Project Habakkuk was an aircraft carrier plan by the British during World War 2.

"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 decided that the answer was ice, which could be manufactured for just 1% 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."

Built using a mixture of wood pulp and ice at 600 meters
long, and 2.2 million tons of displacement it would have been 142
meters longer and have 1,542,981 more tons of displacement than the
Seawise Giant, the largest vessel ever built to date. The hull would
have been 12 meters thick and the rudder 30 meters long, 20 meters
longer than the longest rudder ever made.

The plan was cancelled as it became unnecessary and it was realized
the amount of steel required to build the refrigeration plant to maintain
the structural integrity of the vessel would have been larger than
just building using steel for the vessel in the first place.

https://en.wikipedia.org/wiki/Project_Habakkuk
https://www.youtube.com/watch?v=QcD2BiSx6sI