­A submarine or a ship can float because the weight of water that it displaces is equal to the­ weight of the ship. This displacement of water creates an upward force called the buoyant force and acts opposite to gravity, which would pull the ship down. Unlike a ship, a submarine can control its buoyancy, thus allowing it to sink and surface at will.

To control its buoyancy, the submarine has ballast tanks and auxiliary, or trim tanks, that can be alternately filled with water or air (see animation below). When the submarine is on the surface, the ballast tanks are filled with air and the submarine's overall density is less than that of the surrounding water. As the submarine dives, the ballast tanks are flooded with water and the air in the ballast tanks is vented from the submarine until its overall density is greater than the surrounding water and the submarine begins to sink (negative buoyancy).

The basic construction of a submarine consists of the pressure hull, the tanks which are built around the pressure hull, and the superstructure. The pressure hull, which includes the conning tower, is the principal part. This hull must be watertight and airtight and able to resist the water pressure when submerged. The hull is circular for maximum strength and is reinforced by steel frames over its entire length. Fitted around the pressure hull are the tanks. There are ballast tanks, which control the buoyancy of the boat, and fuel tanks.



More information:
https://science.howstuffworks.com/tra...
https://en.wikipedia.org/wiki/Submarine

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