Noise Addict blog has a neat post about some of the strangest and most mysterious sounds from the Earth (some with audio clips). Like this one, nicknamed "The Bloop," for instance:
At various times during the summer of 1997, an ultra-low frequency sound that rose rapidly in frequency over about one minute was detected at 50 degrees S, 100 degrees W. The sound was detected by the US National Oceanic and Atmospheric Administration with the Equatorial Pacific Ocean autonomous hydrophone array (which was U.S. Navy equipment originally designed to detect Soviet submarines), and was loud enough to be heard on multiple sensors, up to 5000km apart. Scientists dubbed it the "Bloop" (not to be confused with the "Boing ".)
Although the sound matches the profile of a living animal, it is much louder than any known creature can produce. Any creature that could produce such a sound would have to be many times larger than the largest whale.
You can hear a very short recording of the sound here . The recording is short because it’s been sped up 16x to make it audible to you and I.
Some people link the Bloop to Cthulhu, a mythical creature from an H.P. Lovecraft story as the noise originated from an area near the mythical sunken city of R’lyeh from the same story.