Wilmington on Wednesday night, July 9 — prompting the rescue of 31 employees and a halt to work for an undetermined amount of time.