Bloop is an ultra-low-frequency and extremely powerful underwater sound detected by the U.S. National Oceanic and Atmospheric Administration (NOAA) in 1997. The sound is consistent with the noises generated by icequakes in large icebergs, or large icebergs scraping the ocean floor.
The sound's source was roughly triangulated to Coordinates: (a remote point in the south Pacific Ocean west of the southern tip of South America), and the sound was detected several times by the Equatorial Pacific Ocean autonomous hydrophone array. This system was developed as an autonomous array of hydrophones that could be deployed in any oceanographic region to monitor specific phenomena. It is primarily used to monitor undersea seismicity, ice noise, and marine mammal population and migration. This is a stand-alone system designed and built by NOAA's Pacific Marine Environmental Laboratory (PMEL) to augment NOAA's use of the U.S. Navy Sound Surveillance System (SOSUS), which was equipment originally designed to detect Soviet submarines.
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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 (3106.86 miles)." The NOAA's Dr. Christopher Fox did not believe its origin was man-made, such as a submarine or bomb, nor familiar geological events such as volcanoes or earthquakes. While the audio profile of Bloop does resemble that of a living creature, the source was a mystery both because it was different from known sounds and because it was several times louder than the loudest recorded animal, the blue whale. A number of other significant sounds have been named by NOAA: Julia, Train, Slow Down, Whistle and Upsweep.
Dr. Christopher Fox of the NOAA initially speculated that Bloop may be ice calving in Antarctica. A year later journalist David Wolman paraphrased Dr. Fox's updated opinion that it was probably animal in origin:
Fox's hunch is that the sound nicknamed Bloop is the most likely to come from some sort of animal, because its signature is a rapid variation in frequency similar to that of sounds known to be made by marine beasts. There's one crucial difference, however: in 1997 Bloop was detected by sensors up to 4800 kilometres (2982.582 miles) apart. That means it must be far louder than any whale noise, or any other animal noise for that matter. Is it even remotely possible that some creature bigger than any whale is lurking in the ocean depths? Or, perhaps more likely, something that is much more efficient at making sound?
— David Wolman
The NOAA Vents Program has since then attributed the sound to that of a large icequake. Numerous icequakes share similar spectrograms with Bloop, as well as the amplitude necessary to spot them despite ranges exceeding 5000 km. This was found during the tracking of iceberg A53a as it disintegrated near South Georgia island in early 2008. If this is indeed the origin of Bloop, the iceberg(s) involved in generating the sound were most likely between Bransfield Straits and the Ross Sea; or possibly at Cape Adare, a well-known source of cryogenic signals.
According to NOAA seismologist Robert Dziak, the idea the Bloop was possibly emanating from an animal was due to the fact the audio was being played at 16 times the original speed. He stated that, sped-up, the sound was far more characteristic of an animal, rather than the sound of an icequake, which it resembled when played at the original speed. 
- In Fluke, or, I Know Why the Winged Whale Sings by Christopher Moore, the source of Bloop is a living colony known as the "Goo".
- In The Loch by Steve Alten, the source of Bloop is the sound made by large Conger Eels, one of which is the true identity of the Loch Ness Monster.
- In Frank Schätzing's novel The Swarm, Bloop is the speech of the intelligent species the Yrr.
- The roughly triangulated origin of Bloop is approximately 950 nautical miles (1,760 km) from the more precisely-described location of R'lyeh, a sunken extra-dimensional city written of by H. P. Lovecraft in his popular short story "The Call of Cthulhu".
- In the SCP Foundation Web Site, Bloop is generated by SCP-169 also known as the "Leviathan."
- Writer/essayist John Jeremiah Sullivan briefly discusses Bloop in his 2008 GQ article: "Violence of the Lambs."
- A fictional TV show in the form of a documentary titled Mermaids: The Body Found used the bloop sound as evidence for the existence of mermaids or an unknown species in the oceans. NOAA posted a refutation on their web site.
- "Acoustics Monitoring Program - Icequakes (Bloop)". Pacific Marine Environment Laboratory. NOAA.gov. Retrieved 2012-11-17.
- Wolman, David (2002-06-15). "Calls from the deep". New Scientist. Retrieved 2012-01-16.
- "Animal Records". Smithsonian National Zoological Park. Retrieved 2010-09-26.
- "Tuning in to a deep sea monster". CNN. 2002-06-13. Retrieved 2010-01-06.
- "Spectrograms - VENTS program". Pacific Marine Environment Laboratory. NOAA.gov. Retrieved 2010-01-06.
- David Wolman (2002-06-15). "Calls from the deep". New Scientist. Retrieved 2012-01-16.
- "Scientists tune in to sounds of the sea". CNN. 2001-09-07. Retrieved 2012-01-16.
- Stefan Dziemianowicz. "Terror Eternal: The enduring popularity of H.P. Lovecraft". Publishers Weekly. Retrieved 2014-04-26.
- Jonathan Strickland. "Cthulhu goes Bloop". How Stuff Works. Retrieved 2010-10-07.
- Sean Michael Ragan (2009-11-16). "The Bloop of Cthulhu". Retrieved 2010-10-07.
- Zabarenko, Deborah (6 July 2012). "This just in: Mermaids are NOT real, U.S. agency says". Reuters. Washington. Retrieved 27 May 2014.
- "Are mermaids real? No evidence of aquatic humanoids has ever been found.". Ocean Fact. National Oceanic and Atmospheric Administration. 27 June 2012. Retrieved 27 May 2014.