By Kathleen Magramo, CNN

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

Those rescued didn’t have any visible injuries, the LAFD said in a statement, shortly after an operation that saw several of those trapped in the industrial tunnel hoisted up to safety by crane.

More than 100 Los Angeles Fire Department responders raced to the scene in the city’s Wilmington neighborhood to assist in the operation, including Urban Search and Rescue teams, Los Angeles Mayor Karen Bass said on X.

The collapse occurred approximately five to six miles from the sole entry and exit point of the tunnel and about 400 feet below ground, officials said. The tunnel is part of a municipal wastewater project.

The trapped workers managed to climb over a 12- to 15-foot-high pile of loose soil to reach coworkers on the other side, preliminary reports indicate. From there, they were transported by tunnel vehicle to the access point, the fire department said.

Firefighters used cranes and rescue cages to hoist several workers out, according to video from CNN affiliate KCBS/KCAL.

Los Angeles Mayor Karen Bass met with the rescued workers and emergency responders on the scene, according to a video posted to her X account. “Thank you again to all our first responders working to get Angelenos to safety,” Bass said.

The failure occurred when workers were operating the tunnel boring machine and a section they had already built collapsed due to “squeezing ground,” Robert Ferrante, chief engineer and general manager of the Los Angeles County Sanitation Districts, said at a news conference late Wednesday.

Squeezing ground occurs when the ground or soil deforms significantly during an excavation.

Work on the site will be stopped and inspected, Ferrante added, but he did not elaborate on how long it will take.

Los Angeles County Board Supervisor Janice Hahn said the incident serves as a reminder of the risks involved in these projects and that the sanitation districts would “look into” what caused the incident.

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CNN’s Josh Campbell contributed to this report.