Viggo Mortensen Not Able To Breathe During A Scary On-Set Diving Sequence In 'Thirteen Lives

Thirteen Lives star Viggo Mortensen, 63, opens up in this week's issue of PEOPLE about his harrowing experience playing real-life hero Rick Stanton in director Ron Howard's new film chronicling the dramatic 2018 Tham Luang cave rescue.

Mortensen understood to dive to film many complicated underwater sequences in which his character and other cave divers risk their lives to save 12 members of a youth soccer team and their assistant coach. They got trapped inside a flooded cave after venturing inside to explore.

Rick Stanton, who spent hours on Zoom with Mortensen to talk about his experience ahead of filming, was also on hand during production as a technical advisor. He offered guidance and, it turns out, life-saving advice to the actors.

During one scene sequence, Mortensen was diving in one of the Australian sets built to look like a Thai cave. When he wriggled through a narrow spot, the valve of one of his oxygen tanks switched off, a scenario uncommon in those conditions.

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Mortensen recalls, "All of a sudden I couldn't breathe," adding, "It seemed like a long time, but it was only a matter of seconds. I panicked."

That is until he remembered Stanton's training, switched the mouthpiece, and breathed using his second tank. Mortensen explains, "It's not that complicated. But at the moment, it's hard to think clearly."

He's certainly no stranger to on-set scares. When Mortensen was filming a scene where his character floats down a river in the 2002 movie The Lord of the Rings: The Two Towers, a current pulled him to the bottom.

The actor said, who pushed off the riverbed and emerged on the surface, gagging, "I remember thinking, 'This is it. This is the end.'"

Mortensen has had other near-death experiences off-camera too. The actor recalled a time in the 1970s when he decided to sail from England, where he was living, to Denmark, the country his family hails from.

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The actor said, who bought a "small dinghy" unsuitable for the trip, "There was this huge storm that I didn't know was coming."

He and his sailing companion, "this guy I talked into coming," set out on the water, but it wasn't long before disaster struck.

Mortensen continues, "The wind was up. It got dark. And suddenly there was this wall of water," adding, "We were going up, up, up, up, and then down. It was not the type of boat to be doing that in."

Ultimately, the mast snapped, and the two were stuck in the open water until a ferry spotted them and picked them up. "We barely survived," says Mortensen, who lost everything, including his U.S. passport. When he returned to England, "I couldn't prove who I was!"

Thirteen Lives is playing in select theaters beginning July 29 and streaming on Prime Video on August 5.