How do submarines sink below the water and then rise?

The body of a submarine is uniquely constructed. Under its strong outer hull are huge ballast tanks that surround its working core. The tanks can be filled with and emptied of seawater and air, which allows the submarine to sink or rise in the water. When a submarine travels on the surface, its ballast tanks are filled with air, which makes it less dense than the seawater it displaces, and it floats. But when a submarine needs to submerge or dive below the surface, its ballast tanks are flooded with seawater. This action makes the submarine sink; now equal in density to the water that surrounds it, it can move about below the surface. Motor-driven propellers are used to move the vessel along (its streamlined shape creates as little water resistance as possible), and swiveling fins (called hydroplanes) located on its sides direct it up and down. When a submarine needs to return to the surface, compressed air stored in tanks is blown into the ballast tanks. This air forces out the seawater, and the vessel begins to rise, aided by the hydroplanes. Once again lighter than the seawater it displaces, the submarine is able to float on the surface.