Colin Farrell says he developed panic attacks while filming intense scenes for his new film, Thirteen Lives.
The movie, directed by Ron Howard, tells the story of the real-life rescue of 12 young football players and their coach from the flooded Tham Luang cave in Thailand four years ago.
Farrell, who plays rescue diver John Volanthen, told Entertainment Tonight filming the underwater scenes was "terrifying".
"It was scary. I'm not a great swimmer anyway, not that we were swimming, not that we were treading water - we had to stay on the surface - but they built a really impressive network of caves," he said.
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"It was about four or five different caves that were based on the topography of the caves, the Tham Luang caves in Thailand, and they filled them full of water, and we'd go down and there was no up."
Farrell said not being able to see the water's surface caused him to experience panic attacks underwater, which was a "new experience".
Filming has proved stressful for Farrell's co-stars too, with Viggo Mortensen previously sharing he also felt panicked at one point.
He told People the moment of panic came while diving underwater in one of his scenes as rescuer Richard Stanton.
"All of a sudden, I couldn't breathe. It seemed like a long time, but it was only a matter of seconds. I panicked."
He said he eventually remembered his training and switched his mouthpiece to the oxygen tank.
Thirteen Lives will be available to stream on Amazon Prime Video from August 5.