Editor’s Note: “The Richest Reef” follows members of a scientific dive team as they attempt to pinpoint the center of the most biologically diverse marine ecosystem in the world. Long considered our planet’s most species-rich piece of ocean real estate, the Western Pacific’s “Coral Triangle” is a continent-sized patchwork of habitats, populations, and communities. Expedition scientists are trying to identify exactly which section of this rich mosaic is most diverse, but their effort carries with it far more meaning than a simple dot on a map would suggest. Along the way, they’ll discover dozens of new species, visit dive sites and depths no one has seen before, and gain a better understanding of the factors that promote biodiversity and the role these species-rich areas play in sustaining healthy ecosystems. See below for a list of all posts in the series.

All I can see is the thin white line disappearing into the blue-black below. That, and a diffuse stream of bubbles escaping back toward the surface—which is exactly the direction instinct tells me I should be heading. For a moment, the thought of aborting the dive flashes through my mind. But there’s someplace I’m supposed to be. I do my best to override fear by focusing on plan and procedure and push on—or rather fall, effortlessly, heavier than saltwater, and picking up speed.

The meeting I’m scheduled to attend was planned a couple of hours earlier. The proposed rendezvous point, 110 feet below the surface, made my palms sweat just to hear it. Among this crowd, 110 feet is pedestrian. Well within “recreational” dive limits, it’s a depth that makes deep divers feel comfortable, like they’re out of the woods. It doesn’t sound very recreational to me. Then again, I’ve never been anywhere near the 500-foot envelope these guys push every time they enter the water to venture into some of the least-explored terrain on the planet.

We’re here at the southern tip of the Philippines’ largest island, Luzon, at a spot chosen for its combination of steep drops and shallow reefs that climb all the way to the surface. The nucleus of the group is a five-member, multidisciplinary team of deep-reef divers, including Bart Shepherd, the aquarium director at the California Academy of Sciences, Luiz Rocha, the Academy’s curator of ichthyology, Hudson Pinheiro, Rocha’s tireless PhD student, Elliott Jessup, head of the Academy’s scientific diving program, and Brian Greene, a highly experienced deep diver and fish collector from Hawaii’s Bishop Museum.

For the past two weeks, while other members of the expedition have been exploring the Verde Island Passage’s “shallow” reefs—anywhere from zero to 130 feet—the deep team has been plunging into the “twilight zone,” a region of depth between 200 and 500 feet below the surface. This narrow band—well beyond recreational dive limits and far above the deep trenches scientists explore when they have submarines and ROVs at their disposal—has been visited by fewer people than have walked on the moon. That fact and the twilight zone’s unique conditions and habitats make this a treasure trove of novel biodiversity. The rate of new-species discovery on a twilight zone dive can top 10 per hour, which, in my mind, almost makes up for the inherent risks and the hours divers have to spend decompressing on their way back up after a 30-minute tour of this cold, bizarre, light-starved world.

A plan is hatched
As if it’s not enough to spend your days exploring a place no human has ever seen before, the twilight zone team is devoting its final week in the Philippines to a truly audacious plan. They will collect live animals at depth and then attempt to keep them alive and healthy on the impossibly long journey from a deep reef in the Philippines to the public aquarium at San Francisco’s California Academy of Sciences. As you might imagine, the scheme is chock-full of risks, for both humans and non-humans alike. Some aspects of the plan—like the design and construction of a fish-sized, portable decompression chamber—have been in the works for months, while others are standard operating procedure for any deep dive. No detail is spared and nothing is intentionally left to chance. Yet, out of the dizzying list of safety protocols and if-then scenarios discussed during the morning pre-dive meeting, just two simple-sounding objectives emerge: “Stay alive” and “catch fish.”

While the second objective gives the briefest nod to what the team came on this expedition to do, the first is akin to an athlete reminding himself to keep his eye on the ball—except this ball can kill you if you lose sight of it. “There are maybe a dozen people on the planet doing what we’re doing,” Greene says, “because it’s f#@%ing hard and people die.”

That might sound like hyperbole or bluster, but it’s no joke. Although humans cleared the whole breathing-under-water hurdle many decades ago, the deeper you go, the more complicated things get and the less certain you can be of a safe return. As popular as scuba diving might be, most divers wouldn’t dream of going to 400 or 500 feet. The inherent risks are simply too great, as is the investment of time and money required to bring those risks into reasonable proportion relative to the gains. And there’s a lot more to the risks than the distance a deep dive places between the diver and that unlimited supply of air at the surface.

Where oxygen becomes toxic
The pressure that goes hand in hand with depth does bizarre things to one’s physiology, particularly in relation to the air we breathe. Every 33 feet of depth stacks one whole atmosphere’s worth of pressure on top of a diver. That pressure compresses everything, gases in particular—so much so that a lungful of air at 500 feet contains 12 times as many molecules as the same breath at the surface. That’s 12 times the number of molecules pushing into a diver’s tissues and being absorbed by the blood, which is almost exactly 12 times too many.

At depth, the 21-percent concentration of life-giving oxygen we breathe on land quickly becomes toxic and seizure-inducing. The other chief component of air, nitrogen, starts to have a narcotic effect somewhere beyond about 90 feet. Seizures and narcosis are two conditions you most definitely do not want to experience hundreds of feet underwater. To prevent both, the deep divers use an advanced breathing system that dilutes the standard percentages of oxygen and nitrogen with helium, an inert gas that has none of the toxicity and narcotic effects of the other two gases. This closed-circuit system, known as a “rebreather,” also scrubs carbon dioxide from air the diver exhales and recycles oxygen, which allows for longer dive times. A sophisticated onboard computer monitors the mix of gases in real-time and meters out just enough oxygen to keep a diver conscious, clear-headed, and seizure-free.

Of course all of the extra gas absorbed into the tissues while at depth has to go somewhere as a diver heads toward the surface. The deeper you dive, the longer you stay, and the harder you work while down there, the more gas the tissues absorb and the more will be looking to escape on your way back up. Ascend too quickly, and dissolved nitrogen boils out of solution and forms bubbles in the tissues and blood, causing the painful and sometimes-life-threatening condition known as decompression sickness or “the bends.” The only reliable prevention for the bends is a slow ascent. That means hours spent decompressing at increasingly shallow depths in exchange for 30 or 40 minutes mucking around in the twilight zone.

Another world altogether
While it’s arguable that anything you might see in the half-light hundreds of feet below the surface is worth all this risk and preventative effort, you can guess where the deep divers might land on this debate. “It’s an extraordinary place, with a completely different community of organisms,” Shepherd says with infectious enthusiasm. “Half of what we see down there is new to science.”

Interestingly, there’s usually a pretty clear line distinguishing the extraordinary “down there” from all the rest, and in the Philippines that line falls consistently between about 270 and 300 feet. En route to this magical depth, the team sees more or less what they would expect: a steady transition from one community of organisms to another. That’s followed by a strange moonscape of sand and rubble that slopes far too gently for divers who can’t afford to waste time on descent. But then something amazing happens. They reach a sharp ledge that opens steeply toward the center of the Earth. As they spill over the edge, they feel the temperature drop as precipitously as the slope and watch as their dive lights cut through the near darkness to reveal a world unlike anywhere else on the planet.

Gorgeous soft corals and sea fans wave in the current, perfectly adapted to life without the benefit of photosynthesis; multicolored comb jellies, which swim freely in other parts of the ocean, cling to wire corals and strands of discarded monofilament like socks hanging on a line; and bizarre crabs, urchins, and nudibranchs cruise the bottom in search of their next meal. But it’s really the fish we’re here for and it’s easy to see why.

If anything can convince me that all this effort is worthwhile, it’s the fish. Like the daydream creations of a child left with nothing but the brightest crayons in the box, these are some of the most beautiful fish you’ve ever seen. In full light, they practically glow in hues of red, pink, orange, and yellow. Most are known only to the world’s most knowledgeable ichthyologists and a handful of hardcore aquarium enthusiasts. Many have yet to be described and don’t even have proper names yet. If Rocha has anything to do with it, it won’t be long before they do. But first, the dive team has to get today’s catch up to the surface.

Keeping the pressure on
We find the team just where they said they’d be, at the other end of the line, 110 feet below an inflatable buoy shot to the surface a few minutes earlier. At this point, they’re just a little over an hour into a four-hour dive, and have paused briefly at one of their many decompression stops to hand off their precious cargo before it gets too warm.

Rocha, Shepherd, and Greene begin unclipping and handing over decompression chambers containing an unknown number of brightly colored fish as soon as the first two surface-support divers arrive. The divers, Matt Wandell and Nick Yim, are biologists at the Academy’s Steinhart Aquarium and spend their days caring for rare and unusual marine organisms. They take the handoff and attach the chambers to themselves without a hint of nerves, which is surprising, given that they’ve just assumed responsibility for the wellbeing of some of the rarest creatures of them all. They’re also just beginning what they surely know is a vigil that will play out across more than 7,000 miles and over the next several dozen critical hours in these fish’s lives.

The reason for the decompression chamber in the first place is that fish, like humans, carry a tremendous amount of gas inside their bodies, particularly inside organs called swim bladders. These internal sacs enable the fish to maintain neutral buoyancy underwater. The deeper a fish resides, the more gas that fish’s swim bladder will contain. Although fish don’t get the bends, if you bring a deep-water fish up too quickly without taking any precautionary measures, its swim bladder can expand so much that it can push the fish’s stomach completely out of its mouth. That’s the sort of fish trauma that gives aquarium biologists nightmares.

Until recently, scientists and fish collectors would poke hypodermic needles into the swim bladder to vent the gas, but Shepherd was never too keen on this technique. “I don’t like poking holes in fish,” he says. So he challenged Wandell, a full-time biologist and part-time inventor, to develop a portable decompression chamber that could be taken down to the twilight zone, sealed at depth, and would then allow the biologists to bring deep-reef fish "up" (meaning down to surface-level pressures) over the course of two or three days, rather than just a few hours.

With three of Wandell’s latest, third-iteration chambers in tow, he and Yim make their way to the boat as quickly as their obligatory safety stops will allow. Once there, they hand the chambers up and climb onboard. Time is critical. Every minute the chambers are sitting on the deck, the water inside is getting warmer and more contaminated with the fish’s own waste.

Dripping wet and still clad in neoprene, they scramble to get the three chambers hooked up to a system of tubes, pumps, valves, and gauges—also devised by Wandell and powered by a car battery—that will flush the chambers with clean, chilled seawater, while still maintaining the pressure inside. After a brief, dicey moment inherent to situations involving electricity and saltwater, the system hums to life as soon as the second jumper cable makes contact. Wandell and Yim breathe a sigh of relief that lasts all of 30 seconds before they’re back on their feet to make sure conditions inside the chambers are holding steady at levels that match those where the fish came from.

And that’s precisely the point of this whole endeavor—to recreate a small piece of the twilight zone. What might appear on the surface to be a stunt or a challenge to simply do something that’s never been done before has grown from a deep scientific root. Sure, there will be bragging rights when the team successfully transports 15 rare and beautiful twilight zone fish to San Francisco’s Golden Gate Park. But the knowledge they will gain about the biology, behavior, and life histories of these animals through the process of caring for them and recreating their habitats will be far more valuable. As will the ability to share what little they know about this, the least explored place on Earth.

Other posts in this series:

The Richest Reef: Exploring the Most Diverse Marine Ecosystem on Earth

The Richest Reef: No Such Thing as Packing Light

The Richest Reef: Life in Layers

The Richest Reef: A Symbiotic Society

The Richest Reef: A Bagful of New Species

The Richest Reef: Deep Diving into the Twilight Zone

The Richest Reef: Where Have All the Predators Gone?