Case file · Unidentified submerged object

Upsweep

A repeating series of rising tones recorded by NOAA's underwater listening network from August 1991, coming from a remote part of the South Pacific.

1991 CE · -54.00, -140.00 · status: unexplained

The account

The sound, heard across the whole equatorial Pacific array, changes with the seasons and peaks in spring and autumn. Researchers at first suspected fin whales.

The sceptical view

NOAA scientists think it comes from undersea volcanic activity; the biological idea was dropped by 1996.

The researched reading

A real, instrument-recorded ocean sound whose likeliest source is undersea volcanism. Only the exact mechanism is still uncertain.

Explanation	Weight	Why
A known natural phenomenon	40%	NOAA located the sound in a remote, volcanically active part of the South Pacific; its seasonal waxing and waning and long-term decline fit sustained seismic or volcanic activity, and NOAA scientists favour that source.
A poorly understood natural phenomenon	24%	The exact mechanism is unconfirmed: no vent has been directly tied to the sound, so it remains a natural signal of uncertain origin.
Undetermined	22%	The share a generic explanation had beyond what has been shown for this case: it fits many cases like this one, but has not been demonstrated for this one.
Something known, misidentified	8%	Researchers first suspected fin whales; a biological source was abandoned because the sound is far too regular and long-lived.
Secret human technology	3%	A military sonar source is unlikely for a sound that has persisted for decades in a remote area.
Perception, memory or mind	1%	It is an instrumental recording, not a perception.
A hoax or fabrication	1%	Recorded by NOAA’s network; not a fabrication.
Genuinely anomalous	1%	Nothing about it requires unknown physics.

Official position: NOAA classes it as probably volcanic in origin.

Connections in the atlas

Sources

Open it in the living atlas