The bathyscaphe is made up of two main components: the cabin, which is made of steel, which is resistant to sea pressure and the float, a light container, filled with gasoline that is lighter than water. When on the surface, the ballast tanks are filled with air to provide enough lift to keep the vessel afloat. When the tanks are opened, the air escapes and is replaced by water, making it heavy to start its descent. The gasoline is in contact with the sea water so it is pressed almost exactly in proportion to the depth of the ocean. As it descends, the bathyscaphe gradually loses buoyancy and the speed of its descent tends to increase rapidly. To slow down or to begin the reascent, the pilot releases ballast that consists essentially of iron shot stored in silos and held in place by electromagnets.