This is a list of sounds that are currently unknown or were not identified in the past. All of the NOAA sound files included in this article have been made faster by at least 16 times to improve clarity. This was done by shortening the sounds and increasing their pitch from very low frequencies to a range that is easier to hear and record.

Currently unidentified sounds

The Hum is a constant and annoying low-frequency noise, such as a hum, rumble, or drone, that many people can hear but not everyone. Possible causes include nearby machines, often from factories, as well as sounds from the body, like tinnitus or other hearing-related issues.

The Longfellow Boom is a loud and mysterious sound reported in the Longfellow neighborhood of Minneapolis, Minnesota, United States. These booms usually happen during summer nights and are described by residents as extremely loud, shaking houses, and having a lower tone, different from the sound of a car crash or a gunshot.

Skyquakes or mistpouffers are a type of sound that occurs near large bodies of water. The sound is similar to distant cannon fire or thunder. This phenomenon has been reported worldwide, with many examples recorded in the 19th century.

The Ping is a "hum" or "beep" detected by sonar equipment on ships in the Fury and Hecla Strait of northern Canada during the summer of 2016. Canadian military officials investigated the sound but found no unusual activity on the ocean floor, where the sound came from. Possible causes include nearby sonar surveys or the way sound travels through Arctic ice.

Formerly unidentified sounds

Scientists from the U.S. National Oceanic and Atmospheric Administration (NOAA) have identified several unknown sounds using its Equatorial Pacific Ocean autonomous hydrophone array. Researchers believe these sounds may come from volcanic activity or glaciers.

The Upsweep is an unknown sound detected by NOAA’s equatorial autonomous hydrophone arrays. It was first recorded in August 1991 by the Pacific Marine Environmental Laboratory’s Sound Surveillance System (SOSUS). The sound consists of long, narrow-band upsweeping tones lasting several seconds. It is loud enough to be heard across the Pacific Ocean. The sound appears to occur seasonally, peaking in spring and autumn, though the reason for this pattern is unclear. Its source is estimated to be near 54°S 140°W, between New Zealand and South America. NOAA scientists suggest it may be caused by underwater volcanic activity. Since 1991, the sound’s volume has decreased, but it is still detectable today.

The Whistle was recorded on July 7, 1997, by an eastern Pacific autonomous hydrophone located at 08°N 110°W. NOAA notes that this sound is similar to those produced by volcanoes in the Mariana volcanic arc. Locating the source of such sounds typically requires data from at least three recording instruments. Since the Whistle was only recorded on one hydrophone, it may have traveled far from its source before being detected.

The Bloop is a very loud, ultra-low-frequency underwater sound discovered by NOAA in 1997. It matches the sounds made by icequakes in large icebergs or icebergs scraping the ocean floor. The sound’s origin was estimated to be in the southern Pacific Ocean, west of South America. It was detected multiple times by NOAA’s Equatorial Pacific Ocean autonomous hydrophone array. The sound rises quickly in frequency over about one minute and was loud enough to be heard on sensors more than 5,000 kilometers apart. NOAA’s Christopher Fox ruled out man-made sources like submarines or bombs. While the Bloop’s sound pattern resembles that of an animal, its source remained unknown because it was much louder than any known animal, including the blue whale. NOAA’s Vents Program later linked the Bloop to a large icequake. Similar sounds were recorded during the disintegration of iceberg A53a near South Georgia Island in 2008. This suggests the sound likely originated between Bransfield Straits and the Ross Sea, or near Cape Adare in Antarctica.

Julia is a sound recorded by NOAA on March 1, 1999. Scientists believe it came from a large iceberg that ran aground near Antarctica. It was loud enough to be heard across the entire Equatorial Pacific Ocean autonomous hydrophone array and lasted about 2 minutes and 43 seconds. Due to uncertainty about the direction of the sound’s arrival, its origin was narrowed to an area between Bransfield Straits and Cape Adare.

Slow Down is a sound recorded on May 19, 1997, in the Equatorial Pacific Ocean by NOAA. It is likely caused by a large iceberg becoming grounded. The sound’s name comes from its gradual decrease in frequency over about seven minutes. It has been detected multiple times each year since 1997. One theory suggests it was caused by moving ice in Antarctica. Sound spectrograms from ice friction match the Slow Down’s pattern, supporting this idea.

The Sea Train is a sound recorded on March 5, 1997, by NOAA’s Equatorial Pacific Ocean autonomous hydrophone array. It rises to a nearly steady frequency. NOAA believes the sound was produced by a very large iceberg that became grounded in the Ross Sea near Cape Adare.