The deep sea is a forbidding place, inhabited by strange, monstrous creatures that haunt its pitch-black waters. Now researchers have discovered an eerie new attribute of this little-known region: a subtle low humming sound that emanates from its depths every day around dawn and dusk.

“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, co-author of the study, in a statement.

The source of the hum remains a mystery, to say the least. Researchers suspect that it may be coming from an organism, or perhaps many organisms chanting in unison, but no known marine creature could be matched to the noise. It might be coming from a species yet to be identified, or it might be evidence of a new capability of an already-known creature. Then again, it might be coming from a non-living source too.

There's one clue, however. The sound comes from the ocean's mesopelagic zone, a region between 660 to 3,300 feet below the surface that's too dark for photosynthesis to occur. Since food is scarce there, many of the bizarre organisms that call this region home must migrate up and down the water column en masse on a daily basis to feed. These migrations typically happen at dawn and dusk, which coincides with the weird humming sound.

Researchers have theorized that the hum might be serving as some sort of "dinner bell" for the scores of marine creatures, a signal that tells them when to rise up or down in depth depending on the time of day. Or perhaps the sound is just the wholesale noise of the migration itself, the cacophony of billions of creatures moving through the depths simultaneously.

The daily migration of organisms that inhabit the mesopelagic zone is no small matter. The region is home to an unfathomable — and largely unstudied — number of sea creatures, which are estimated to weigh around 10 billion tons all combined. The planet's carbon cycle is likely tied in many fundamental ways to this global daily migration.

That we're just now detecting this omnipresent ocean hum is proof that there's much for us to discover about this little-known but vitally important region.

Though it's difficult to pick out from the background noise, you can listen to the hum yourself here, thanks to NPR: