“Terrifying in a word. Terrifying,” Farrell said in a new interview with Entertainment Tonight. “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.”

Farrell explained that the film’s cave setting meant that the underwater scenes were being filmed indoors in the dark. It wasn’t the water that scared him as much as not being able to see where it ended. “There’s a lot to be said for being able to look up in water and see the surface,” he said. “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.” Even with the help of an accomplished safety team and several consultants who worked on the actual rescue, Farrell found himself having panic attacks. “We had amazing assistants,” he said. “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.” He continued: “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.’ Sign Up: Stay on top of the latest breaking film and TV news! Sign up for our Email Newsletters here.