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

None of the 31 construction workers rescued in the tunnel sustained visible injuries, according to LAFD. Firefighters said construction workers climbed over a roughly 12 to 15-foot pile of dirt to escape.

An industrial tunnel in Los Angeles collapsed partially. None of those rescued had major injuries, authorities said.