The Bloop is the name given to an ultra-low frequency underwater sound detected by the U.S. National Oceanic and Atmospheric Administration several times during the summer of 1997. The source of the sound remains unknown. According to the NOAA description, it “rises rapidly in frequency over about one minute and was of sufficient amplitude to be heard on multiple sensors, at a range of over 5,000 km.” According to scientists who have studied the phenomenon it matches the audio profile of a living creature but there is no known animal that could have produced the sound. If the sound did come from an animal, it would reportedly have to be several times the size of the largest known animal on Earth.
You can listen to the bloop here.