Scuba diving may be a mode of underwater diving where the diver uses a self-contained underwater breathing device (scuba), which is totally independent of surface supply, to breathe underwater.Scuba divers carry their own source of breathing gas, usually compressed gas ,allowing them greater independence and freedom of movement than surface-supplied divers, and longer underwater endurance than breath-hold divers. Although the utilization of compressed gas is common, a replacement mixture called enriched air (Nitrox) has been gaining popularity thanks to its advantage of reduced nitrogen intake during repetitive dives. circuit scuba systems discharge the breathing gas into the environment because it is exhaled, and contains one or more diving cylinders containing breathing gas at high which is supplied to the diver through a regulator. they’ll include additional cylinders for range extension, decompression gas or emergency breathing gas. Closed-circuit or semi-closed circuit rebreather scuba systems allow recycling of exhaled gases. the quantity of gas used is reduced compared thereto of circuit , so a smaller cylinder or cylinders could also be used for the same dive duration. Rebreathers extend the time spent underwater compared to circuit for an equivalent gas consumption; they produce fewer bubbles and fewer noise than circuit 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.