Submarine buoyancy

Submarines adjust depth by taking seawater into ballast tanks through vents and forcing the water out using compressed air, thus adjusting seawater/air ratio inside the submarine. Since seawater is denser than air, higher seawater/air ratio makes it heavier, causing the submarine to sink, and higher air/seawater ratio makes it lighter, causing it to rise. The buoyancy of the submarine actually remains the same, because the volume is not changing, its just the net buoyancy-weight of the submarine that is changing.

The air in the submarine that is used for blowing water out of the buoyancy tanks is greatly compressed, to between 3000 and 6000 PSI. The volume displaced by the compressed air stored in the tanks inside the pressure hull is lots less than than of uncompressed air, and this pressure is great enough to empty the tanks even at the submarine's maximum service depth.

Note also that a submarine can increase its buoyancy by pumping water out from inside its pressure hull. In so doing, the air pressure inside the hull will go down but this effect can be countered by venting high pressure air into the hull as the pumps are running.