Thirteen Lives Star Viggo Mortensen 'Couldn't Breathe' During a Scary On-Set Diving Sequence

Vince Valitutti/Metro Goldwyn Mayer Pictures

Thirteen Lives star Viggo Mortensen may very well have nine lives himself.

In this week's issue of PEOPLE, the 63-year-old actor opens up about his harrowing experience playing real-life hero Rick Stanton in director Ron Howard's new movie chronicling the dramatic 2018 Tham Luang cave rescue.

Mortensen learned to dive in order to film several 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 who got trapped inside a flooded cave after venturing inside to explore.

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.

At one point while making the movie, Mortensen was diving in one of the Australian sets built to look like the Thai cave. When he wriggled through a narrow spot, the valve of one of his oxygen tanks switched off, a scenario not uncommon in those kinds of conditions.

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

RELATED: Boys Freed from Thailand Cave Seen for the First Time in Hospital Photos Taken After Ordeal

Vince Valitutti/Metro Goldwyn Mayer Pictures Rick Stanton and Viggo Mortensen

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

He's certainly no stranger to on-set scares. When Mortensen was filming a scene in which 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. "I remember thinking, 'This is it. This is the end,' " says the actor, who pushed off the riverbed and emerged on the surface, gagging.

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 at the time, to Denmark, the country from which his family hails. "There was this huge storm that I didn't know was coming," says the actor, who bought a "small dinghy" unsuitable for the trip.

He and his sailing companion — "this guy I talked into [coming]" — set out on the water, but it wasn't long before disaster struck. "The wind was up. It got dark. And suddenly there was this wall of water," continues Mortensen. "We were going up, up, up, up, and then down. It was not the type of boat to be doing that in."

Eventually, 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 got back to England, "I couldn't prove who I was!"

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