Flotation (also spelled floatation) involves phenomena related to the relative buoyancy of minerals. Early scuba divers dived without a buoyancy aid. Underwater divers are a common example of the problem of unstable buoyancy due to compressibility. The hull is fiberglass with internal buoyancy tanks. The foam provides strength, buoyancy, and sound deadening. The physics mostly relates to gases under pressure, buoyancy, heat loss, and light underwater. They have heavy bones that act as ballast to counteract the buoyancy of their blubber. An object whose weight exceeds its buoyancy tends to sink. This similarly reduces or eliminates the need to consider variable fuel weight in buoyancy calculations. The bones show osteosclerosis, increasing their density to reduce buoyancy. The motion of the fluid is sustained by convection, motion driven by buoyancy. Neutral buoyancy in a diver is an unstable state. The ship also featured a set of 21 vertical watertight compartments for extra buoyancy. This means that the maximum downward motion is associated with the integrated negative buoyancy. Discovery of the principle of buoyancy is attributed to Archimedes.