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An aqualung, also called a scuba (self-contained underwater breathing apparatus), is a device that allows divers to breathe and move about freely underwater for extended periods. It consists of a mouthpiece, a connecting tube, a valve, and at least one compressed-air cylinder. The key component is the demand valve which allows the diver to breathe air at the pressure prevailing in the surrounding water however great the pressure in the supply cylinder. Used air is vented into the water. Other equipment used for scuba diving includes a wet suit to insulate the diver from the cold, weighted belt, face mask, and flippers. Professional divers use scuba to inspect vessels below the waterline and to search underwater. The aqualung was invented by Jacques Cousteau and Émile Gagnan in 1943.

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