A submarine has several huge tanks inside it that are filled with air when the ship is floating on the surface of the water. It is this air, which is lighter than water, that keeps the sub afloat, or buoyant, just like any other kind of ship.
But these tanks are different from those on ships that just float on the surface, for the tanks on submarines have doors at the top and bottom. These doors, called valves, can be opened to let sea water or air in.
When the bottom valve is opened, water fills each tank, and the air that was in the tank is forced up and out through another valve at the top. The water inside the tanks increases the weight of the submarine and causes it to sink below the surface.
In order to bring the sub back to the surface again, pumps are used to force the water out of the tanks, and air is blown in again. This new air is compressed air which has been stored in huge metal containers and forced into the tanks under great pressure. With the tanks again full of air, the sub rises.
A submarine can dive to a depth of 100 feet in less than a minute. Some of today’s research subs have been designed to dive to depths of 5,000 feet. These subs have thick steel hulls to withstand the crush of the water pressure at that depth.
History was made in 1960 when the Triton traveled underwater around the world, a voyage of 41,500 miles, in 84 days.
The main ballast tanks do not have valves at the bottom, they are open to sea.
There are no pumps to pump the water out.
Either high pressure air stored in air cylinders (used mainly in an emergency) or low pressure air from an air compressor like machine is used to force the water out of the open bottom and fill the top (progressing until the tank is almost completely empty of water) with air. This added bouyancy is what allows the ship to raise to the surface or stay there. In a non-emergency the ship is driven to the surface where the air compressor is then lined up to blow air into the tanks.