During the Cold War, the United States built a fence from Greenland, through Iceland, to the United Kingdom — an underwater fence, that is, comprised of listening posts called hydrophones. The goal of this innovation: to detect Soviet submarines as they entered the North Atlantic — which the hydrophones successfully did beginning in the early 1960s. But as technology advanced in the half-century following, their military importance waned. In 1991, their data and listening tools were made available to civilian researchers.
Since then, researchers, particularly those at the National Oceanic and Atmospheric Administration (“NOAA”), have deployed hydrophones throughout the Earth’s oceans. And they started listening — for anything and everything.
Sound waves captured by hydrophones are visually mapped into something called a spectrogram, an example of a special one (explained below) is pictured above. As one researcher noted, the “sound waves are almost like voice prints. You’re able to look at the characteristics of the sound [as visualized as a spectrogram] and say, ‘There’s a blue whale, there’s a fin whale, there’s a boat, there’s a humpback whale and here comes an earthquake.'”
Except not all sound waves are easily identifiable — if identifiable at all.
On May 19, 1997, the hydrophones picked up a sound, dubbed “Slow Down,” which lasted seven minutes and produced the spectrogram pictured above. (You can listen to the sound, sped up to 16 times its original speed, here.) The exact location of the sound’s source is unknown — but it’s believed to be, roughly, in the South Pacific. What caused the sound? We don’t know. And we may never know, because, as NOAA states, the “signal has not been heard before or since.”
This makes “Slow Down” interesting, but hardly unique. There are about a half dozen others which, thus far, defy explanation. One of which, named “Bloop,” also occurred in 1997. But unlike “Slow Down,” “Bloop”‘s spectrograph resembles that of a sound made by an animal. One problem: “Bloop” was several times louder than any (other?) animal has made before.
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Related: A 99 cent mp3 of underwater sea sounds, recorded via hydrophone.