Submarine Science

A submarine does what no regular ship can: control whether it floats or sinks by adjusting its average density on demand. Ballast tanks fill with seawater to dive and empty to surface.

To surface, compressed air blasts water out of the ballast tanks. The sub's average density drops below water's, and buoyancy pushes it upward. Modern nuclear submarines can stay submerged for months, adjusting trim with precision that keeps them at any chosen depth.