A submarine is fundamentally an air space contained by a hull that is designed to withstand deep ocean pressures and to move easily underwater. The hull is a double steel shell. The inner, or pressure, hull contains all the machinery for propelling and guiding the vessel, plus living quarters for the crew. The outer hull holds the ballast tanks. When the vessel submerges, these tanks are opened and flooded with seawater. For surfacing, the seawater is forced out of the ballast tanks and replaced by compressed air.