The following is a list of sounds that have not been identified, either currently or previously. All of the NOAA sound files in this article have been sped up by at least a factor of 16 to make them easier to understand. This was done by shortening the sounds and raising their frequency from infrasound to a more audible and reproducible range.

Currently unidentified sounds

The Hum is a constant and annoying low-frequency humming, rumbling, or droning sound that many people can hear, but not everyone. Possible reasons include nearby machines, often from factories, or health-related hearing issues like tinnitus.

The Longfellow Boom is a loud and unexplained noise 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, enough to shake houses, with a lower pitch that is different from a car crash or gunshot.

Skyquakes, also called mistpouffers, are sounds that occur near large bodies of water. They are often compared to distant cannon fire or thunder. These sounds have been reported worldwide, with many cases recorded in the 1800s.

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 features on the ocean floor, where the noise originated. Possible reasons include nearby sonar activities or the way sound travels through Arctic ice.

Formerly unidentified sounds

The following previously unknown sounds have been discovered by the U.S. National Oceanic and Atmospheric Administration (NOAA) using its Equatorial Pacific Ocean autonomous hydrophone array. Scientists have suggested that these sounds may come from volcanoes or glaciers.

The Upsweep is an unknown sound detected by the American NOAA's equatorial autonomous hydrophone arrays. This sound was recorded when the Pacific Marine Environmental Laboratory started using its sound surveillance system, SOSUS, in August 1991. It consists of a series of long, narrow-band sounds that rise in pitch over several seconds. The sound is strong enough to be heard across the entire Pacific Ocean.

The Upsweep appears to occur seasonally, with peaks in spring and autumn. However, it is unclear if this pattern is due to the source itself or changes in how sound travels through water. The sound’s location is estimated to be near 54°S 140°W, between New Zealand and South America. NOAA scientists believe the sound may be caused by underwater volcanic activity. Since 1991, the sound’s volume has decreased, but it is still detectable on NOAA’s hydrophone arrays.

The Whistle was recorded by an eastern Pacific autonomous hydrophone at 08°N 110°W on July 7, 1997, at 07:30 GMT. According to NOAA, the Whistle is similar to sounds caused by volcanic activity in the Mariana volcanic arc of the Pacific Ocean. NOAA explains that locating a sound’s source usually requires at least three recording instruments. Since the Whistle was only detected by one hydrophone, the sound may have traveled a long distance from its source before being recorded.

The Bloop is an extremely loud, ultra-low-frequency underwater sound detected by NOAA in 1997. The sound matches noises created by icequakes in large icebergs or icebergs scraping the ocean floor. The Bloop’s source was roughly located in the south Pacific Ocean, west of the southern tip of South America. It was detected multiple times by NOAA’s hydrophone array.

According to NOAA, the Bloop rises quickly in pitch over about one minute and was loud enough to be heard on multiple sensors, even over 5,000 km (3,100 mi) away. NOAA’s Christopher Fox stated the sound was not man-made, such as from a submarine or bomb. While the Bloop’s sound pattern resembles that of a living creature, its source remained unknown because it was much louder than the loudest known animal, the blue whale.

NOAA’s Vents Program linked the Bloop to a large icequake. Many icequakes have similar sound patterns and loudness to the Bloop. This was observed when iceberg A53a broke apart near South Georgia Island in 2008. The iceberg responsible for the Bloop was likely located between Bransfield Straits and the Ross Sea, or near Cape Adare in Antarctica, a known source of ice-related signals.

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

Slow Down is a sound recorded by NOAA on May 19, 1997, in the Equatorial Pacific Ocean. The sound likely came from a large iceberg that became grounded. The name "Slow Down" describes how the sound’s pitch gradually decreases over about seven minutes. It has been recorded multiple times each year since 1997. One theory suggests the sound was caused by moving ice in Antarctica. Sound patterns from ice friction match the Slow Down’s spectrogram, indicating the sound may have been created by a large ice sheet moving over land.

The Sea Train is the name given to a sound recorded on March 5, 1997, by NOAA’s Equatorial Pacific Ocean hydrophone array. The sound reaches a steady pitch. NOAA believes the sound was created by a very large iceberg that became grounded in the Ross Sea near Cape Adare.