During the Cold War, the U.S Navy set up a system called SOSUS (the Sound Surveillance System). It's an array of underwater microphones positioned all around the world, which they used to track Soviet submarines by the sound of their engines. The microphones sit several hundred yards below the surface of the water, at a depth where sound waves become trapped in a layer of water known as the "deep sound channel." At this depth, a combination of temperature and pressure cause sound waves to keep traveling without being scattered by the ocean surface or seafloor. Nowadays, NOAA has expanded the concept, installing additional hydrophones in 1996, and using the array to record underwater sounds for scientific study in the fields of biology, seismology, etc.

Through the years, several unexplained and unidentified sounds have been recorded, some repeatedly. Today, I've been reading about them and listening to the sound clips provided by NOAA. It's kind of spooky.

One significant sound is known as 'Bloop' (Wikipeda page). It's relatively recent, recorded in 1997, just a year after NOAA installed the additional hydrophones. They don't know what made the sound, but they've determined based on its properties that it most likely came from some biological source — some sea creature. The odd thing about it, though, is that it was incredibly loud — the signal was picked up by two stations 5000 km apart. Scientists say that a creature would need to be much larger than a Blue Whale to create a sound that loud at those frequencies. Some mysterious sea monster? We know so little about what's going on in the deeper oceans that it wouldn't be surprising at all to discover that some large creature has gone thus far undetected.

Another sound that's pretty spooky is known as Slow Down (Wikipedia page), because it does just that — the frequency steadily drops down from about 30 Hz to zero in about 7 minutes. This one was also very loud, being picked up by three separate stations, thousands of miles apart. They've traced its origin to an area out in the middle of the Pacific ocean. It's an interesting feeling to listen to that recording and wonder what monstrous beast or machine could have created it.

I think I need to watch The Abyss again.