It's called an underwater plane
With only a few weeks left on their Kickstarter, Hawkes Ocean Technologies' DeepFlight Super Falcon -- essentially an underwater aircraft with wings (!), tail surfaces (!!), and ailerons (!!!...?) that's theoretically faster and more facile than traditional manned submersibles -- is still a ways off from reaching its funding goal. Which of course means we might never know if it can achieve the first-ever underwater loop. Drop on by their page to check out their video, and throw 'em some coin if you want to help "open-source" underwater exploration... or just land yourself a stint as a co-pilot when they test it in Lake Tahoe.