Scuba Diving Regulators deliver air from an oxygen cylinder at a breathable pressure while you are underwater.
This is done this in two stages:
First, the regulator breaks down the high pressure from inside the tank to a 'medium' pressure, normally 140 psi above ambient water pressure.
It then reduces the medium pressure to a breathable pressure that is equal to that of the surrounding water pressure.
As you descend, the air pressure increases to match the increasing water pressure.
