Upsweep (1991-present)

Upsweep is one of the ocean's most enduring acoustic mysteries, detected since 1991 by hydrophone arrays across the Pacific. This unidentified sound consists of long, narrowband upsweeping tones lasting several seconds each, occurring seasonally with higher intensity in spring and fall. Ranging from diffuse to sharply defined, the sounds appear related to volcanic activity yet defy exact explanation. Scientists speculate Upsweep may originate from seabed volcanic processes or interactions between magma and seawater, but its precise source remains unknown. This persistent phenomenon continues to intrigue oceanographers as one of Earth's most enigmatic natural sound recordings.