Just below the sunlit surface of the sea lies a middle ground between light and shadow—the ocean’s “twilight zone.” Animals are abundant in this mesopelagic zone, which sits 200 to 1000 meters below the surface. But this dark region has no plant life, forcing many of its denizens to make a daily pilgrimage to the surface waters to forage for food. Now, researchers have shown that this mass migration makes a very distinctive sound. The low-frequency hum is just 3 to 6 decibels louder than background noise—too faint for the human ear to detect, but loud enough to be picked up by sensitive acoustic equipment, researchers will report Monday at the American Geophysical Union’s Ocean Sciences Meeting in New Orleans, Louisiana. To capture the noise, the team spent several nights recording underwater sounds in the San Diego Trough. They found that the animals let off their hum twice each day: once as they head up to feed at dusk and again as they return to the depths at dawn. The sound may act as a kind of “dinner bell” for predators, letting them know their prey is nearby. Which creatures are responsible for the hum isn’t yet clear, but the layer’s many small bony fish are a likely culprit, the scientists say. They add that using acoustics to monitor the tiny creatures could offer a new window into their ecosystem and help illuminate the consequences of climate change, commercial fishing, and other human activity on their dark world.