During World War II, Lord Mountbatten and Geoffrey Pyke approached Winston Churchill with a novel plan to reach German U-boats in the mid-Atlantic, where land-based planes couldn’t reach them. They wanted to build an aircraft carrier out of solid ice.

It sounds crazy today, but if they’d gone through with it Project Habbakuk might well have lived up to its biblical billing (“be utterly amazed, for I am going to do something in your days that you would not believe, even if you were told”). Mountbatten and Pike planned to assemble 280,000 blocks of ice into a ship 600 meters long, with a displacement of 2 million tons (today’s Nimitz-class carriers are 333 meters long and displace 100,000 tons). It would carry electric motors, anti-aircraft guns, an airstrip, and a refrigeration unit to keep everything from melting.

Pro: It would be practically unsinkable.

Con: It would take 8,000 people eight months to build it, at a cost of $70 million.

In the end they made a little prototype in Alberta, but the project never got any further. Still, it’s a credit to Churchill that he even considered such an outlandish idea. “Personally I’m always ready to learn,” he once said, “although I do not always like being taught.”