Humans can adapt to a suprisingly large range of different pressures - everything from 0.15 bar to 70 bar is possible when breathing a correct gas mixed from oxygen, nitrogen, helium and/or hydrogen. However, any given breathing gas is good only for a small range in pressures. In ordinary SCUBA diving with just air, the lower limit is around 50 meters. Divers who want to go deeper using the more sophisticated techniques of technical diving will quickly notice that using just one gas mix from the surface to the bottom is not practical. Either the decompression obligation becomes too large or the gas mix is just plain toxic at some depths. Thus the divers will find themselves carrying bottom gas, travel gas and decompression gas or equipment that can change the mix on the fly.

Always breathing the right mix is vital for survival underwater.