Upsweep
Upsweep is a rising underwater sound NOAA first recorded in 1991 in the South Pacific. Its source is unresolved, with volcanic activity the leading guess.
Upsweep is an unexplained underwater sound recorded by NOAA hydrophones. It is a long train of narrow-band, rising tones, each lasting several seconds. Nobody has proven what makes it, but the evidence points to the seafloor and not to a creature.
What NOAA reports
The NOAA Pacific Marine Environmental Laboratory (PMEL) says the sound was already present when it began recording the US Navy’s SOSUS hydrophone arrays in August 1991. Its source level is high enough to be recorded across the Pacific. PMEL says it appears to be seasonal, peaking in spring and fall, but it is unclear whether that reflects changes at the source or changes in how sound travels through the ocean. The source can be roughly located at 54 degrees south, 140 degrees west, in a remote area between New Zealand and South America near inferred volcanic seismicity. NOAA says the overall level has declined since 1991, but the sound can still be detected on its equatorial autonomous hydrophone arrays. PMEL offers a recording sped up 20 times, so people can hear it.
Explanations
Early on, some thought a whale might be responsible. Wikipedia reports that this biological idea was dismissed by 1996, because the tone varied too little to fit an animal. The leading explanation is underwater volcanic or seismic activity near a seafloor spreading centre. That is a hypothesis, because the source location is only approximate and no volcano has been tied to the sound. NOAA’s own page says the origin is unresolved.
How to read it
Upsweep is a real recording, not a legend, and it has no known link to anything paranormal.
Sources
- Upsweep (NOAA PMEL Acoustics Program) - NOAA’s own description: first recorded August 1991, rough location, seasonal peaks, declining level
- Upsweep (Wikipedia) - summary of the early biological guess and the volcanic hypothesis