Instability is caused by the centre of mass of the ship rising above the metacenter resulting in the ship tipping on its side or capsizing. To remain buoyant, the hull of a vessel must prevent water entering the large air spaces of the vessel (known as downflooding). Clearly for the ship to float, the normally-submerged parts of the hull will be watertight, but the upper parts of the hull must have openings to allow ventilation to compartments, including the engine room, for crew access, and to load and unload cargo. In a capsize, water can enter these openings if not watertight. If a ship sinks after capsizing, or as a consequence of a leak in the hull or other water ingress, it is often described as having "foundered" or "foundering". Large ships are designed with compartments to help preserve the necessary buoyancy.