|Credit: Ocean Explorer/Peter Lindberg|
There are plenty of theories about what the USO (unidentified sunken object) might be. Lindberg offered a "new Stonehenge," though some suspect it's a natural formation, such as the rim of a small underground volcano, which certainly create a very round, prominent ring.
Others see clear evidence that the object, whatever it is, is too perfectly round to be anything but man-made. Of course, one of the most popular - and most outlandish - theory is that it's a crashed extraterrestrial spacecraft.
Yet there's another theory: the USO is neither an extraterrestrial craft nor a natural feature but instead a rotating gun turret from a World War II era battleship.