Implosion is the collapse of an object into itself from a pressure differential or gravitational force. The opposite of explosion, implosion reduces the volume occupied and concentrates matter and energy. Implosion involves a difference between internal (lower) and external (higher) pressure, or inward and outward forces, that is so large that the structure collapses inward into itself, or into the space it occupied if it is not a completely solid object. Examples of implosion include a submarine being crushed by hydrostatic pressure and the collapse of a star under its own gravitational pressure.