List of unexplained sounds

The following is a list of unidentified, or formerly unidentified, sounds. All of the sound files in this article have been sped up by at least a factor of 16 to increase intelligibility by condensing them and raising the frequency from infrasound to a more audible and reproducible range.
NOAA (unidentified).
The following unidentified sounds have been detected by the U.S. National Oceanic and Atmospheric Administration (NOAA) using its Equatorial Pacific Ocean autonomous hydrophone array.
Upsweep.
Upsweep is an unidentified sound detected on the American NOAA's equatorial autonomous hydrophone arrays. This sound was present when the Pacific Marine Environmental Laboratory began recording its sound surveillance system, SOSUS, in August 1991. It consists of a long train of narrow-band upsweeping sounds of several seconds in 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 autumn, 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 , between New Zealand and South America. Scientists/researchers of NOAA speculate the sound to be underwater volcanic activity. The Upsweep's level of sound (volume) has been declining since 1991, but it can still be detected on NOAA's equatorial autonomous hydrophone arrays.
Whistle.
This mysterious sound, dubbed the Whistle, was picked up by one NOAA hydrophone in 1997. The location of this sound is unknown, and the source is somewhere in the Pacific Ocean. According to NOAA, the Whistle is similar to volcanogenic sounds previously recorded in the Mariana volcanic arc of the Pacific Ocean, but since it was only recorded on one hydrophone rather than the three required to triangulate a location, it is considered "unidentified".
NOAA (formerly unidentified).
Bloop.
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 sound is consistent with the noises generated by icequakes in large icebergs, or large icebergs scraping the ocean floor.
Analysis.
The sound's source was roughly triangulated to 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. 
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 ." 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.
The NOAA Vents Program has attributed Bloop to a large icequake. Numerous icequakes share similar spectrograms with Bloop, as well as the amplitude necessary to spot them despite ranges exceeding . 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 in Antarctica, a well-known source of cryogenic signals.
Julia.
Julia is a sound recorded on March 1, 1999, by the U.S. National Oceanic and Atmospheric Administration (NOAA). NOAA said the source of the sound was most likely a large iceberg that had run aground off Antarctica. It was loud enough to be heard over the entire Equatorial Pacific Ocean autonomous hydrophone array, with a duration of about 2 minutes and 43 seconds. Due to the uncertainty of the arrival azimuth, the point of origin is most likely located near the equator by South America, in the Pacific Ocean. Location (15°S 98°W)
Slow Down.
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 was most likely a large iceberg as it became grounded.
Analysis.
The name was given because the sound slowly decreases in frequency over about seven minutes. 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 vibrations caused by friction closely resemble the spectrogram of the Slow Down. This suggests the source of the sound could have been caused by the friction of a large ice sheet moving over land.
Train.
The Train is the name given to a sound recorded on March 5, 1997, on the Equatorial Pacific Ocean autonomous hydrophone array. The sound rises to a quasi-steady frequency. According to the NOAA, the origin of the sound is most likely generated by a very large iceberg grounded in the Ross Sea, near Cape Adare.