Scuba diving is an underwater diving technique in which the diver breathes underwater using a self-contained underwater breathing apparatus (scuba), which is fully self-contained and independent of surface supply. Scuba divers have their own breathing gas, usually compressed air, which gives them more independence and freedom of movement than surface-supplied divers and allows them to dive for longer periods of time underwater than breath-hold divers. Although compressed air is widely used, enriched air or nitrox, a mixture of air and oxygen, has grown in popularity due to the benefit of reduced nitrogen intake during long or repetitive dives. Nitrogen narcosis can be alleviated by inhaling helium-dilute breathing gas.
Open circuit scuba systems consist of one or more diving cylinders containing high-pressure breathing gas that is supplied to the diver via a diving regulator, and they discharge the breathing gas into the environment as it is exhaled. Extra cylinders for range expansion, decompression gas, or emergency breathing gas may be included. Rebreather scuba systems with closed or semi-closed circuits allow exhaled gases to be recycled. Since the amount of gas used is less than in an open circuit dive, a smaller cylinder or cylinders may be used for the same dive time. Rebreathers extend the time spent underwater with the same gas use as open circuit scuba; they create less bubbles and less noise than open circuit scuba, making them appealing to clandestine military divers who want to escape detection, research divers who don’t want to disturb marine animals, and media divers who don’t want to be distracted by bubbles.