limpet mine 
noun
A mine designed to be attached magnetically to a ship’s hull and set to explode after a certain time.East Rand members of Cosas were given hand grenades and a limpet mine of which timing mechanisms had been reduced to zero seconds....
- The car's 250 kilograms of plastic explosives, with a powerful limpet mine attached as a detonator, was defused, Smith said.
- For underwater sabotage missions, each diver can carry a limpet mine to attach to the hull of enemy watercraft or docks.