I read about this in the latest Fortean Times: During WWII, eccentric genius Geoffrey Pyke invented a form of ice that melted so slowly you could use it as a building material. It was as strong as concrete and floated, just like regular ice. So you could build a literally unsinkable ship out of it: a ship whose hull was made of a material that floated. And if you wanted you could make that ship with a forty foot thick hull that would just slowly chip away bit by bit even under the force of an artillery barrage, with no real damage to the ship. There were plans underway to build aircraft carriers out of Pykrete. A relatively small test ship was built on a Canadian lake. It took over a year to melt even after it was abandoned completely.
I was like, there has to be something about Pykrete on the internet!
Pyke was a fascinating character. Grab the current issue of Fortean Times and read up on him.
UPDATE: some are skeptical of many of the details of the Pykrete stories.