According to Dr Hans Van Tillburg, "[the I-201 submarine]was nothing like anybody had in the Second World War". It had a streamlined body and conning tower, retractable guns and three catapult-launched bombers. They just found it in Hawaii.
According to Van Tillburg, it looks more like a Cold War submarine than a WW2 ship, built for high speed underwater travel. The I-401 was its sister vessel. That one was located in 2005, and it was capable of carrying 144 people through 59,545km, it was three times the size of modern submarines. Japan wanted to use these ships to launch aerial attacks in the continental United States. To do this, it used three Aichi light bombers, each capable of carrying an 800kg bomb. When their mission was complete, the bombers returned to the submarine, landing on the water using floats.
The Japanese also had another dirty trick in their pockets: use the submarines to deploy rats infected with the plague and insects loaded with cholera, dengue and typhus. Fortunately, they never were able to put that in practice before the Americans delivered to them a fat tin-can full of atoms.
These submarines were so advanced that, after being captured and inspected by the United States, they were sunk so the Soviets couldn't inspect them. The Japanese were the pioneers in developing advanced aerial attack technology for submarines. The sea-to-air cruise missile was developed later, during the Cold War period. Nowadays, the US Navy is working on submarine-launched drones which can take off from underwater. [National Geographic via Daily Mail]