Some of the ocean’s largest predators are up to something, and marine scientists want to shed light on it — literally.

New research into great white sharks and swordfish has revealed that these massive fish regularly dive several kilometers below the surface. They reach pitch-black depths known as the twilight zone (200 to 1,000 meters) and the midnight zone (1,000 to 3,000 meters).

Why is that news, exactly? Because marine biologists had no idea that those species explore these hidden ocean habitats as part of their normal behavior. The discovery was explained in a November 6 study in the journal PNAS.

Researchers gathered data from 12 species of formidable ocean hunters, including sharks, billfish, and tunas. They first tagged 344 predatory fish, among them great white sharks, tiger sharks, whale sharks, yellowfin tuna, and swordfish.

Swordfish. Photo: Shutterstock

Sonar technology recorded the animals descending regularly to the twilight and midnight zones. Now scientists want to find out why they went so deep.

“How, when, where they access the deep ocean certainly varies, but the clear anecdotal answer is that the deep ocean seems like an important habitat — regardless of the predator species,” study lead Camrin Braun of the Woods Hole Oceanographic Institution told Live Science. “It’s clear there are good reasons for these animals to dive deep. Otherwise, why would they all do it?

An overlooked feeding ground?

The findings unveil a compelling correlation between the fishes’ deep dives and the presence of the deep scattering layer (DSL). This is a mysterious oceanic stratum that teems with small fish and various marine organisms.

This layer is so densely populated that it’s sometimes mistaken for the seafloor. The creatures residing in the DSL ascend to the surface during the night to feed before retreating back to the twilight zone during the day.

Whale sharks were observed diving nearly two kilometers under the ocean. Photo: Shutterstock

This new study suggests that marine predators target these denizens of the DSL as a primary food source. But that’s only a partial explanation for their behavior. Many of these predators venture even deeper than the DSL, reaching previously uncharted depths.

Great white sharks, for instance, were observed diving as deep as 1,128 meters. Meanwhile, whale sharks plummeted to 1,912 meters, and swordfish explored depths as far as 2,000 meters.

Many of the species that inhabit this zone are commercially fished, making it imperative to comprehend and safeguard this ecosystem, researchers said.

“We also find several cases where we can pretty definitively say the use of the deep ocean is not for feeding — or if it is, it represents a totally different kind of predator-prey interaction or mysterious prey resource,” said Braun.

It’s just another example that the deep ocean remains a mysterious and unexplored realm.