The way we shot [this scene] is we build the dormitory above a tank. And on action […] we lowered the set into the pool (it’s all real, there is so CG) and it was a disaster on the first day of filming. […] We said let’s just shoot it and we’ll do it very controlled. […] And we say action and the computer went wrong and the whole thing went 25 feet (7,62m) down. And the fear you see in these kids is not acting, they were terrified. It just went down and we lost all our cameras, sound and from that point on that look of fear was real.
–Matthew Vaughn, director