First detected in 1991 by NOAA’s Pacific Marine Environmental Laboratory, “Upsweep” is a repeating underwater sound consisting of long, narrow-band upsweeping tones. The source has never been identified, but it appears strongest in the central Pacific Ocean, near volcanic and tectonic activity along the mid-ocean ridge. The signal peaks in spring and autumn, suggesting a natural but unexplained link to seasonal changes. While many scientists suspect undersea volcanic activity, paranormal enthusiasts frame Upsweep as evidence of massive, unknown life forms calling across the ocean depths.