Scuba diving is a mode of underwater diving where the diver uses a self-contained underwater breathing apparatus (scuba), which is completely independent of surface supply, to breathe underwater. Scuba divers carry their own source of breathing gas, usually compressed air, allowing them greater independence and freedom of movement than surface-supplied divers, and longer underwater endurance than breath-hold divers. Although the use of compressed air is common, a mixture of air and oxygen called enriched air or nitrox has become popular due to its benefit of reduced nitrogen intake during long or repetitive dives. Breathing gas diluted with helium may be used to reduce nitrogen narcosis.
