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These are three forms of diving where you always have a physical ceiling above you, meaning you cannot make emergency ascents. Another form of diving that is considered by many to be overhead environment diving is decompression diving. When you make a decompression dive, requiring several stops on the way to the surface, you do not have a physical ceiling but a 'virtual ceiling'. If you for example make a trimix dive to 80 meters, for 20 minutes, you cannot skip decompression stops without an extremely high risk of getting decompression sickness. Your mandatory stops have then become your 'virtual ceiling', and if you want to stay healthy, you better respect these stops as if they were real ceilings. Therefore decompression diving is often approached as being a form of overhead environment diving. These four forms of overhead environment diving all have their unique risks. Cave-diving requires a good sense of orientation, because of the risk of getting lost. Ice-diving has a high risk of hypothermia, that needs to be avoided. With wreck-diving the chance to get entangled in a fishing line or net is bigger then with open water diving. And when you do a decompression dive you need to know what can cause an uncontrolled ascend and how this can be avoided. Because of the unique aspects these forms of overhead environment diving all require specific training. If you do not know the specific dangers of an environment you cannot prepare yourself to avoid them. Without the right training you are not prepared for overhead environment diving and when you are not prepared "accidents" happen.
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