Mysterious Upsweep Sound

First detected in 1991 by NOAA's underwater microphones, the Upsweep is an ongoing, unexplained sound phenomenon that has echoed through the Pacific Ocean for over three decades. Characterized by long, narrow-band frequency bursts that spike in loudness for several seconds, the sound is powerful enough to be picked up across vast distances. Unlike one-time anomalies like the now-explained "Bloop," Upsweep recurs in a seasonal pattern, particularly in spring and autumn, and remains active to this day-though it appears to be gradually weakening.

Despite extensive monitoring, scientists still don't know the source of the Upsweep. Theories range from undersea volcanic activity to the movements of glaciers or even unknown marine life. But none of these explanations account for the seasonal timing or the sheer duration of the phenomenon. The idea of a volcano erupting consistently for over 30 years, and only during certain seasons, defies geological norms. With no clear answers yet, the Upsweep remains one of the ocean's most persistent and perplexing acoustic mysteries.

Previous Fact Next Fact