The “Upsweep” sound was first recorded by an autonomous array of hydrophones used to catch specific phenomena such as undersea earthquakes, ice cracking, marine mammal population migration, and detection of submarines (U.S. Navy SOund SUrveillance System (SOSUS) in August 1991. Although the source of “Upsweep” is approximately known to be at 54° S 140° W, near an active volcanic and seismic region, it still remains a mystery.
As revealed in the picture, the sound spectogram of “upsweep” presents narrow-band upsweeping sounds of several seconds duration each. The strange sound can be heard throughout the Pacific and its source is.
The “Upsweep” sound seems to be seasonal (spring and fall) but the reason why is not well understood: is it because of source changes or of seasonal changes.
I think this sound is one of the most scary unexplained sounds I have heard until now. Just listen to a sample of it here.