5. The Japanese I-400 submarine class were designed to launch aircraft
During the war the Japanese devised a class of submarine from which three aircraft could be launched against targets after the vessel surfaced, making them in essence submerged aircraft carriers. The Japanese developed plans to use the vessels in an attack on the Panama Canal, going so far as rehearsing the mission in waters off Japan. Later they planned to use them in a surprise attack on the American fleet anchorage at Ulithi, but the war ended while the mission was still in the planning phase. When the Japanese surrendered the three completed I-400 class submarines were taken by the Americans, many of whom were stupefied by the sheer size of the vessels.
When the Soviets demanded the right to inspect captured Japanese submarines of all classes, the Americans decided that they did not want the advanced Japanese technology in the hands of Stalin, who was proving intransigent. The US Navy removed the submarines to classified positions and sank them, with several including the I-400 and I-401 being sunk by torpedoes from the USS Trumpetfish, which was ordered not to report the exact location of the sinking. In March, 2005, the I-401 was discovered in the waters off Hawaii. In 2013, the I-400 was discovered by the same research team from the Hawaii Undersea Research Laboratory, southwest of Oahu. The massive Japanese submarines had remained undetected since June of 1946.