The abyss of death, because you can't talk underwater, you can only communicate with gestures. If something goes wrong, the time left for you to correct it can only be calculated in seconds. In cave diving, all standards will be different from our usual moral standards because they are above the limit. Understanding this point is of great significance for understanding the film story, because several important turning points in the film story deviate from the conventional legal and moral standards, which is an extreme experience of human nature. It is for this reason that this film was rated as R abroad. Because there is no film grading system in China, I strongly urge parents not to take their children to see this film.