How does a submarine submerge?

Submarines stay on the surface by keeping their ballast tanks filled with air. To submerge, the submarine opens special valves at the top of the ballast tanks. When the valves open, air escapes out the top of the tanks as seawater enters the tank from the bottom. Since the seawater entering the tank is heavier than the air it replaces, the submarine becomes heavier and submerges.