The Bloop, which has its own Wikipedia page (OK, what interesting subject doesn’t, the Wikipedia Paradox notwithstanding?)

Aaaaanyway …

[T]he sound is believed to be coming roughly from 50oS; 100oW. After reading that, I wondered how close that was to the coordinates given in “The Call of Cthulhu”. Allow me to quote: “Then, driven ahead by curiosity in their captured yacht under Johansen’s command, the men sight a great stone pillar sticking out of the sea, and in S. Latitude 47°9′, W. Longitude l23°43′, come upon a coastline of mingled mud, ooze, and weedy Cyclopean masonry which can be nothing less than the tangible substance of earth’s supreme terror – the nightmare corpse-city of R’lyeh, that was built in measureless aeons behind history by the vast, loathsome shapes that seeped down from the dark stars.”

Also: NOAA: A Collection of Sounds from the Sea

Spectrographs of underwater sounds, including The Bloop