The Upsweep is a sound that was found by the U.S. National Oceanic and Atmospheric Administration's (NOAA) special underwater microphones located near the equator. The sound was recorded in August 1991 using the Pacific Marine Environmental Laboratory's underwater sound surveillance system, called SOSUS. It is loud enough to be heard all across the Pacific Ocean. By 1996, early ideas that the sound came from a living thing were not accepted. The sound includes a long series of sounds that rise in pitch, and these sounds happen every few seconds. The sound happens and changes with the seasons, leading NOAA scientists to think it comes from underwater volcanoes.

Sound profile

The sound comes from a location near 54°S 140°W, in a far area of the Pacific Ocean between New Zealand and about 2,500 miles west of the southern tip of South America. The sound is louder in spring and fall, but it is not clear if this is because the source changes or because the environment around the sound changes with the seasons. The sound has a pattern of repeated sweeps that go from low to high pitch, each lasting about three seconds. It was loud enough for a system of underwater microphones across the Equatorial Pacific to detect it. The sound has a strange, echoing tone similar to a siren or ambulance.

The sound was recorded by a system of underwater microphones operated by NOAA's Sound Surveillance System (SOSUS) program. This program monitors the northeast Pacific for small earthquakes and volcanic activity. At first, scientists thought fin whales made the sound, but they changed their minds because the tone didn’t vary enough to be from a living creature.

Scientists believe the sound comes from an area where volcanic activity is suspected. Since 1991, the sound has gotten quieter, but it is still picked up by NOAA's microphones.

Volcanic origin

A leading theory about the origin of Upsweep connects the sound to underwater volcanic and seismic activity. Submarine volcanic eruptions are common in the creation of rift zones found in all of Earth's major ocean basins. These areas are also called seafloor spreading centers, where the SOSUS program was created by NOAA to monitor underwater earthquakes and volcanic activity. The Monterey Bay Aquarium Research Institute described the sound features of these events as follows:

Underwater volcanoes produce a range of sounds during eruptions, including short, sharp cracks, loud explosions, and deep, low rumbles. Scientists do not fully understand what causes all these different types of sounds.

The estimated location of the sound has led researchers to believe it came from an area with underwater volcanic and seismic activity. However, the exact location of the sound remains unknown.