How do submarines stay afloat?

Submarines stay afloat by using a combination of buoyancy and ballast control. When they want to surface, they use compressed air to force water out of ballast tanks, making the submarine lighter and allowing it to float. To dive, submarines take in water into these tanks, increasing their weight and causing them to sink. By adjusting the amount of water in the ballast tanks, submarines can maintain their desired depth in the water.