Between the ocean’s bright blue surface and its blackest depths — 660 to 3,300 feet below — is a mysterious, dark span of water. Welcome to the twilight zone.

Evidence suggests there are more animals here by weight than in all of the world’s fisheries combined. But who lives here, and in what quantities?

Since August, a group of scientists has been using new technology to better understand the twilight zone’s strange inhabitants. They hope their findings will lead to a more sustainable approach before the fishing industry tries to harvest some of its abundant life as fisheries closer to the surface are diminished.

“The time is right to get this knowledge before it’s too late,” said Heidi Sosik, a biologist at the Woods Hole Oceanographic Institution. “This twilight zone region of the ocean is really, very barely explored, but the more we learn, the more interesting and more important it seems to be in playing a role in the whole ecosystem.”

Each animal in the ocean has its own auditory signature that ships usually detect by sending out sound waves that bounce or scatter off their bodies. But the acoustic fingerprints of twilight zone animals are still mysterious because shipboard sonar don’t have the bandwidth to distinguish the many organisms living far below the surface.

Around 250 different species of myctophids, or lantern fish, make up much of this layer. Though abundant enough to trick sonar, individually they are no bigger than an index finger.

Scientists hope to match frequencies detected with new technology with pictures and DNA to create an auditory dictionary of inhabitants.

Sosik said, “Most of the animals that live in the twilight zone are really small.”

This size adaptation is vital where food is scarce. One fang-toothed monster, the Sloane’s viperfish (Chauliodus sloani), could fit inside your hand. But it also has one of the biggest teeth-to-body-size ratios in the animal kingdom. “This guy would be considered one of the big, bad predators down there,” said Paul Caiger, a biologist at Woods Hole.

When the Navy started using active acoustics to monitor the sea in the 1930s, it noticed lots of sound bouncing off a layer they thought was the bottom. But its depth changed from night to day. This “false bottom” turns out to be a mass of animals that journey hundreds to thousands of feet from the depths to the surface nightly in a living wave that wraps around the planet.

During the day, surface dwellers like sharks, tunas and swordfish dive to the depths to eat, evidence shows. “The organisms that live in one layer are influenced by the organisms that live in another,” Sosik said.

Scientists working to untangle this multilayered food chain think it may play a major role in regulating climate by keeping carbon in the ocean. “If you imagined erasing the life in the twilight zone, it wouldn’t affect just that layer,” Sosik said. “It would affect the ocean and the whole planet.”