Project Habakkuk

Project Habakkuk or Habbakuk (spelling varies; see below) was a plan by the British during the Second World War to construct an aircraft carrier out of pykrete (a mixture of wood pulp and ice) for use against German U-boats in the mid-Atlantic, which were beyond the flight range of land-based planes at that time. The idea came from Geoffrey Pyke, who worked for Combined Operations Headquarters. After promising scale tests and the creation of a prototype on a lake in Alberta, Canada the project was shelved due to rising costs, added requirements, and the availability of longer-range aircraft and escort carriers which closed the Mid-Atlantic gap the project was intended to address.

Geoffrey Pyke was an old friend of J.D. Bernal and had been recommended to Lord Mountbatten, Chief of Combined Operations, by the Cabinet minister Leopold Amery. Pyke worked at Combined Operations Headquarters (COHQ) alongside Bernal and was regarded as a genius by Mountbatten.

Pyke conceived the idea of Habbakuk while he was in the United States organising the production of M29 Weasels for Project Plough, a scheme to assemble an elite unit for winter operations in Norway, Romania and the Italian Alps. He 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 aluminium were in short supply, and were required for other purposes. Pyke realized that the answer was ice, which could be manufactured for only 1 percent of the energy needed to make an equivalent mass of steel. He proposed that an iceberg, natural or artificial, be levelled to provide a runway and hollowed out to shelter aircraft.

...
Wikipedia