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The sets were built in an enormous airplane hangar-sized building using double Olympic-size tanks and long sections of tunnel which would be flooded with 20 feet of water. The cast would practice for several hours each day to work out their movements through each passage. Each successive section was more difficult to navigate, and was built while the first one was in use for filming. The cast would film their underwater scenes then train on the next section. Viggo Mortensen said, "If they'd given us the fourth or fifth section to do the first week, we wouldn't have known to do it. It would've been too hard."
Diver Rick Stanton praised the film's accuracy, saying the one cinematic change was that the cave water was muddy. The real divers had zero visibility, but "That would be impossible to demonstrate because then the viewers would not see anything."
Colin Farrell stated he experienced frequent panic attacks while underwater, and is hesitant to film any aquatic scenes in the future.
Viggo Mortensen approached Ron Howard to suggest he and Colin Farrell do their own diving shots rather than using doubles; they'd been training with Rick Stanton and John Volanthen and didn't want to leave it to stunt doubles. Howard said it made his editing and storytelling easier.
At the suggestion of Viggo Mortensen, Ron Howard added a scene during production of the boys left alone in the dark after the divers have discovered them.
Colin Farrell and John Volanthen frequently spoke over Skype. Farrell took up distance running, a hobby of Volanthen's. At the end of the shoot, Farrell completed the Brisbane Marathon. Around 33 minutes of film that the governor announces to stop the rescue attempts until the rain stops, Farrell is wearing a Spartathlon T-shirt.