Submarines: How They Work - Archimedes' Principle

Whether a submarine is floating or submerging depends on the ship's buoyancy. Buoyancy is controlled by the ballast tanks, which are found between the submarine's inner and outer hulls.

A submarine resting on the surface has positive buoyancy, which means it is less dense than the water around it and will float. At this time, the ballast tanks are mainly full of air.

To submerge, the wsubmarine must have negative buoyancy. Vents on top of the ballast tanks are opened. Seawater coming in through the flood ports forces air out the vents, and the submarine begins to sink.

The submarine ballast tanks now filled with seawater is denser than the surrounding water. The exact depth can be controlled by adjusting the water to air ratio in the ballast tanks. Submerged, the submarine can obtain neutral buoyancy. That means the weight of the submarine equals the amount of water it displaces. The submarine will neither rise nor sink in this state.

To make the submarine rise again, compressed air is simply blown into the tanks forcing the seawater out. The submarine gains positive buoyancy, becomes less dense than the water and rises.