Pressure

One atmosphere every ten meters

At the surface you carry one atmosphere of pressure. At ten meters, two. At a hundred, eleven. Your lungs don't feel the weight — they feel the squeeze.

0 m    1 atm    100% lung volume
10 m   2 atm     50%
20 m   3 atm     33%
30 m   4 atm     25%
40 m   5 atm     20%   ← the freefall starts
100 m  11 atm     9%   ← residual volume
120 m  13 atm     8%   ← beyond every CWT record

Boyle's law, felt

Pressure and volume are inverses. Halve the depth, double the pressure. This table is why you equalize early — and why the last ten meters are the longest.