Scuba diving ("SCUBA" originally being an acronym for self contained underwater breathing apparatus,now widely considered a word in its own right) is a form of underwater diving in which a diver uses a scuba set to breathe underwater.
Unlike early diving,which relied exclusively on air pumped from the surface,scuba divers carry their own source of breathing gas (usually compressed air),allowing them greater freedom than with an air line.Both surface supplied and scuba diving allow divers to stay underwater significantly longer than with breath-holding techniques as used in snorkelling and free-diving.
According to the purpose of the dive,a diver usually moves underwater by swimfins attached to the feet,but external propulsion can come from an underwater vehicle,or a sled pulled from the surface.
Early diving experimenters quickly discovered it is not enough simply to supply air to breathe comfortably underwater.As one descends,in addition to the normal atmospheric pressure,water exerts increasing pressure on the chest and lungs—approximately 1 bar (14.7 pounds per square inch) for every 33 feet (10 m) of depth—so the pressure of the inhaled breath must almost exactly counter the surrounding or ambient pressure to inflate the lungs.It generally becomes difficult to breathe through a tube past three feet under the water.[3]
By always providing the breathing gas at ambient pressure,modern demand valve regulators ensure the diver can inhale and exhale naturally and virtually effortlessly,regardless of depth.
Because the diver's nose and eyes are covered by a diving mask; the diver cannot breathe in through the nose,except when wearing a full face diving mask.However,inhaling from a regulator's mouthpiece becomes second nature very quickly.