On a balmy Caribbean evening in August, crew members aboard the 184-foot exploration vessel the Alucia tied dead fish to the front of a small yellow submarine.

They tightly wound the fish to a metal pole extending out from the undersea craft to tempt whatever might be lurking, three thousand feet below.

But Dean Grubbs, one of the researchers preparing the bait, didn’t intend to catch anything. Grubbs, a shark scientist at Florida State University, only hoped to attract a little-seen creature that largely dwells in the lightless ocean depths: the sixgill shark.

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"They're one of the oldest lineages of living sharks. That, by itself, makes them cool," Grubbs, who with his long black hair and dark beard looks like he would fit right in at an Iron Maiden concert, said.

Unlike the charismatic sharks often spotted near the surface — hammerheads, great whites, and tiger sharks — the sixgill spends most of its life in the deep ocean, some 700 feet to 3,200 feet (200 to 1,000 meters) below the surface. It's not easy to understand the sixgills, though Grubbs has glimpsed the sharks' mysterious existence by tagging their dorsal fins with GPS devices.

OceanX lead scientist Vince Pieribone and sub pilot Lee Frey accompany Mashable's Mark Kaufman on an earlier dive. Image: Bubby Pavlo/OceanX Media

Far under the sea, the sixgill has carved out a niche as the biggest, dominant predator of the deep tropical and temperate latitudes — a huge swath of ocean.

It's mostly lightless down there, at least to humans. But the sixgills, and their creepy, vivid green eyes, are adapted to this black world.

"It's pitch dark to us, but to them, it's daylight," said Grubbs.

The three species of sixgill sharks are also ancient. At some 200 million years old the sixgills — so named for their sixth gill when most sharks have five — predate most dinosaurs.

Beyond their mystique, Grubbs has good reason to seek out these sharks.

For years he's been tracking where these ancient creatures go, why they go there, and the role they play in the deeps. But this requires catching the massive beasts, hauling them to the surface on a fishing line, and attaching a GPS tracker to their dorsal fin before releasing them back into the water.

It rattles them, said Grubbs.

So he's come aboard the exploration vessel Alucia, operated by the deep sea exploration organization OceanX, to try a new idea. He'll meet the sixgills where they live, thousands of feet beneath the surface. As the sharks swoop by to investigate the dead fish attached to the submersible, Grubbs will attempt to tag them with a GPS dart.

Credit: Cape Eleuthera Island School/OceanX Media

On that evening at sea in August, Grubbs climbed through the hatch atop the Alucia and sat down inside the craft’s big bubble, which is sandwiched between two yellow slabs holding cameras and propellers.

The bubble may initially appear vulnerable, but it's built out of seven-inch thick plexiglass, designed to withstand the unrelenting weight of water pressing down on the craft, and the three occupants inside.

Using a great hook dangling from the Alucia's crane, submarine crew members raised the submersible into the air before gently plopping it into the Caribbean waters off of Eleuthera, a long, thin island in the eastern Bahamas.

A wild-haired diver leapt off the Alucia's nearby dinghy to unhitch the bobbing submersible from the crane, and then Grubbs, along with another scientist and submarine pilot, began to sink beneath the surface, and soon disappeared.

Credit: OceanX Media

The submersible dropped down to the ocean floor like a space capsule parachuting down to Earth in slow motion.

All is still in this forever wilderness, save the robotic sounds of the submersible. At first, an omnipresent blue glow pervades everything, dying human skin an alien, indigo color. Then, the light dims to dusk as the craft continues its descent. Eventually, there’s little to no light. Here, the sixgills dwell.

Down in the dark, one realizes why the sixgills evolved eons ago, but remain unchanged. They had no need to evolve.

“They’ve been living in a pretty constant environment for a very, very long time,” Chip Cotton, a marine scientist who also researches sixgills, but took no part in this expedition, said in an interview.

On the surface, volcanoes rumble, continents collide, ice ages pass, and warfare ensues. But the sixgill shark, who holds dominion over this distant black realm, doesn’t flinch.

A bluntnose sixgill's serrated teeth and vivid green eye. There are three known species of sixgills. Image: Dean Grubbs

The shark has spent millions years passing lethargically through the deep sea, said Cotton. And for good reason.

“If you think about energy expenditures, food is kind of a luxury down there,” explained Cotton, saying that the creatures don’t needlessly waste energy by zipping around the sea floor.

The sixgills are masters of eating the dead. Their teeth, which have remained mostly unaltered for some 200 million years, are uniquely designed for twisting and tearing off big chunks of fallen whales, or large dead fishes.

“It’s a good way to make a living,” said Cotton.

Down in the dark, Grubbs waited patiently for the sixgills to arrive at the submersible.

The night before, a curious sixgill swam right in front of him, just beyond the glass bubble. But he couldn’t get off a safe shot to tag the shark on its cartilaginous fins. The shark only exposed its underbelly, an area Grubbs didn’t want to risk harming.

Still, when Grubbs returned to the surface, he considered the mission a success. It almost worked.

Now, down in the depths again for five hours, Grubbs hoped other sixgills would be tempted by the easy meal perched directly in front of the submersible, and within sight of the dart guns.

Credit: Cape Eleuthera Island School/OceanX Media

But on this night, no sixgills came to visit the bait.

Grubbs mused they needed to bring a larger hunk of meat, perhaps a pig.

Yet, the mission wasn’t a failure. It’s precisely the type of experiment that interests OceanX, which in 2012 captured the first and only footage of the legendary giant squid wrapping its tentacles around part of the very same submersible Grubbs sat in.

“We’re into trying unprecedented things out there,” Vincent Pieribone, a Yale neuroscientist who oversees OceanX’s science operations, said in an interview. “What's interesting to us is the untested, high-risk, high-reward type stuff.”

Grubbs hopes to return to realm of the sixgills again, and give the mission another shot.

Protecting the sixgills

Sharks that live in deep waters are generally vulnerable to overfishing. They get caught in nets like other fish, and hauled to the surface.

But not the sixgill. These large sharks have been mostly safe in their dark realms. Here, they're numerous, but hard to find.

"We don’t go to their house often looking for them, so they’re perceived as rare," said Cotton.

They're generally too big to catch, and too strong for hooks and lines, said Grubbs.

But every once in a while, someone accidentally snags a sixgill, and they take the sharks' valuable livers.

Credit: Edie Widder and Dean Grubbs

"They throw the rest overboard," said Grubbs. "The rationalization was the sharks were going to die anyways," due to the trauma of being caught and taken from their usual waters.

But Grubbs wondered, is that true?

In 2005, he decided to do something that had never been done to find an answer. He wanted to catch the elusive sixgill sharks, to see if they could survive the trauma after being forced out of the water.

Grubbs was told that it simply could not be done. Capturing a large deep sea shark is a daunting task. Doing it many times is beyond reason.

"We took that as a challenge," said Grubbs.

Grubbs set out at sea, and has since caught 23 sixgills in an ongoing project that continues today.

After releasing them into the water with GPS tags, he found 90 percent of them survived, and continued roaming the depths.

It seemed sixgills needn't be slaughtered just because they were hauled to the surface.

"Lo and behold, that assumption was totally wrong," said Cotton.

Dean Grubbs in Hawaiian waters with a sixgill shark. Image: dean grubbs

In their dark ocean homes, the sixgills might be king, but it's not as if other predators aren't lurking in these waters.

Tiger sharks, large dominant predators near the coast, sometimes venture into the sixgills' realm. It's likely they chew up smaller sixgills said Cotton.

"I would be surprised if they didn't," he said, emphasizing that it might be the sixgills' territory, but there are no walls keeping other predators out.

"None of these things exist in a vacuum," said Cotton. "Everything is interconnected in some way."

And in the a cold, lightless world where food is scarce and one eats what is available, both Grubbs and Cotton said the sixgills also hunt each other.

Even the monstrous 17-foot long sixgill Grubbs once hauled aboard a research vessel needs to look over its shoulder.

"There’s always a bigger predator," he said.