Upsweep
NOAA/PMEL source audio is presented in an accelerated form for audibility.
Record status: A recurring low-frequency signal recorded by NOAA/PMEL since the SOSUS data became available for research in 1991. The source remains unresolved; even the seasonal pattern may reflect propagation rather than the source itself.
What is established
NOAA describes Upsweep as a long train of narrow-band rising tones, strong enough to be recorded across the Pacific. Its approximate source region lies near 54°S, 140°W, close to inferred volcanic seismicity. The overall source level has declined since 1991, but the signal has remained detectable.
What changed in this archive
The old version treated the spring/autumn amplitude peaks as evidence against a geological source. That was too strong. NOAA explicitly notes that the seasonality may come either from changes in the source or from seasonal changes in propagation. The archive now preserves that uncertainty instead of turning it into mystery.
Record parameters
| First recorded | August 1991 |
| Approx. source region | 54°S, 140°W |
| NOAA status | Origin unresolved |
| Playback | NOAA clip is speeded up ×20 |