Underwater diving
Scuba diving

Scuba diving is diving with a self-contained underwater breathing apparatus, which is completely independent of surface supply. Scuba gives the diver mobility and horizontal range far beyond the reach of an umbilical hose attached to surface-supplied diving equipment (SSDE). Scuba divers engaged in armed forces covert operations may be referred to as frogmen, combat divers or attack swimmers.

Open circuit scuba systems discharge the breathing gas into the environment as it is exhaled, and consist of one or more diving cylinders containing breathing gas at high pressure which is supplied to the diver through a diving regulator. They may include additional cylinders for decompression gas or emergency breathing gas.

Closed-circuit or semi-closed circuit rebreather scuba systems allow recycling of exhaled gases. The volume of gas used is reduced compared to that of open circuit, so a smaller cylinder or cylinders may be used for an equivalent dive duration. They greatly extend the time spent underwater as compared to open circuit for the same gas consumption. Rebreathers produce fewer bubbles and less noise than scuba which makes them attractive to covert military divers to avoid detection, scientific divers to avoid disturbing marine animals, and media divers to avoid bubble interference.

A scuba diver moves underwater primarily by using fins attached to the feet; external propulsion can be provided by a diver propulsion vehicle, or a towboard pulled from the surface. Other equipment includes a diving mask to improve underwater vision, a protective diving suit, equipment to control buoyancy, and equipment related to the specific circumstances and purpose of the dive. Scuba divers are trained in the procedures and skills appropriate to their level of certification by instructors affiliated to the diver certification organisations which issue these diver certifications. These include standard operating procedures for using the equipment and dealing with the general hazards of the underwater environment, and emergency procedures for self-help and assistance of a similarly equipped diver experiencing problems. A minimum level of fitness and health is required by most training organisations, and a higher level of fitness may be needed for some applications.
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