In military and naval operations, a stationary explosive device placed in the water and designed to destroy ships that touch or approach it. Submarine mines have been used since the mid-19th century. They consist of an explosive charge fitted with a device that detonates the charge when a ship or submarine is nearby. Placed by vessels called minelayers or dropped by aircraft, they are anchored to the sea floor by a cable. Various types of submarine mines are detonated by contact, by an approaching ship's magnetic field, by changes in water pressure, or by the sound of a ship's propellers. The mine was the most effective antisubmarine weapon in World War I. Its role was even greater in World War II, when mines sank 1,118 Allied ships and 1,316 Axis ships. See also land mine, minesweeper