The first “Limpet” mines were developed during World War II for use by the British Navy. They had two rows of 3 heavy-duty horseshoe magnets, one set along each side of a cylindrical container of high explosive.
They were attached to the hull of a ship below the waterline using a special fastening device on the end of a long pole.
There have been a number of copies and modifications made since. Among them are the British “clam” mine, the Soviet SPM, and MPM type 158 mines. They operate by means of a time pencil, a time-delay firing device with a primer and non-electric detonator on one end. The time pencil contains a cocked, spring-loaded firing pin, set into action by withdrawing a safety pin.
Securesearch manufactures a number of replicas of these mines, with inert time pencil fuzes, heavy duty magnets and a simulated high explosive charge.