The 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 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:
For reference here - Estimates made by Cummings and Thompson (1971) suggest the source level of sounds made by blue whales are between 155 and 188 decibels when measured relative to a reference pressure of one micro-pascal at one meter. All blue whale groups make calls at a fundamental frequency between 10 and 40 Hz; the lowest frequency a human can typically perceive is 20 Hz. Blue whale calls last between ten and thirty seconds. (This was no whale, imho.)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? — David Wolman
The Train is an unidentified sound recorded on March 5, 1997 on the Equatorial Pacific Ocean autonomous hydrophone array. The sound rises to a quasi-steady frequency. The origin of the sound is unknown.
The Slow Down is a sound recorded on May 19, 1997, in the Equatorial Pacific Ocean by the U.S. National Oceanic and Atmospheric Administration. The source of the sound remains unknown.
The name was given because the sound slowly decreases in frequency over about 7 minutes. The sound was detected at 15°S 115°W. It was recorded using an autonomous hydrophone array. The sound has been picked up several times each year since 1997. One of the hypotheses on the origin of the sound is moving ice in Antarctica. Sound spectrograms of ice-friction closely resemble the spectrogram of the Slow Down. Antarctica is very far away from the equator, however.
The Whistle is an unidentified sound recorded by the autonomous hydrophone deployed at a location in the Pacific Ocean with coordinates 8°N 110°W. It was recorded on July 7, 1997. The origin of the signal is unknown, and it was not detected on any other hydrophone. The band of energy between 1 and 6 Hz represents strumming of the mooring in mid-water currents.
The Upsweep is an unidentified sound detected on NOAA's equatorial autonomous hydrophone arrays. This sound was present when PMEL began recording its sound surveillance system SOSUS in August, 1991. It consists of a long train of narrow-band 'up-sweeping' sounds of several seconds duration each. The source level is high enough to be recorded throughout the Pacific.
The sound appears to be seasonal, generally reaching peaks in spring and fall, but it is unclear whether this is due to changes in the source or seasonal changes in the propagation environment. The source can be roughly located at 54° S 140° W, near the location of inferred volcanic seismic activity, but the origin of the sound is unresolved. The overall source level has been declining since 1991, but the sounds can still be detected on NOAA's equatorial autonomous hydrophone arrays.
And last, but certainly not least -
"Julia" is a sound recorded on March 1, 1999 by the U.S. National Oceanic and Atmospheric Administration. NOAA said the source of the sound was unknown, but sufficiently loud to be heard over the entire Equatorial Pacific Ocean autonomous hydrophone array. It lasted for about 15 seconds, and its origin was in the equatorial Pacific Ocean at approximately 15° S 98° W.
It is very impressive that most of these sounds occur in the same year (1997) and in the same, relative, geographical area (at remote points in the south Pacific Ocean, west of South America.)
One can reduce the listening time by following the links to hear sped-up versions of these sounds. This will come with the consequence of a lessening in the breadth of the sound quality, however.