They couldn't imagine some of the strange sounds they would record deep underwater. One was recorded in 1997 and is still a mystery to this date. They dubbed it "The Bloop". This sound existed for about a minute, picked up by multiple sensors over an area of 3,000 miles in the south Pacific Ocean west of the southern tip of South America and appeared several times.
There have been theories concerning its origins. Some have been ruled out like the possibility of it being man-made, a volcano or earthquake. Scientists have determined it possesses the similar frequency hallmark of a marine animal. Although, the audio profile of "The Bloop" is several times louder than the loudest known animal, the blue whale. There is a theory it may be a giant squid as they are rather elusive creatures. However, Phil Lobel, a marine biologist at Boston University, Massachusetts is not quite so convinced, "Cephalopods have no gas-filled sac, so they have no way to make that type of noise. Though you can never rule anything out completely, I doubt it."
I'm sure the sea monster theory will likely persist until they discover its actual origins partly because of H. P. Lovecraft. What does H. P. Lovecraft have in common with "The Bloop"? The triangulated origin of the sound is roughly 950 nautical miles from the more precise location of R'lyeh, a sunken extra-dimensional city in Lovecraft's famous short story The Call of Cthulhu and the location where the creature Cthulhu awakens. Despite the distance, the two have been frequently linked.
For now, "The Bloop" has never been heard again and remains a mystery.