Upsweep is an unexplained sound localized somewhere in the Pacific Ocean. First heard in 1991, it can periodically be detected throughout the Pacific Ocean by the National Oceanic and Atmospheric Administration's autonomous hydrophone array.  Upsweep is particularly interesting because it was never detected before 1991, even though the U.S. Navy most likely would have picked it up in decades of searching. 
Upsweep is sometimes attributed to volcanic activity, although this cannot be proven. It does not appear consistent with most volcanic activity. 
Another possible explanation is that Upsweep is really the sound produced by marine life, but the signal is particularly loud and can be picked up easily across the Pacific Ocean.
One possibility is that Upsweep is created by some sort of cryptid, perhaps a massive undersea creature.