How does a submarine work?

A submarine controls whether it floats or sinks by adjusting its buoyancy using ballast tanks. To dive, the submarine opens valves to let seawater flood into these tanks, increasing its weight until it becomes denser than the surrounding water and sinks. To surface, compressed air is blown into the tanks, pushing the water out, which makes the submarine lighter and lets it rise. To maintain a steady depth, the crew balances the tanks so the submarine has neutral buoyancy, neither rising nor sinking, and uses angled fins called hydroplanes to fine-tune its depth while moving. Propellers drive it forward. Modern military submarines, especially nuclear-powered ones, can stay submerged for months, producing their own oxygen and fresh water, and use sonar to navigate and detect other vessels since light does not travel far underwater.