National Geographic Explorer David Gruber was studying coral while diving at dusk off an uninhabited volcanic island in the South Pacific in 2013 when fellow explorer Brennan Phillips swam up to him excitedly. Turn your dive lights off and follow me, Phillips signaled.

Gruber swam after his colleague through the dark waters around Mborokua Island, which were known to be home to dangerous saltwater crocodiles. When they reached a cave-like cove along the jungle-covered island, they were stunned.

“It was like a scene out of Avatar,” says Gruber, a biologist at City University of New York’s Baruch College and the American Museum of Natural History.

Hundreds, if not thousands, of small flashlight fish—glowing blue—began emerging from the underwater cave. They