The Upsweep
Since 1991, underwater microphones have recorded a long series of rising sounds from a remote corner of the Pacific.
What was reported
In 1991, the US National Oceanic and Atmospheric Administration (NOAA) began recording a strange sound in the Pacific: a long series of rising tones, each lasting several seconds. It was loud enough to be picked up across the whole Pacific, and it came from a remote area between New Zealand and South America.
The sound was strongest in spring and autumn, and has slowly faded since the early 1990s. It became known as the Upsweep.
Explanations offered
NOAA's best explanation is volcanic activity on the seabed, which can produce long-lasting sounds. The source has never been precisely located, so this has not been confirmed.
- When
- Since 1991
- Where
- Remote South Pacific Ocean
- Witnesses
- Underwater microphones run by the US government
- Status
- Mostly explained
Famous cases are real, widely reported events. Each file sets out what witnesses said and the explanations that have been offered, without taking a side.