At night. “There’s a whole lot of nothing,” he said. “There’s no bottom, no walls, just this space that goes to infinity. And one thing you realise is there are a lot of sea monsters there, but they’re tiny.”

Of course, there are big monsters, too, like sharks. But the creatures Milisen is referring to are part of a daily movement of larval fish and invertebrates, which rise from the depths to the surface each evening as part of one of the largest migrations of organisms on the planet. The emerging hobby of taking pictures of them is known as blackwater photography.

Most of the larvae are no bigger than a fingernail; others are even smaller. And they can easily be mistaken for bits of seaweed or drifting detritus. But up close, when captured with a camera using a special lens called a macro, the animals can appear to loom as large as wild animals on a safari — a safari on another planet.

Five years ago, Milisen began sharing his photos in a Facebook group, and there he discovered a community of passionate night-time adventurers who were capturing images of living things rarely seen before. Perplexed and astonished by what they were photographing, Milisen and others in the community, called the Blackwater Photo Group, began contacting fish scientists, asking for help in identifying what they were seeing.

Even the most seasoned specialists responded with incredulity. “The No. 1 thing people, even scientists, ask is: ‘What the hell is that?’” said Ned DeLoach, an experienced underwater photographer, who, with his wife, Anna, and the writer Paul Humann, has published eight books on marine fishes. “Why these images are so spectacular and so popular is they’re so otherworldly. People have never imagined that creatures like this exist, and that has attracted photographers.”

David G. Johnson, curator of fishes at the Smithsonian National Museum of Natural History, was one of the first scientists to be contacted by members of the Facebook group. He said he was immediately smitten by the images. As the blackwater hobby has taken off, gaining adherents around the globe, more and more photographers have captured stunning images and videos that reveal a secret world of bizarre, tiny animals that scientists have struggled for decades to better understand. Many of the images have gone viral on social media, and some recently won major underwater photography awards.

Now, scientists like Dr. Johnson want to formalise the collaboration with blackwater photographers.

So far, the scientists leading the effort have recruited about a dozen divers, who have collected more than 60 specimens for analysis. More are in the pipeline. “We’re building a collection that for the first time has a live image,” Dr. Johnson said. “We get the specimen and create a DNA record tied to it.” He expects scientists with a knack for underwater photography to join the effort as well. Marine researchers hope that examining images of animals photographed in their natural surroundings and pairing those images with data drawn from techniques such as dissection and DNA bar-coding will significantly expand the knowledge of how these animals change over time and why they behave as they do.