Britannica Sources used in this answer

How do submarines use ballast tanks to dive and return to the surface?

Created from Britannica articles using AI. AI can make mistakes, so verify using Britannica articles.

USS Nautilus — submarine The former USS *Nautilus*, launched in 1954, being towed to Groton, Connecticut, to become a museum, May 1985.

Submarines utilize ballast tanks to control their depth by manipulating their overall density relative to the surrounding water.

To dive, the submarine opens flood valves at the bottom of the main-ballast tanks and air-vent valves at the top. This allows seawater to displace the air, increasing the vessel's total weight. Once the submarine's average density exceeds that of the water, it submerges.12

To return to the surface, the air-vent valves are closed, and compressed air is used to force the water out of the ballast tanks. This process decreases the submarine's average density to less than that of the water, providing the necessary buoyancy to rise.12

Additionally, submarines use variable-ballast tanks to make fine adjustments to their weight, ensuring it remains equal to the weight of the water displaced. This is essential for maintaining neutral buoyancy and compensating for changes in water density, such as when moving between salt water and brackish water.2