Lovecraft said that R'lyeh
is located at 47°9′S, 126°43′W in the southern Pacific Ocean. August Derleth, however, placed R'lyeh at 49°51′S, 128°34′W in his own writings. Both locations are close to the Pacific pole of inaccessibility, the point in the ocean farthest from any land. Derleth's coordinates place the city approximately 5100 nautical miles (5900 statute miles or 9500 kilometers), or about ten days journey for a fast ship, from Pohnpei (Ponape), an actual island of the area. Ponape also plays a part in the Cthulhu Mythos as the place where the "Ponape Scripture", a text describing Cthulhu, was found.
In summer 1997, the U. S. National Oceanic and Atmospheric Administration’s autonomous hydrophone array in the Equatorial Pacific Ocean repeatedly recorded a peculiar sound of a nature suggesting its biological origin. Yet, the sound’s amplitude was too large to be produced by any known animal species, and its source remains a mystery. According to NOAA, the readings yield a general location of the sound’s source “near 50° S 100° W”. The sound was given the name “Bloop”.
Charles Stross's novella 'A Colder War' implicitly locates R'lyeh in the Baltic Sea: it describes Cthulhu as being "scraped from a nest in the drowned wreckage of a city on the Baltic floor" . This is presumably because a Baltic location was more convenient for Stross's plot.
The sound, traced to somewhere around 50° S 100° W (South American southwest coast), was detected repeatedly by the Equatorial Pacific Ocean autonomous hydrophone array, which uses U.S. Navy equipment originally designed to detect Soviet submarines. According to the NOAA description, it "rises rapidly in frequency over about one minute and was of sufficient amplitude to be heard on multiple sensors, at a range of over 5,000 km." Though it matches the audio profile of a living creature, there is no known animal that could have produced the sound. If it is an animal, it would have to be, reportedly, much larger than even a Blue Whale, according to scientists who have studied the phenomenon.
The site of the Bloop is remarkably close to the site of the fictional city of R'lyeh from H. P. Lovecraft's short story The Call of Cthulhu, where an ancient undersea monster (Cthulhu) lies sleeping. Lovecraft said that R'lyeh is located at 47°9′S, 123°43′W in the southern Pacific Ocean, with the bloop also being targeted somewhere in that range .