As the team lowered Nadir, a bulbous three-seat submarine, into the waters off of the Bahamian island of Eleuthera on June 29th, the rain that had pummeled them that afternoon began to wane. They were eager to interpret the improving weather as a good omen; a lot was riding on this dive.

For the past year, a group of researchers from Florida State University, the Florida Museum of Natural History, the Cape Eleuthera Institute, and OceanX, a marine exploration initiative established by hedge fund billionaire Ray Dalio, had been trying to tag a shark with a satellite tracker from a submarine for the first time in history.

Their target was the bluntnose sixgill, a massive creature with glittering emerald eyes and six gills to most sharks’ five. Believed to be descendants of sharks that inhabited Earth’s waters even before pterodactyls and T. rexes roamed its lands, bluntnose sixgills spend most of their time at profound depths, sometimes swimming to 4,500 feet below the ocean’s surface.

This predilection for deep water makes bluntnose sixgills a difficult bunch to study. When asked what remains mysterious about them, Simon Thorrold, a senior scientist at the Woods Hole Oceanographic Institution laughs, explaining: “It will be way quicker to go over what we do know. And that is almost nothing.”

The International Union for Conservation of Nature’s Red List of Threatened Species classifies bluntnose sixgills as “near threatened,” but there is not enough data to estimate their population or discern whether it is increasing or dwindling. Females tend to be larger than males—measuring up to 18 feet at maturity—and give birth to 40 to 110 pups at a time, but we don’t know where they breed or how long they gestate. While bluntnose sixgills appear to be rather cosmopolitan, having been spotted near the coasts of every continent but Antarctica, it is unclear whether they’re highly migratory or merely spread out.

Subs and spearguns

In 2005, Dean Grubbs, now the Associate Director of Research at Florida State University’s Coastal and Marine Laboratory, was the first scientist to affix a bluntnose sixgill with a satellite tag, which gathers data about light, temperature, and depth. He used a mix of bait fish to hook the shark and a long fishing line to pull it to the surface, where he placed the tag below the shark’s dorsal fin.

He went on to tag more than twenty bluntnose sixgills this way in five different regions of the world. But eventually he noticed that, though the sharks survived the experience of being hauled to the ocean’s surface, they didn’t seem to resume normal behavior for a few days. Grubbs began wondering about something audacious: Why not try tagging one from a sub, in their natural deep-water environment?

There were lots of reasons why not. According to the Marine Technology Society, as of 2017, there are only 14 crewed underwater submersible vehicles dedicated to science in the world. Conducting research on one of those submarines would be orders of magnitude more expensive than tagging sharks from a boat. While OceanX declined to comment on its costs, Woods Hole Oceanographic Institution, which wasn’t involved in the bluntnose sixgill tagging project, reports that use of their 55-year-old submarine, Alvin, costs $45,000 a day. Plus, the endeavor would be tricky: With the shark unrestrained, it would be much harder to correctly position the tag.

Eventually, Grubbs, Edd Brooks, and Brendan Talwar—both of whom are researchers at the Cape Eleuthera Institute—convinced OceanX to back the mission, which also received funding from the Moore Bahamas Foundation and Bloomberg Philanthropies’ Vibrant Oceans Initiative. They would mount two spearguns on Nadir—giving them two shots at tagging a shark per night—which they would use to target bluntnose sixgills in the Exuma Sound off of Eleuthera.

But despite outside support and resources, things did not immediately look promising. After practicing in a pond with a shark carcass, the team’s first trip was scuttled when a giant female sixgill turned her belly to the submarine at the exact moment when they prepared to shoot a tag at her. They didn’t want to damage her organs and abstained from firing. A speargun malfunction ruined their second trip a few months later. Finally, in June, they embarked on their third—and final— trip, which would last a total of four nights.

Again, things began inauspiciously. On the first night, a sixgill knocked into the speargun, causing it to misfire. The next night, no sharks appeared. On the third night, a pesky grouper swam by at exactly the moment they had fired at a sixgill, intercepting the precious tag.

Last chance

June 29th represented the team’s last chance to tag a bluntnose sixgill before Nadir and the research boat that housed it would move on to another project. As the researchers inside the submarine waited for sixgills to come investigate the bait dangling outside, they marveled at phosphorescent Siphonophores, Cuban dogfish, and larval fishes usually only seen in textbooks. (Related: See nine photos of deep-sea creatures.)

“I was happy as a kid in a candy store just seeing these invertebrate, these little shrimp” recalls Gavin Naylor, who directs the Florida Program for Shark Research at the Florida Museum of Natural History and was tasked with manning the spearguns that night.

Around 9:50 p.m., which the scientists had taken to calling “sixgill-o’clock” after they noticed shark activity consistently increase then, a male sixgill swam past the sub, sending silt from the ocean floor swirling. Though he couldn’t see well, Naylor felt confident he could predict where the shark had gone. He asked Lee Frey, the sub’s pilot, to arm the speargun and fired into the sediment.

A tense 20 seconds passed before Grubbs and the other scientists sitting in the research boat’s mission control area heard Frey's voice come in over the radio.

Deep-sea mysteries

The tag that Naylor applied to the sixgill has a three-month lifespan. In late September, if all goes to plan, the tag will detach from the shark, float to the surface and upload data about how deep the shark swam, how much light there was, and what water temperatures the creature spent time in. Grubbs and his team will use that information to recreate the shark’s movements.

Shawn Larson, the curator of research at the Seattle Aquarium who has long studied bluntnose sixgills in the Puget Sound, is excited to see what new evidence the tag holds. “These sharks are really a mystery. I’m hoping that the tag will shed light on the shark’s home range and maybe, since it’s a male, the location of breeding grounds.”