Underwater diving is the practice of descending below the water's surface to conduct underwater activities. In ambient pressure diving, the diver is exposed to the pressure of the surrounding water, and uses breathing apparatus for scuba diving or surface supplied diving, or when freediving, will breath-hold. The saturation diving technique reduces the risk of decompression sickness after long duration deep dives. Atmospheric diving suits may be used to isolate the diver from the effects of high ambient pressure. Although not usually considered to be diving, crewed submersibles can extend depth range, while remotely controlled or robotic diving machines can reduce the risk to human divers.
The diving environment exposes the diver to a wide range of hazards, and though the risks are largely controlled by appropriate diving skills, training, types of equipment and breathing gases used depending on the mode, depth and purpose of diving, it remains a relatively dangerous activity. Diving activities are restricted to relatively shallow depths ranging from around 40 m (130 ft.) maximum for recreational Scuba diving to commercial saturation diving maximum around 534 metres (1,752 ft) and 610 metres (2,000 ft) wearing atmospheric diving suits. Diving is also restricted to conditions which are not excessively hazardous, though the level of risk acceptable to the diver can vary considerably. Occasionally diving may be done in liquids other than water.
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