Air volume underwater: Volume changes proportionally with pressure.
If you descend to 10m, you double the pressure ( at sea level) so 2 bar, and halve the volume.
20m, tripple it so 3 bar, then you have a third volume.
When you double the presssure and halve the air volume, the volume reduction comes from squeezing the same amount of air molecules into half the space. Therefore the density doubles!
0 m 1 bar 1 air volume x1 density container = FULL
10 m 2 bar 1/2 volume x2 density cont. = 1/2 FULL
20m 3 bar 1/3 volume x3 density cont.…