Penetration diving is the practice of divers to enter a confined space where there is no possibility of direct ascent to the surface. There are generally two types of dive sites offering penetration diving: caves and shipwrecks (or other man made structures). The two are similar in most aspects of diving, but both require special attention.
Training in special techniques and equipment is essential. A redundant breathing gas supply and a line leading to the exit are the two most important differences from regular open-water diving. This type of diving is generally referred to as technical diving.
Because of the often prolonged dive time and/or increased depth, other breathing gases than air are often used. Nitrox is used for shallow diving and Trimix for deeper dives.
See also: scuba diving
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