The Japanese managed to build three of these behemoths during the war, vessels which hold the record as the largest conventionally powered submarines ever built. As part of the Japanese plan to dominate the Pacific Ocean — including the Western seaboard of the United States — they were designed to attack the Panama Canal.
The subs were equipped with three Aichi M6A1 airplanes which could carry a torpedo or up to 1,763 pounds (800 kg) of bombs. They were housed in a watertight, pressure-resistant hangar. The planes were launched from a catapult located in front of the tower, and all three could be assembled, fueled, armed and launched within 45 minutes of surfacing.
It even featured a German-inspired snorkel that was coated with a thick, rubber-like substance designed to absorb radar and sonar signals. The war ended before the Japanese could use them in combat. In 1946, the I-400 was surrendered to the USA and sunk off the coast of Hawaii.Researchers only re-discovered it last year.