I'm asking this question on this website because I want an evolutionary reason rather than a reason of how the human brain works. In one episode of "The Nature of Things," I heard David Suzuki say he was terrified when he went in a deep mine because he's afraid of depths rather than because of the higher temperature there, so I assume other people feel the same way. I'm also scared to go into a submarine that will go really deep into the ocean because I know the laws of Physics and realize that there might be a mistake and it's not as strong as the designers thought and it might implode under the extreme outside pressure. Maybe for some people, it's a fear for a good reason like me and for other people, it's a fear for no reason. I want to know a reasonable theory on what the evolutionary reason for it is. Is it similar to the reason why the human selection of nonaggressive dogs led to dogs with floppy ears? I think it was so rare for somebody to be able to kill themselves with depths by implosion that the fear of depths wasn't selected against directly. For example, in our evolutionary history, by the time somebody swam deep enough that they could not swim back up to the surface without drowning first, the pressure still wasn't extreme enough to implode their lungs. I think that instead, there were many ways people could kill themselves but there was also a pattern between the different ways they could kill themselves so it was easier for them to evolve a simpler way to avoid killing themselves in any of those ways, and although death by being crushed under extreme pressure at great depths wasn't one of the ways, it fit into the pattern of the other ways people could kill themselves so the natural selection for people who don't kill themselves those other ways came with the trait of fear of depths automatically. Could the fear have actually been selected for directly after all because people could not think ahead and by the time they've swum to the deepest depth they can and still make it back to the surface, they're like "I still don't yet have the desperate urge to breathe so I can swim even deeper" so instead, natural selection selected for people who were afraid of swimming that deep below the surface of the water?