Colin Farrell suffered panic attacks under water while filming 'Thirteen Lives'

Asked about the underwater scenes, he told 'Entertainment Tonight':

"Terrifying in a word. Terrifying. 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.

"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.

"We had amazing assistants. 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."

Colin admitted that even the diving experts working on the film said it was the most "dangerous" shoot they have ever been involved with.

He said: "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 20 underwater films.'

"I said, what was the most dangerous - and you know where this is going- they went, 'This one.

"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.' "

Despite the scary experience, Colin - who is joined in the movie by Viggo Mortensen and Joel Edgerton - insisted it was a "pleasure" to be involved with the production.

He added: "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."