What’s special about overhead environments?
“Normal” recreational scuba diving is surprisingly safe. What helps make it this way is the fact it takes place in open water. This means there is nothing overhead.
If a diver in open water runs out of air, and there is no one with whom he can share air, he can still make an independent emergency ascent. People usually call this a Controlled Emergency Swimming Ascent or CESA.
Overhead environments include not only caverns and caves, but wrecks, mines and ice. Divers in overhead environments may lose the ability to make independent emergency ascents. Cave divers always do. This robs them of one of sport diving’s greatest safety factors.
Any form of overhead-environment diving requires special training. The greater the risk, the more training you need. As cave diving entails the greatest risk, it requires the most training.