Plants and Animals

Mysterious Deep Hum Found Within The Ocean's "Twilight Zone”

February 24, 2016 | by Robin Andrews

Photo credit: It's almost impossible to hear with human ears. Rich Carey/Shutterstock

A group of researchers has recently stumbled across an unusual buzz, emerging only at dusk and dawn, within the dark reaches of the ocean. As they will suggest when their research is presented at the annual Ocean Science Meeting in New Orleans this week, this noise could be the “dinner bell” for a huge collection of creatures moving up and down the water column in order to feed en masse.

Using highly sensitive listening equipment, the team picked up an unidentified, low-frequency hum at a sea depth of around 200 to 1,000 meters (660 to 3,300 feet). No single type of marine creature could be matched to the noise, which was a mere three to six decibels above the background noise of the ocean.

You can listen to it below via NPR.

“It’s not that loud, it sounds like a buzzing or humming, and that goes on for an hour to two hours, depending on the day,” said Simone Baumann-Pickering, an assistant researcher at UC San Diego and co-author of the study, in a statement.

This sound was coming from the mesopelagic zone, a dark world with limited food sources for many larger types of marine animal. Unlike surface waters, the light levels are insufficient for photosynthesis, so the general types of lifeforms found here are either herbivores or scavengers feeding on the remnants of organisms or descending fecal matter.

What mysterious creature, or creatures, could be making this low-frequency noise? superjoseph/Shutterstock

It’s not just a mostly dead region of the sea, though: A 2015 study estimated that up to 90 percent of the world’s fish may reside in this aquatic twilight zone, including many species that are as-of-yet unidentified. Is this novel, mysterious hum generated by an unknown species? According to the researchers, the sound may actually be the result of a huge number of creatures acting in unison.

It’s already known that large crowds of fish, shrimp, and squid move upwards towards the surface waters to feed on surface plankton and nutrients. They do this as the sun sets, using the darkness to mask their approach. When the sun rises, these small critters retreat back down into the darkness, hiding overnight for protection from sizable predators.

The researchers are convinced that this ascent and descent is responsible for generating the unusual noise – although at present, they can’t be sure which specific animal, or group of animals, is actually responsible. There’s a chance that this noise could signal when it is safe for a colony of fish to rise up and feed. If these creatures are indeed emitting this sound as a form of communication, it will completely change our understanding of how various, deep-sea ecosystems fit together.

This enigmatic sound is the latest in a series of mysterious ocean noises. The Bloop, an ultra-low frequency sound heard on hydrophones (submerged microphones), was briefly thought to be the call of an unknown sea creature. Although nowadays it’s thought to have been made by the cracking of an ice shelf, the oceans are still clearly providing plenty of curious noises for scientists to mull over. The Upsweep in the Pacific, for example, still remains unexplained.

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