Actors must not only focus on performance, but also have enough physical fitness to cope with swimming, diving and climbing. Among them, only learning to use a rebreather requires 6 weeks, and the physical energy is extremely exhausted.
In order to make the film more credible, the creative team hired a number of cave divers as consultants. Jill Haynes, who served as the film's underwater technical consultant, was responsible for designing some underwater techniques for the film and training the actors to use rebreathers. Whether in the pool or on the set, she strives to lead the actors to spend as much time as possible under the water, so that the actors' underwater performance is as real and professional as possible
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In order to create a more realistic sense of the scene for the audience, the main cave scenes in the film were shot in Romania. The air bubbles generated by the traditional scuba will agitate the sand. In order to prevent the water quality from becoming turbid, the filming will be affected. The personnel used a closed rebreather. In the later period, some thrilling scenes were filmed in caves in Mexico. However, in order to protect the personal safety of these natural products and the staff, some of the more dangerous scenes were not shot on the spot, but were completed in a 700,000-gallon tank built. During the filming period, the diving time of the underwater crew members exceeded 3,500 hours
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