Colin Farrell Had Panic Attacks Underwater Filming ‘Thirteen Lives’ Biopic on Thailand Cave Rescue (Exclusive)

ET spoke to Farell at the premiere of his new biopic Thursday night.

Colin Farrell went deep for his latest role in the Ron Howard-directed Thailand cave rescue biopic, Thirteen Lives. ET's Will Marfuggi spoke to the Irish-born actor at the premiere of the film Thursday night, where he got real about the panic attacks he experienced while filming the movie's intense underwater scenes.

"Terrifying in a word. Terrifying," Farrell said when asked what it was like to film underwater. "It really was -- 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, 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."

While Farrell said they weren't actually swimming, going down into this network of caves built for the film and not being able to look up and see the surface "wreaked" havoc on his mind.

"I mean, there's a lot to be said for being able to look up in water and see the surface, so when you can't and there's actually a ceiling over your head, and there's no air at all, it just wreaks havoc on my mind," he continued.

The biopic tells the story of the rescue mission assembled in Thailand, to save a group of 12 young boys and their soccer coach, after they got trapped in a system of underground caves that began flooding.

With the help of two of the men who were part of the real-life five-man rescue team, Farrell said he got through it, but not without a few underwater panic attacks along the way.

"We had amazing assistants," Farrell maintained. "We had Rick Stanton there, we had Jason Mallinson, who were two of the gentlemen who were part of the five that swim the team out, but man, I had panic attacks underwater, that's a new experience. A panic attack underwater is a new experience for me."

As far as his swimming skills have improved, the Batman star admitted that they have not, stressing that it's a "very particular" thing those divers do and that he and the rest of the cast did the best they could to replicate it.

"No, it's a very particular thing that they do. It's a very particular thing, and we replicated it as close as we could," Farrell explained, before revealing that one of the safety divers he worked with on the film said that Thirteen Lives is the most dangerous underwater movie he's ever shot. "I mean, at the end of the day, we did have amazing safety divers and a team of safety divers, but I asked one of the safety divers on one of the days we were -- he spends a lot of time in the tanks floating and talking, when we weren't shooting I said, 'How many films have you done,' and the safety diver said, 'About twenty underwater films.' I said, what was the most dangerous -- and you know where this is going -- they went, 'This one.'

The reason why, is if just one thing goes wrong, they could have a problem.

"And they explained why, and it was very rational what they said. They said, 'If this happens, at this point, we might have a problem,'" the 46-year-old actor shared. "So, that was like week three, but anyway, look, it was amazing, we had a great team of people, and it was a pleasure to be a part of."

See Farrell and Viggo Mortensen, Joel Edgerton and more when Thirteen Lives opens in theaters exclusively in New York, Los Angeles and Chicago, for one week on July 29, before launching globally on Amazon Prime Video on Aug. 5.