It was just another day for the Aquanauts exploring the warm, crystal waters of Conch Reef when a storm hit. The divers registered the 10-degree temperature drop on their skin a moment after the visibility went from 60 feet down to a dozen.

But everything was calm topside—this was an underwater storm. The bad weather was driven by a wave pushed up from the depths.

This phenomenon is called an internal wave and it was here, at Aquarius, a few years ago, that Aquanaut Dale Stokes and Jim Leichter discovered that they were critical for reef life. The waves reach 20-40 meters tall without breaking the surface. And they are filled with important nutrients vital to the sea life. Their three-dimensional movement was understood using a mesh network of hundreds of little thermal loggers. When waves like this passed over the sensors, they lit up the electronics like rolling traffic lights. The resulting data visualization discovered that they moved just like a breaking wave on a beach. Only bigger.

This discovery would have been near impossible without Aquarius.

Installing this number of sensors requires the kind of dexterity robots can't manage—even with today's state-of-the-art in manipulator arms and cameras—and the kind of bottom time available only to saturation divers who can stay in the deep for hours a day.

The Extreme Difficulty of Leaving an Underwater BaseS

This is the kind of science the Aquanauts have been doing over this past week. Mark Patterson installed a device on the reef that costs about $50,000 and measures pH and oxygen in coral to see how they react to the stress of temperature changes. The sensors' electrodes are only as thick as a human hair and take two hours to position correctly. A day later, Patterson noted that a school of tiny gobies had taken up residence in the device's many flexible arms. "Stay away from the electrode please, the tip is not a tasty morsel of food." he muttered to himself, worried one would bite off the machine's business end. If the sensor had been compromised, he would have had to go back out and fix it. Again, a task that only a saturation diver could accomplish.

Large-scale planned experiments like these are part of what makes living underwater so necessary, but it's the spontaneous moments that provide the best argument for keeping underwater exploration a manned endeavor.

I asked Sylvia the most impressive thing she saw on one of her last dives, which was 254 minutes long. "Ten giant permit fish swept in from the reef and began careening around the pillars supporting Aquarius, sometimes all together in a fast-moving circle, sometimes splitting into two groups of 4 and 6, then swirling back together again, a blurring silver carousel of fins, eyes and sleek bodies." While this didn't solve any checklisted mysteries about the fish and their behavior, nobody can predict its importance going forward. These observations are cataloged and shared, and might lead to more research in the future. The chance to witness such displays only increases with time below. It's like creating the equivalent of a land-based nature photographer's blind.

To survive on the reef for many hours, the aquanauts use tethered air systems or scuba tanks that they can refill at various recharge gazebos a few hundred feet from the base. These gazebos allow the aquanauts to stand up in the little polygonic tee-pees filled with air, radio back to base, and eat lunch, too, which is important. Just moving around at a normal rate in the body heat-draining ocean can cause an aquanaut to burn 8,000 calories a day. This job is exhausting—it's common for crew to spontaneously pass out in the habitat, like some deepwater narcolepsy—which is one of the reasons why they only stay down for about a week.

And coming back up is not as simple as just swimming to the surface.

The Extreme Difficulty of Leaving an Underwater BaseS

Decompression is the process by which the soaked up nitrogen in the tissues of a human living under high pressure is slowly released. Depressurize too quickly and the body will bubble like a shaken soda when it is opened suddenly. The bends. It is supposedly excruciatingly painful. The safe thing for an Aquanaut to do is to get rid of that gas through breathing it out over a number of hours. Significant bubbling can cripple or kill someone and the only cure is to re-pressurize and decompress properly.

But before decompression, there is prep.

Two days before, some veterans start eating a liquid diet so they don't have to use the habitat's toilet for anything more than urination. Defecation during decompression is frowned upon because the air does not circulate well there. And when you're decompressing, you can't leave Aquarius.

Just before decompression, everyone has to get a checkup by the dive medical officer. If they all get green-lit, the process begins

It starts with aquanauts laying down and donning pure oxygen masks for a few 20-minute intervals. At enough depth, O2 can set off seizures. So there's a person who stays saturated in the habitat during decompression to watch the rest of the crew and make sure everything is OK. The chamber has a tube that leads to the surface buoy and releases air from the chamber.

The next morning the base is pressurized back to 2.5x atmospheres, which allows the outer door to be opened without flooding. The place heats up like a sauna from all additional air molecules being introduced to the confined space, and people sweat. The door, which is kept unlocked, swings open as the air equalizes on the inside and out. From there, the aquanauts have a few minutes to get back to the surface before too much nitrogen is absorbed back into their bodies.

As the aquanauts step through to the wet porch no one says a word. I hear that no aquanaut ever feels like leaving the habitat at the end of a mission.

Climbing through the moonpool one last time, they ascended up a white, taut line of rope tied to a waiting boat, bobbing in rough seas. The boat, the George F. Bond, named after a forefather of saturation diving, has its own decompression chamber in case anyone gets the bends.

Breaking through to the surface, the first things the aquanauts notice is the true wind and the sunlight, full of oranges, reds and yellows. Then, the ease with which they can breath and speak. But the next thing an aquanaut feels is a sense of regret that they can no longer swim under sea for as long as they'd like—as they could on Aquarius' final mission.




Mission Aquarius is our week-long trip to the world's last remaining undersea habitat: Aquarius Reef Base. After this mission, Aquarius is scheduled to close, but you can donate to keep Aquarius open through the Aquarius Foundation on Indiegogo.

Brian Lam is an ocean exploration journalist and the editor of The Scuttlefish and The Wirecutter. He is a Gizmodo alum and a Wired Magazine contributor. Videos provided by One World One Ocean, a campaign dedicated to telling the story of the ocean through multimedia.