By Mark Kaufman

As midnight neared, we bobbed around in the black Caribbean Sea aboard a rubber dinghy. There were five of us out there, peering down into the undulating, forever darkness. We scoured the water for signs of a telltale light, coming from below.

A yellow submarine—the same one that seven years previous captured the first deep sea footage of a giant squid—was expected to return to the surface after spending five hours in the ocean depths off of Eluethera, a snake-shaped island in The Bahamas.

“There!” yelled a crewmember, pointing 50 feet off the dinghy. And there the water began to glow, an emerald radiance amid the black sea. The shine grew brighter and brighter until the submersible’s bubble-like capsule, holding three humans, popped out of the water. On cue, a crewmember balanced on the edge of the dinghy, lunged into the water and swam over to the exploration craft, preparing to hook it to a looming 184-foot vessel called the Alucia, which would soon hoist the yellow submarine from the sea, and end the night’s mission.

Earth’s deep sea is famously uncharted, and on this August night the exploration group OceanX sought out sixgill sharks—the dominant shark of the dark fathoms—at depths of 3,000 feet. The surface of the moon is better mappedthan these lightless realms, a place where sealife awaits dead, sinking creatures for their meals.