During the First World War, steel shortages forced President Wilson to commission the construction of 24 cement war ships. The war ended less than a year later, at which time the first 12 ships were still under construction. These were eventually completed and sold to private companies. Many of them can now be seen slowly decaying off the coast of the U.S., and one has even been converted to a 10 room hotel off the coast of Cuba.

Almost 25 years later, Geoffrey Pyke recommended the construction of a huge aircraft carrier – the equivalent of a floating island – made from ice and wood pulp, during the Second World War. The material, dubbed Pykrete, possesses incredible tensile strength – more than concrete – at less than half the density. The project, named Habakkuk after a biblical quote (“I am going to do something in your days that you would not believe“), underwent a number of prototypes and tests, from secret lairs under Smithfield Market in London, to Lake Louise in Canada, but was never fully realised due to a number of engineering and cost concerns.

Wikipedia recounts an entertaining anecdote about a demonstration of Pykrete’s strength:

… at the Quebec Conference of 1943, Lord Mountbatten brought a block of Pykrete along to demonstrate its potential to the bevy of admirals and generals who had come along with Winston Churchill and Franklin D. Roosevelt. Mountbatten entered the project meeting with two blocks and placed them on the ground. One was a normal ice block and the other was Pykrete. He then drew his service pistol and shot at the first block. It shattered and splintered. Next, he fired at the Pykrete to give an idea of the resistance of that kind of ice to projectiles. The bullet ricocheted off the block, grazing the trouser leg of Admiral Ernest King and ended up in the wall.