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.

Recent 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 who is leading The Ocean Twilight Zone project. “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.”