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Currently unidentified sounds

The Hum

The Hum is a persistent and invasive low-frequency humming, rumbling, or droning noise audible to many but not all people. Different causes have been attributed, including local mechanical sources, often from industrial plants, as well as manifestations of tinnitus or other biological auditory effects.

Longfellow Boom

The Longfellow Boom is a loud and unexplained phenomenon reported in the Longfellow community of Minneapolis, Minnesota, United States. The booms reportedly generally take place on summer nights, and have been described by residents as "house-shakingly loud" and at lower tone, distinct from a car crash or gunshot.

Skyquakes

Skyquakes or mistpouffers are a sound phenomenon generally occurring near large bodies of water, described as comparable to distant cannon fire or thunder. The sound has been reported around the world, with several cases documented in the 19th century.

The Ping

The Ping is a "hum" or "beep" detected by shipboard sonars in the Fury and Hecla Strait of northern Canada during the summer of 2016. It was investigated by Canadian military authorities, who did not detect any anomalies on the seabed, where the sound originated. Possible causes include local sonar surveys or the acoustics of Arctic ice.

Formerly unidentified sounds

NOAA

The following previously unidentified sounds have been detected by the U.S. National Oceanic and Atmospheric Administration (NOAA) using its Equatorial Pacific Ocean autonomous hydrophone array. Researchers have since posited volcanic and glacial origins for the sounds.

The 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 54°S 140°W, 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.

This sound, dubbed the Whistle, was recorded by the eastern Pacific autonomous hydrophone deployed at 08°N 110°W on July 7, 1997 at 07:30GMT. According to NOAA, the Whistle is similar to volcanogenic sounds previously recorded in the Mariana volcanic arc of the Pacific Ocean. NOAA also stated that locating the source of an event requires at least three recording instruments, and since the Whistle was only recorded on the NW hydrophone, the sound could have traveled a great distance from its source volcano before detection.

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 sound is consistent with the noises generated by icequakes in large icebergs, or large icebergs scraping the ocean floor.

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.

Other identified sounds

Bio-duck, a quacking-like sound produced by the Antarctic minke whale.

Moodus noises, strange sounds heard in Moodus, Connecticut, later attributed to microquakes.

Western Pacific Biotwang - Vocalization produced by Bryde's whales.