Scuba diving is a mode of underwater
diving in which a scuba diver uses a
self-contained underwater breathing
apparatus (scuba) to breathe
underwater. Unlike other modes of
diving, which rely either on breath-hold
or on breathing supplied under pressure
from the surface, scuba divers carry
their own source of breathing gas,
usually compressed air, allowing them
greater freedom of movement than with
an air line or diver's umbilical and longer
underwater endurance than breath-
hold. Scuba equipment may be open
circuit, in which exhaled gas is expelled
to the surroundings, or a closed or
semi-closed circuit rebreather, in which
the breathing gas is scrubbed to
remove carbon dioxide, and the oxygen
used is replenished from a supply of
feed gas before being re-breathed