How Do Submarines Dive and Surface?

A submarine controls its depth using the principle of buoyancy. To dive, it opens valves on its ballast tanks, allowing seawater to flood in and replace the air, making the submarine heavier than the water it displaces (negative buoyancy). To surface, compressed air stored in high-pressure bottles is blown into the ballast tanks, forcing the water out and making the submarine lighter (positive buoyancy). For fine depth adjustments while submerged, submarines use smaller trim tanks and adjustable hydroplanes (fin-like control surfaces) that angle the submarine up or down. Nuclear submarines can remain submerged for months, limited only by food supplies, because their reactors provide unlimited power and generate oxygen from seawater through electrolysis.