In the 1940s, German U-Boats were causing chaos in the Atlantic, prompting inventor Geoffrey Pyke to propose a giant, unsinkable aircraft carrier made of ice, called Project Habakkuk. Pyke developed pykrete, a mixture of wood and ice, as a stronger alternative. Despite setbacks and incidents, a 1,000-ton prototype was built on Patricia Lake in Canada, but the full-scale ship was ultimately scrapped due to high costs and changing wartime technology. The prototype proved durable, taking three summers to melt. The remains lie at the bottom of the lake, a reminder of the ambitious and innovative Project Habakkuk.
https://99percentinvisible.org/article/project-habbakuk-britains-secret-ice-bergship-aircraft-carrier-project/