By Kathleen Magramo, Alaa Elassar, Stephanie Elam, Norma Galeana, CNN

(CNN) — Thirty-one men working on a tunnel about 400 feet beneath the ground in Los Angeles were operating a machine to carve through the earth.

Without warning, a portion of the tunnel collapsed around them Wednesday night.

Trapped five miles from the tunnel’s only entry and exit point, the men scrambled to escape.

The tunnel, roughly 18 feet wide and seven miles long, had been under construction for years as part of a complex municipal wastewater project, one that demands highly skilled workers capable of managing intricate technical challenges – and responding effectively to potential emergencies.

As anxiety gripped officials, families and loved ones, rescue teams raced against time to reach the men.

Meanwhile, stranded deep underground, the men climbed over a 12- to 15-foot-high mound of loose soil to try and reach their coworkers on the other side.

From there, they were transported by tunnel vehicle to the access point, the fire department said.

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.

Three of those trapped workers were Arally Orozco’s brothers. She was attending church when her phone rang over and over again.

Moments later, she received a text from her son, alerting her that the tunnel where her brothers worked had collapsed, The Associated Press reported. “It was sad and scary,” she told AP in Spanish. “We feared the worst.”

As officials, families and loved ones of the workers waited anxiously, rescue workers tried to reach the men.

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

One of Orozco’s brothers was finally able to reach her. They were able to squeeze through a tight space to get out, he told her in tears.

“My brother was crying,” Orozco said, according to AP. “He told me he thought he was going to die underground.”

Those rescued didn’t have any visible injuries, the LAFD said in a statement.

The rescued workers were traumatized but thankful to be alive, Los Angeles County Board Supervisor Janice Hahn said at a news conference late Wednesday.

“We got to meet many of the men who were alive and happy, but they were all shaken up,” Hahn said. “It was quite traumatic for them to go through that.”

“The first thing they did was call their family members,” she added. “They were also light about it, like you would expect when you’re in a situation that’s very tense that you know could have ended in a tragedy, and it didn’t. There’s a celebratory sense that people made it out.”

Cause of tunnel collapse under investigation

The project will be placed on hold indefinitely until officials complete their investigation and safety and engineering assessments for structural integrity, Los Angeles County Sanitation Districts spokesperson Michael Chee told CNN.

The failure occurred when workers – mostly engineers, electricians and mechanics – 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 the news conference.

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

“I have never seen a collapse of structural integrity of this nature,” Los Angeles Fire Department Capt. Danny Wu told CNN. “We’ve had small accidents with vehicles inside, or small fires or whatnot, but nothing this catastrophic where a section of the tunnel has failed.”

With only a single entry and exit point, Wu said he was “expecting the worst” and rescuers were braced to handle severe trauma injuries during the operation.

To escape the tunnel, the men had to return to the point of collapse and walk through it before getting into a vehicle that brought them out to the shaft site, according to Ferrante.

“It was a tense situation. Anytime you have a collapse in a tunnel behind you, there was only one way out at that time, it’s to come back here to the shaft, so they had to come back and make their way through the damaged section of the tunnel,” Ferrante said.

“It was very scary, as I just want to reiterate, we’re very fortunate,” he said.

The workers were highly skilled, well-trained and able to immediately recognize what steps to take in the emergency situation, Tim McOsker, the current councilman for Los Angeles City Council District 15, said at the news conference.

“This is a highly technical, difficult project, and they knew exactly what to do. They knew how to secure themselves,” McOsker said. “They knew how to get to the train that brought them back. They knew all of the signals as we spoke to them.”

McOsker and Hahn have requested an investigation into the cause of the failure and a full report on the safe repair of the tunnel to the LA County Sanitation Districts.

The tunnel being built is part of the Los Angeles County Sanitation Districts’ Clearwater Project, which is designed to replace two large tunnels that carry treated wastewater to the ocean.

The current tunnels, which are 60 and 80 years old, are not up to current earthquake standards and “cannot be taken out of service because they must continuously carry flow,” according to the county’s website.

These tunnels are part of the county’s main sewer system that treats wastewater from more than five million people in the Los Angeles Basin. The new tunnel will be seven miles long and “constructed almost entirely underneath public right-of-way (streets),” with completion expected in 2027, according to the website.

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CNN’s Josh Campbell contributed to this report.