Air pressure decreases with altitude.
The bathyscaphe Trieste set the world depth record in 1960. BC - see buoyancy compensator. In multi-level diving, the time between descending below the surface and beginning the safety stop. Boyle's law - at a fixed temperature and for a fixed mass of gas, pressure times volume is a constant value. Charles's Law - at a constant volume, the pressure of a gas varies inversely with absolute temperature.