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