Geoffrey Pyke had a simple idea. Geoffrey Pyke had been considering the problem of how to protect seaborne landings and Atlantic convoys out of reach of aircraft cover. The problem was that steel and aluminum were in short supply and required for other purposes.
Pyke realized that the answer was ice, which could be manufactured for only 1% of the energy needed to make an equivalent mass of steel. His idea, which he named after himself, was Pykrete, a mixture of water and wood pulp frozen together. While it sounds ridiculous, in fact Pykrete was much stronger than plain ice, and took much longer to melt, months if not years, out on the open ocean. And of course Pykrete, being lighter than water, would never sink. (An episode of Mythbusters demonstrated this idea.) Among the other benefits were that 'ice ships' might be much cheaper and more durable than standard ships, and they wouldn't attract magnetic mines.
Pyke sent the idea of his invention of Pykrete via a proposal he had composed in a diplomatic bag to COHQ with a label forbidding anyone apart from Lord Mountbatten from opening the package. Mountbatten in turn told Churchill about Pyke's proposal, who was enthusiastic about it.
According to the (likely apocryphal) stories Pyke demonstrated this idea to Winston Churchill by bursting into his bathroom while Winston was bathing and dropping a massive chunk of Pykrete into his bath.
Operation Habbakuk' was Pyke's idea carried out. Together the British, Americans and Canadians, constructed a prototype aircraft carrier out of pykrete (a mixture of wood pulp and ice), for use against German U-boats in the mid-Atlantic, out of range of land-based planes.
In 1943 fifteen men spent two months on Patricia Lake building a 1-to-50-scale model prototype of the planned Pykrete boat. It was considered seaworthy but full-size versions were never built, and not long after the war ended.
With the spring thaw the ship melted and the wooden forms and refrigeration equipment sank, thus ending Geoffrey Pyke's dream of a fleet of ice ships, and leaving Pykrete a odd footnote in the history of WWII.