On May 19, 1997, the U.S. National Oceanic and Atmospheric Administration detected an unprecedented sound in the eastern Pacific Ocean. Deep and vast, it descended in frequency over about 7 minutes. Here’s a sound file, sped up 16 times.
A few months later, and about 2,500 miles closer to Cape Horn, a Navy hydrophone picked up a mysterious “bloop.” This one matched the audio profile of a living creature; if so, it must have been gigantic, as the sound was audible more than 5,000 km away. Here’s a sound file of that one, similarly sped up.
No one knows whether the two are related; if something huge was headed for Chile, it never arrived. Neither sound has been heard since.