Upsweep
UnsolvedSummary
In August 1991, when the Pacific Marine Environmental Laboratory began listening to the Pacific through the SOSUS hydrophone network, a sound was already there: a long, repeating train of narrow-band tones sweeping upward in frequency, loud enough to be heard across an entire ocean. Its siblings have all been explained. This one has not. It is still sounding, and it is getting quieter.
What is documented
The sound. A long train of narrow-band upsweeping sounds, each of several seconds' duration, repeating. On a spectrogram it appears as a sequence of rising strokes. The source level is high enough for it to be recorded throughout the Pacific Ocean.
The date. This is the fact that matters most and is stated least. Upsweep did not begin in 1991. August 1991 is when NOAA's Pacific Marine Environmental Laboratory began recording through the SOSUS array. The sound was there when the recording started. The record contains no beginning for it. How long it had been sounding before anyone was listening is not known and, from the acoustic data, is not knowable.
The location. The source is roughly located at 54° S, 140° W, in a remote part of the South Pacific, near a region of inferred volcanic seismicity. NOAA's own summary states plainly that the origin of the sound is unresolved.
The seasonality. The sound peaks in spring and autumn. PMEL is explicit that it cannot tell whether this reflects a change in the source or a seasonal change in the propagation environment, which is to say in the ocean itself. A sound channel that shifts with the seasons will make a constant source appear to pulse.
The decline. The overall source level has been falling since 1991. The sound is still detectable on the equatorial hydrophone arrays. No explanation for the decline has been established.
Not biological. An early suggestion of a biological origin was dismissed by 1996. The sound is too loud, and its structure too regular, for any known animal.
Leading explanations
Submarine volcanism (leading, unconfirmed). The favoured hypothesis, and NOAA's own, is that Upsweep is generated by underwater volcanic activity: lava meeting seawater, producing a repeating acoustic signature. The location supports it, sitting near inferred volcanic seismicity. What is missing is the confirmation. No specific volcanic source has been identified, no eruption has been correlated with the sound, and the mechanism that would produce this particular repeating upsweep structure has not been demonstrated.
Seasonal propagation (partial, and a caution rather than a hypothesis). Some or all of the apparent seasonality may be an artefact of how sound travels through a changing ocean rather than a property of whatever is making it. PMEL says so directly. It is included here because it is a documented caveat on the primary data, and because it is routinely reported as though the source itself were seasonal.
What the popular version gets wrong
"The mysterious NOAA ocean sounds have all been explained." Most have. The Bloop was attributed to a large icequake. Julia and Slow Down were attributed to ice. The Bio-duck was traced to Antarctic minke whales in 2014. Upsweep was not explained, and it is the one that is still going. The list of unexplained ocean sounds is not empty; it is short, and this is what is on it.
"It began in 1991." It was recorded from 1991. Nothing in the record says it began then, and the distinction is the whole point.
"It is a creature." Ruled out by 1996 on loudness and regularity.
"The seasonal pattern proves the source turns on and off." It proves nothing of the sort, and the laboratory that collected the data says so.
Current status
Unsolved. A well-supported hypothesis exists and points at underwater volcanism. It has not been confirmed, the emitting source has not been located, the decline in level since 1991 is unexplained, and the sound has no established start date. This is what an honestly unsolved case looks like: not an absence of ideas, but an absence of confirmation.
Sources
- NOAA Pacific Marine Environmental Laboratory, Acoustics Monitoring Program: Upsweep. pmel.noaa.gov/acoustics.
- NOAA/PMEL Equatorial Pacific Ocean autonomous hydrophone array data, August 1991 onward.
- SOSUS (Sound Surveillance System) records as made available to PMEL.
Last reviewed: July 2026. Records are provisional. Where the evidence changes, the entry changes. Found an error? Tell us.