1. The Bloop
Okay, I can find nothing on this one at all. It's an undersea noise recorded in the Pacific in 1997 and it remains unexplained. It sounds, to me, like a bubble. Perhaps a large clathrate collapse. To date, it has not been assigned any official explanation at all. Chthulu is fiction. Let's not even start on that one.
The most interesting thing about this one, aside from its inexplicability, is that it's not the only unexplained sound to come out of the ocean. Some very strange noises have been recorded.
Some are available here, with whale sounds, boats, volcanoes and earthquakes for comparison.
If your name is Julia, beware. Davy Jones is calling for you.