Detection History
Upsweep was first detected in August 1991 by NOAA's PMEL hydrophone arrays shortly after their reactivation for oceanographic research. The signal consists of stacked narrow-band upswept tones — each sweep rising in frequency over several seconds, repeating in bursts. Detection has been continuous across multiple geographically separated hydrophone stations, ruling out local instrument artefact.
The source location has been roughly triangulated to an area of the South Pacific near known volcanic ridge systems. This has led to speculation about a volcanic or hydrothermal origin, but no confirmed seismic event produces the characteristic repetition rate or the seasonal amplitude cycle.
The Seasonal Anomaly
Upsweep's amplitude peaks in northern hemisphere spring and autumn and weakens in summer and winter. This modulation is consistent across years. A geological source would not be expected to show this pattern. A biological source would imply an organism or population of organisms producing low-frequency tonal sweeps on a planetary scale — no such organism is known. The seasonal modulation remains the most difficult feature to explain within any current model.
Technical Parameters
| Acoustic record ID | AUDIO-002 / "Upsweep" |
| First detection | August 1991 |
| Detection status | Ongoing — confirmed at multiple stations |
| Network | NOAA PMEL hydrophone arrays |
| Frequency range | 15–70 Hz, upswept profile |
| Estimated source location | South Pacific, near volcanic ridge system |
| Seasonal modulation | Confirmed — peak spring/autumn |
| Official classification | Unresolved — probable volcanic, mechanism unclear |
| Audio | Available (×20 speed-shifted) |