The Upsweep is a sound discovered by the U.S. National Oceanic and Atmospheric Administration (NOAA) using underwater listening devices called hydrophone arrays. In August 1991, the sound was recorded with the Pacific Marine Environmental Laboratory's underwater sound system, SOSUS. It is loud enough to be heard across the entire Pacific Ocean. By 1996, scientists no longer thought the sound came from living creatures. The sound includes a series of rising-pitched sounds that happen every few seconds. These sounds change with the seasons, leading NOAA scientists to believe they may come from underwater volcanic activity.

Sound profile

The source of the sound is located at approximately 54°S 140°W, in a remote area of the Pacific Ocean between New Zealand and about 2,500 miles west of the southern tip of South America. The sound changes with the seasons, often reaching its highest levels during spring and fall. However, it is not clear if these changes are caused by the sound source itself or by seasonal changes in how the sound travels through the environment. The sound includes a long series of repeated vertical "sweeps" that move from low to high frequency, lasting about three seconds each. It was loud enough to be detected by the entire Equatorial Pacific Ocean autonomous hydrophone array system. The upward sweep has a strange, echoing tone, similar to the sound of an ambulance or siren.

The sound was recorded by a network of hydrophones operated by NOAA's Sound Surveillance System (SOSUS) program, which monitors the northeast Pacific Ocean for low-level seismic activity and volcanic events along the region's spreading centers. At first, scientists thought the sound might come from fin whales, but this idea was rejected because the sound lacked enough variation in tone to be biological.

Researchers have linked the sound's origin to areas where volcanic seismic activity is suspected. Since 1991, the loudness of the Upsweep has decreased, but it is still detectable by NOAA's hydrophone arrays.

Volcanic origin

A main idea about where the Upsweep sound comes from is that it is connected to underwater volcanic and earthquake activity. Volcanic eruptions under the ocean often happen in areas called rift zones, which are found in all of Earth's major ocean basins. These areas are also called seafloor spreading centers, and the NOAA set up the SOSUS program there to watch for earthquakes and volcanic activity on the ocean floor. The Monterey Bay Aquarium Research Institute explained that the sound's location suggests it may have come from an area with underwater volcanic and earthquake activity. However, scientists do not know the exact place where the sound happened.