Project Habakkuk was Geoffrey Pyke's brainchild: a 2,000-foot-long aircraft carrier made of pykrete—a frozen mixture of water and wood pulp that was far stronger than plain ice and melted much more slowly. The material was named after its inventor and was genuinely remarkable: a block of pykrete could stop bullets and was as strong as concrete.
Lord Mountbatten famously demonstrated pykrete's properties at the Quebec Conference by shooting a block of it in a room full of Allied leaders. The bullet ricocheted and nearly hit Air Chief Marshal Charles Portal. Churchill was said to have watched the demonstration from his bathtub.
A prototype was built on Patricia Lake in Alberta, Canada. The 60-foot model required a one-horsepower refrigeration unit to prevent melting. However, calculations revealed the full-scale version would require 8,000 tons of wood pulp, 25,000 workers, and take about 8 months to build. By 1943, long-range aircraft and escort carriers had reduced the mid-Atlantic gap, eliminating the need for the project.