Take water, mix with wood pulp, and freeze. Now it’s as strong and tough as concrete, as long as it stays frozen. So, in World War II, serious plans were afoot to use it to build battleships out of ice.
The material known as pykrete, after its inventor Geoffrey Pyke, was a mixture of wood pulp – e.g. sawdust – and water in a 6:1 ratio. The wood pulp holds the ice together in the same way that rebar holds concrete together. It was strong enough to sustain shots from a rifle, in one early test.
But what do you do with strong ice? You build an aircraft carrier out of it, of course. The theory was that you could make a reinforced hull with pykrete, install internal freezers to maintain its low temperature, and then build an airstrip on top. Most planes could not reach the mid-Atlantic at this time (the so-called “Mid-Atlantic Gap” or “The Black Pit”), so having an aircraft carrier would be a huge boon to the Allied defence against U-boats.
Churchill apparently loved the idea, after Mountbatten allegedly dropped a block of pykrete into his bathtub. The Canadians even built a prototype 18 metres long. But ultimately there were too many practical obstacles, and the whole project was abandoned.