I'm fascinated by the World War II-era Japanese submarine that was found by researchers off the coast of Hawaii. This is no ordinary submarine. It's an underwater aircraft carrier.

The submarine carried three bombers in a watertight hanger. A large door gave planes access to the deck, where they would be prepared for flight. The planes would launch from tracks on the submarine's deck. More information on this class of submarine is available here and here.

The submarine aircraft carrier seems like a much more practical idea than the flying submarine.