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 and retractable gun." They just found it in Hawaii.
According to Van Tillburg, it looks more like a Cold War submarine than a WW2 ship, build 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 37,000 miles, 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 800-kilogram bomb. When their mission was complete, the bombers returned to the submarine, landing on the water using floats.
The Japanese also had a dirty trick in their pockets: Use the submarines to drop big cans full of rats infected with the plague and insects loaded with cholera, dengue, and typhus. They never were able to put that in practice, however. The Americans delivered a big can full of atoms to them first.