Limpet mines

A limpet mine is a type of naval mine attached to a target by magnets. It is so named because of its superficial similarity to the limpet, a type of sea snail that clings tightly to rocks or other hard surfaces.

A swimmer or diver may attach the mine, which is usually designed with hollow compartments to give the mine slight negative buoyancy, making it easier to handle underwater.

Usually limpet mines are set off by a time fuse. They may also have an anti-handling device, making the mine explode if removed from the hull by enemy divers or by explosions. Sometimes the limpet mine was fitted with a small turbine which would detonate the mine after the ship had sailed a certain distance, so that it was likely to sink in navigable channels or deep water out of reach of easy salvage and making it harder to find what caused the sinking.

In December 1938, a new unit was created in the British military that soon became known as Military Intelligence (Research) — usually abbreviated as MI(R) or occasionally as MIR. MI(R) absorbed a technical section that was at first known as MI(R)c. In April 1939, Joe Holland, the head of MIR, recruited his old friend Major Millis Rowland Jefferis (1899–1963) as director of the technical section and under his leadership the team developed a wide range of innovative weapons.

One of Jefferis' earliest ideas was a type of mine that could be towed behind a rowboat, but which would attach itself to the hull of a ship that it passed. Getting a heavy bomb to stick to a ship reliably was a problem; the obvious answer was to use magnets which should be as powerful as possible.

In July 1939, Jefferis read an issue of the popular magazine Armchair Science, which contained a small article on magnets:

The most powerful permanent magnet in the world—for its size—has been developed in the research laboratories of the General Electric Company in New York. Only half the size of the eraser on a lead pencil, it will lift a flat-iron weighing 5 lb. Its magnetic attraction is several times as strong as that of any previous magnet. The strongly magnetic alloy forming the magnet can be used, too, in electrical equipment to replace electro-magnets that require current.

...
Wikipedia