Caves are hazardous for several major reasons, but this list isn’t everything:
1. It’s a hard overhead environment. Unlike open water, you cannot swim up to safety. You have to solve your problem in place or be prepared to swim out, which can be several hundred yards or more.
2. Orientation is more challenging. There are no sources of natural light. There may be little or no water current to help you establish direction. You need to be extraordinarily competent with mental mapping and setting/following navigation lines and markers without getting tangled.
3. Cave dives are often significantly deeper and longer than a regular dive. We incur decompression obligations and have to manage our ascent/exit carefully so that we do not get the bends. We’re typically using more than one gas mixture (blends of air, oxygen, and helium) and we have to use them properly. The wrong gas at the wrong depth can kill you in less than a minute.
4. Caves often have silty floors or muddy water. We have to move carefully and maintain impeccable technique to ensure that we don’t cause a “silt out” (dust cloud) that obscures everything.
5. Our equipment is more complex. I carry a minimum of 2 of everything I need and 3 of anything that will save my life. My equipment for a cave dive weighs more than I do!