In H.P. Lovecraft's "The Call of Cthulhu," he specifically places the sleeping Cthulhu and the ancient civilization of R'lyeh at 47°9′S 126°43′W. It's odd to pinpoint a random place in the ocean for a fictional monster of yours.
In 1997, long after the story was written, an extremely loud ultra-low-frequency sound was recorded at 50°S 100°W. Scientists described the unknown-source sound (called "Bloop") as organic and resembling a marine beast, but far too powerful for any known animal. You can hear it on the Wikipedia page.
Why would the random ocean spot Lovecraft chose also be where strange vibrations and sounds were later detected? He couldn't have known; "The Call of Cthulhu" was written in the early 1920s, well before NOAA, advanced sonar, or submarines existed. Lovecraft said most of his ideas came from vivid nightly dreams. This story stemmed from one where he was in a museum and saw people secretly worshipping a gigantic monstrous aquatic creature in the basement.
What if it's… all true?