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.
An icy resolution
The mystery of the Bloop persisted for years, leading to speculation about its origins. The location and biological characteristics of the sound led to many theories, but the scientific explanation proved to be terrestrial, not biological. By 2012, NOAA concluded the Bloop was the sound of a cryoseism, or "icequake". This conclusion came after researchers deployed hydrophones closer to Antarctica and studied the sounds of the continent's massive glaciers.
They found that the acoustic signature of large icebergs cracking and breaking away from glaciers was a near-perfect match for the Bloop's spectrogram. Events like the disintegration of iceberg A53a near South Georgia Island in 2008 produced sounds with the same characteristics and amplitude as the 1997 event. The immense energy released when a massive iceberg scrapes the ocean floor or calves from a glacier generates low-frequency sound waves powerful enough to travel across an entire ocean basin. The likely origin of the Bloop was determined to be ice-related activity somewhere between the Bransfield Strait and the Ross Sea off the coast of Antarctica.