For respiration, deep sea divers use a mixture of

To breathe underwater a diver must be supplied air at a pressure equal to that of the water surrounding the diver. Air of course is a suitable breathing mixture for a diver based primarily on nitrogen as the oxygen diluent. Beyond this depth helium is preferred as the diluent and is in fact particularly well suited to the depth. Helium does not cause narcosis at these pressures, is relatively inexpensive and is readily available.