We know so little about the deep, dark oceans that we could consider them an alien world altogether. Each expedition to the bottom of the oceans discovers new and strange species, each with unique adaptations to survive the abyss.
But while the bottom of the ocean lacks light, it is by no means silent. In fact, there is quite a number of strange and unexplained sounds that have been puzzling scientists ever since they were picked up. In the 1960’s, the US Navy established SOSUS (Sound Surveillance System), a network of underwater microphones that kept an ‘ear’ on Soviet submarine activity. At the end of the Cold War, this vast network spanning thousands of miles was repurposed for scientific study and it has been picking up strange, even scary sounds.
While many of the sounds recorded by the hydrophones were identified as underwater volcanic eruptions ore icebergs crashing into each other, there are a number of sounds that have managed to remain unexplained.
One of the strangest ones is called the Upsweep. It was first detected in 1991 and was heard throughout the Pacific. Its sound signature was unlike anything else heard in the ocean. It doesn’t sound like volcanoes or earthquakes and although some have tried to attribute it to whales, it is far too loud to have been made by them. The sound consists of a long sequence of narrow band upsweeping sounds lasting a few seconds each. Its loudness made it audible in multiple places in the Pacific although its exact source cannot be pinpointed. It appears to follow a seasonal pattern and is at its loudest in spring and autumn. Perhaps it’s the mating call of some gigantic underwater beast. For now, nobody knows.
Another strange sound is the Bloop, an extremely powerful ultra-low-frequency underwater sound. Detected in 1997 by hydrophones located roughly 3,000 miles (5,000 kilometers) apart, this sound’s profile is consistent with that of a living creature due t its rapid variation in frequency. It is, however, several times louder than the loudest animal on Earth, the blue whale. The more rational explanation is that it was cause by an icequake taking place off the southern coast of South America. But this doesn’t necessarily mean there aren’t any undiscovered enormous monsters lurking in the deep.
The Whistle was also recorded in 1997 but its origins are completely unknown and no other hydrophone has since detected it. It was picked up in the mid-water currents and some researchers believe it was caused by an underwater volcano. Nevertheless, it remains unidentified.
Another enigmatic sound is Slowdown, first detected in the Pacific on May 19, 1997. This seasonal sound has been heard several times a year ever since by hydrophones located thousands of miles apart. It got its name from its unusual behavior: it gradually slows down over a period of 7 minutes.
Train was detected in 1997 by the Equatorial Pacific Ocean autonomous hydrophone array. Scientists believe it was caused by a large iceberg grinding against the seafloor and it sounds like distant, muffled train.
A similar sound called Julia was detected on March 1, 1999 and it is believed to have been also caused by an iceberg.
It seems the further we study our oceans, the weirder they get.