Boyle's Law

The gas law stating that volume varies inversely with pressure at constant temperature, the reason air spaces shrink on descent and expand on ascent.

Boyle's law says that when temperature stays constant, the volume of a gas is inversely proportional to the pressure on it. For a diver this is the master rule of every air space: descend from the surface to 10 m and the ambient pressure doubles from 1 to 2 ATA, so a flexible volume of gas compresses to half its size. Rise back up and it expands again to its original volume.

This single law explains most of what you manage on a dive. Your ears, sinuses, and mask squeeze on descent and must be equalized. Your wetsuit and BCD compress, costing buoyancy that you replace with air, then vent on the way up. Most seriously, gas trapped in the lungs expands on ascent, which is why the first rule of scuba is to breathe continuously and never hold your breath: an ascent of only a few meters on held breath can cause a lung overexpansion injury.

The proportional change is greatest near the surface, where a swing from 2 ATA to 1 ATA doubles volume, so the last 10 m of every ascent deserve the slowest, most controlled pace.