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Fire Department Chief Ronnie Villanueva said the workers had to make it through the most treacherous part themselves, climbing over more than 12 feet (3.6 meters) of loose dirt before rescuers could reach them and drive them to the only opening.

The tunnel, which is 18 feet (5.5 meters) wide and 7 miles (11.3 kilometers) long, is under the Wilmington neighborhood, a heavily industrial area filled with oil refineries just north of the Port of Los Angeles. It is a nearly $700 million project that's designed to carry treated wastewater to the Pacific Ocean.

The workers were 400 feet (121 meters) underground and as much as 6 miles (9.6 kilometers) away from the only exit, said Michael Chee, spokesperson for the Los Angeles County Sanitation Districts.

A transport vehicle had taken them in to supervise the operations of a machine that digs out the corridor and then builds the tunnel at the same time and uses the panels that are installed to move itself forward, Chee said.

When they learned of the collapse, they ran back and hopped aboard the transport vehicle that had taken them into the tunnel, but it could only move for 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.

After that, the workers — still in the dark, frightened 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 the workers will be taking time to recover and all work has been halted.

Escaping in the dark

Arally Orozco said her three brothers who were in there 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 they heard a hissing sound after they got to their area of work, which took them an hour to get to by the transport vehicle.

As they worked in the dark with only headlamps, "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 her brother feared he would not fit through. Water was rushing in so strong 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.

Digging underground

The project has been underway 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 Wilmington project.

Mohammed said the investigation could take months, if not longer. It will take some time just to make the tunnel safe for investigators to enter. 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.�

Getting out safely

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

“This is a highly technical, difficult project. And they knew exactly what to do. They knew how to secure themselves,� he said. “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. Instead, what we found was victory," Bass said.

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Watson reported from San Diego. Associated Press journalists Damian Dovarganes and Eugene Garcia contributed to this report.