The underwater scenes in Harry Potter and the Goblet of Fire were undoubtedly very trying for the actors, especially Daniel Radcliffe.
The young actor had no particular diving experience and had to learn everything for the occasion.
In total, he had to stay underwater for 42 hours to complete the underwater sequences.
Knowing that to facilitate filming, the actors did not return to the surface between each take, they remained underwater and received oxygen from the divers who ensured their safety.
Filming took place in a huge tank built for the occasion, including a blue backdrop for special effects.
The safety of the actors was obviously a major concern. They had been trained to report the slightest problem to the divers who were supervising them.
All they had to do was wave their hand to alert them in case of trouble.
Fun fact, Daniel Radcliffe made this distress signal without doing it on purpose during the filming of a sequence. The divers then rushed on him to urgently bring him to the surface, causing major panic on the set!
In addition to the stress and fatigue of filming underwater, Daniel Radcliffe also had two ear infections.