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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.
A helium-oxygen breathing gas is often used by deep-sea divers at depths of seawater over 55 m (180 ft) to keep the diver from experiencing oxygen toxemia, the lethal effect of high-pressure oxygen, and nitrogen narcosis, the distracting narcotic effect of the nitrogen in air beyond this partial-pressure threshold.