It's the Titanic and the iceberg

You think hemp is a unique construction material for a boat? During WWII, Winston Churchill authorized the construction of a gargantuan aircraft carrier made of ice. (Actually, it was a blend of ice and wood pulp called pykrete, much stronger and resistant to melting.) The ship, dubbed the Habbakuk, was to be 2,000 feet long and 300 feet wide, displacing 2 million tons. (By comparison, the largest ship around today is a 1,504-foot-long, 565,000 ton supertanker) And, oh yeah, the walls were to be 40 feet thick, making the damn thing invulnerable to gunfire or torpedoes.

It was brilliant idea, reversing the conventional wisdom of shipbuilding... If you build a hull out of metal, it has to be hollow to float, and when pierced by a torpedo, the hull will fill up and sink. So why not build a hull out of a solid material that floats no matter what, and is way more plentiful? That kind of sideways thinking is the hallmark of true innovation.

A prototype was built in Canada, but the Habbakuk itself was never begun, for a few reasons: It would have taken 8,000 men 8 months to complete; the amount of wood pulp needed for that much pykrete was prohibitive; and the war started turning around for the Allies anyway. But you have to give these people credit for thinking outside the icebox. (It hurt just to type that.)

For more about this fascinating project:
The ubiquitous Wikipedia
A great essay from Cabinet magazine
Shipwreck Central explores the remains of the prototype