A Limpet Mine is an explosive device seen in The Man From MI.5. Carl attaches one to the side of Captain Blacker's ship with devastating effect. The mine rips a hole into the side of the ship causing it to explode and sink very quickly.
The mine is activated by radio waves, detected by the two antennas once they have been raised, giving the frogman time to escape.
Carl also used one of these devices in attempt to blow up his own boat, with Lady Penelope still onboard.