The Bloop is the name given to an ultra-low frequency and extremely powerful underwater sound detected by the U.S. National Oceanic and Atmospheric Administration (NOAA) in 1997. The source of the sound remains unknown.
The sound, traced to somewhere around 50° S 100° W, 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 seismic activity, 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 the U.S. Navy SOund SUrveillance System (SOSUS), 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." The NOAA's Dr. Christopher Fox does not believe its origin is man-made, such as a submarine or bomb, or familiar geological events such as volcanoes or earthquakes. While the audio profile of the Bloop does resemble that of a living creature, the source is a mystery both because it is different from known sounds and because it was several times louder than the loudest recorded animal, the blue whale.
Dr. Christopher Fox of the NOAA initially speculated that the Bloop may be ice calving in Antarctica. A year later journalist David Wolman paraphrased Dr. Fox who'd updated his opinion and said 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 kilometers 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?