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How does a submarine use ballast tanks to dive and surface?

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Portrait of Archimedes by Giuseppe Nogari shows an elderly man with white hair and beard, holding a compass, gazing downward, set against a dark background. — Archimedes Archimedes, oil on canvas by Giuseppe Nogari, 18th century; in the Pushkin Fine Arts Museum, Moscow. Archimedes was an ancient Greek mathematician and inventor.

Submarines utilize ballast tanks to control their depth by manipulating their average density relative to the surrounding water, a practical application of the principles of buoyancy.

To dive, the submarine opens flood valves at the bottom of its main-ballast tanks and air-vent valves at the top. This allows seawater to displace the air, increasing the submarine's total weight and making its average density greater than that of the surrounding water, which causes the vessel to submerge.12

To surface, the submarine closes the vent valves and uses compressed air to force the seawater out of the ballast tanks. This process reduces the submarine's weight and lowers its average density below that of the water, providing the necessary buoyancy to rise to the surface. Additionally, submarines use variable-ballast tanks to make precise adjustments to their weight, ensuring it remains equal to the weight of the water displaced, which allows the craft to maintain neutral buoyancy at various depths.12