US submarines are designed for speed and stealth. The Navy takes great pains to keep this technology a secret. The most critical part of that secret is the design of the propellers that drive the subs and keep them quiet. Usually, the propellers are shrouded or covered by a large tent. Bird's Eye found this one out in the open.
Military - Navy and Coast Guard, Sea - Military - Submarines
Note that in most Bird's Eye and Google images, this area is covered by a large tent structure.