A sound heard across an ocean
In the summer of 1997, a network of underwater microphones recorded an astonishingly powerful, ultra-low-frequency sound. The U.S. National Oceanic and Atmospheric Administration (NOAA) had placed the hydrophone array across the Pacific to monitor undersea volcanoes and earthquakes. This particular sound was different. It rose in frequency for about a minute and was so loud it was detected by multiple sensors over a staggering 5,000 kilometers (3,100 miles).
Scientists at NOAA nicknamed the sound "Bloop". Its audio profile was because it resembled vocalizations made by marine animals. NOAA's chief scientist for the project, Dr. Christopher Fox, initially noted the similarity to a biological source. There was a significant problem with this hypothesis: the sheer volume. The sound was several times louder than the call of a blue whale, the loudest animal known on Earth. For a living creature to produce the Bloop, it would need to be immense, far larger than any animal ever documented. The source was roughly triangulated to a remote point in the South Pacific, west of South America's southern tip, at approximately 50°S, 100°W.