LOS ANGELES — After a section of a large industrial tunnel caved in on Wednesday, more than two dozen construction workers who were hundreds of feet beneath the streets of Los Angeles raced for miles in the dark.

Remarkably, all 31 workers escaped Wednesday night, and none suffered any major injuries. The workers had to traverse the most treacherous part — 12 feet of loose dirt, while water rushed into the tunnel — themselves before rescuers could reach them and drive them to the only opening, Fire Department Chief Ronnie Villanueva said.

The tunnel, which is 18 feet wide and 7 miles long, is under the Wilmington neighborhood, a heavily industrial area filled with oil refineries slightly north of the Port of Los Angeles. The nearly $700 million project is designed to create space to carry treated wastewater to the Pacific Ocean.

The workers were 400 feet underground and as many as 6 miles away from the entrance/exit, said Michael Chee, a spokesperson for the Los Angeles County Sanitation Districts. They had traveled for an hour in a transport vehicle to the work area, he added. They were supervising the operations of a machine that digs out the corridor and then builds the tunnel at the same time, he said. Panels that are installed are then used to move the machine forward, he said.

When workers learned of the collapse, they ran back and hopped aboard the transport vehicle, but it could move for only a mile before it encountered the debris.

“What we understand is the men who were in front of the collapse had approximately 6 to 8 feet of space above the debris where they were able to clamber over,” Chee said.

Then the workers — still in the dark and miles from the opening — continued on foot until rescuers were able to reach them and help them onto vehicles to take them to the shaft where a cage carried them out.

Aerial footage showed a crane hoisting workers out of the tunnel in a yellow cage.

“They’re shaken up,” Chess said, adding that all work has been halted and the workers will take time to recover.

Arally Orozco said her three brothers who were in the tunnel are too traumatized to speak to journalists. After escaping, one came out crying.

“He told me he thought he was going to die underground,” she said.

She said they described to her what they experienced: That night, at their work area, they heard a hissing sound.

“They heard like a psss sound, like air was going out, like pressure was escaping, and they didn’t know what it was,” Orozco said.

A while later, a couple of workers were heading back through the tunnel toward the opening when debris started raining down, the brothers told her. One worker ran back to alert the others while the other worker rushed ahead to get to the opening and call for help, Orozco said.

The group had to squeeze through a tiny hole that she said one of her brothers feared he would not fit through. Water was rushing in so forcefully that it pulled at the transport vehicle, she said. Her brother told her at one point the water reached up to his waist and he struggled to breathe because it seemed like the tunnel was losing oxygen.

“They felt helpless,” she said.

The project has been under way for two years without any problems, Chee said.

“The tunnel boring machine has been digging under streets, public right-of-ways, homes, parks, lakes, ponds, golf courses without incident until now,” he said.

Officials will investigate to determine the cause, Chee said.

“What our people and what our contractors and their specialists are going to do is a full assessment,” he said. “Everything from the engineering to the structural integrity to the safety, and obviously a very close inspection and look at the actual collapse point in the tunnel before anything else is done.”

Working so near the shoreline and at such a depth means crews could have been contending with very wet conditions that add challenges during design and digging, said Maria Mohammed, president of the Structural Engineers Association of Southern California.

“You would design not just for the pressure from the soil and the weight of the soil; you have to design for the pressure from the water,” said Mohammed, whose group is not involved in the project.

The investigation could take months, if not longer, Mohammed said. It will take some time just to make the tunnel safe for investigators to enter, she explained. Once inside, they’ll try to determine where the collapse originated, she said.

“It all comes down to: what’s the first element that broke?” Mohammed said. “Usually, a collapse is a propagating thing. One thing fails and it takes other things with it. So, you would try to figure out, of the broken elements, which one broke first.”

City Councilmember Tim McOsker praised the workers for keeping cool heads.

“This is a highly technical, difficult project,” he said. “And they knew exactly what to do; they knew how to secure themselves. Thank goodness for the good people that were down in the tunnel.”

Mayor Karen Bass said at a news conference that she met with some of the workers.

“I know when we raced down here, I was so concerned that we were going to find tragedy,” she said. “Instead, what we found was victory.”

Editor’s note: Watson reported from San Diego. Associated Press journalists Damian Dovarganes and Eugene Garcia contributed to this report.

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