I've Spent More Than 300 Hours in the Deep-Sea Twilight Zone


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I've Spent More Than 300 Hours in the Deep-Sea Twilight Zone

When you're in the Twilight Zone, it's incredible because everything you see is something that has never been seen before. But you have to be careful, because it's very easy to die at that depth.
July 11, 2016, 4:00am

All photos by Dr. Luiz Rocha

On June 10, the California Academy of Sciences opened its latest exhibit, Twilight Zone: Deep Reefs Revealed. It examines and displays sea life that resides in a relatively small band of ocean between 200 and 500 feet below the surface. Most everything above that has been examined by professional and amateur SCUBA divers, while the deeper stuff is the realm of submersible vessels. But the area between the light of the shallow coral reefs and the pitch-black of the deep sea has rarely been explored due to the complexity of the pursuit—only 20 to 30 scientists are qualified enough to make these dives.


Dr. Luiz Rocha, 43, is one of them. He's the curator and Follett chair of Ichthyology at the California Academy of Sciences and has been working at CAS for the past five years. As a diver, he's logged more than 5,000 hours underwater, including 300 hours in this Twilight Zone band. Rocha tells us in his own words what it's like to explore an area of the planet that's never been seen by human eyes.

When you're down there, in the Twilight Zone, everything you see is a novelty. Everything is something you've never seen before. It's incredible and always exciting. Your adrenaline just goes through the roof. But you have to keep an eye on your oxygen and what's going on with your rebreather, because there's no margin for error down there. The dangerousness of it all would probably be stifling for some, but the excitement for scientific discovery is what keeps me going.

I first caught the bug for diving in the mid 80s, when I was in high school in Brazil. I took part in the first SCUBA course offered in my hometown. When you dive for biology, the deeper you go, the more exciting stuff you find. So even when I was diving regular SCUBA, I was pushing the limits, going as deep as I could with the equipment I had, trying to graduate to the next level. When I was in Brazil, the technology wasn't there, because we didn't have the funding. What I do for a living now in California is a very specialized type of diving, so it's hard to find a program that supports it.


The training is specific for the rebreather unit. Rebreathers have been around forever. The first diving units were rebreathers, but they were never reliable. The military used pure oxygen rebreathers, which are efficient, but only up to a depth of six meters. At 400 feet, if you breath too much oxygen or too little, you're dead, and we didn't have reliable sensors until 20 years ago.

With the rebreathers we use, when you exhale, no bubbles come out. Your breath goes back in, and there's a filter that scrubs out the CO2. You keep breathing the same mix, over and over. It's very efficient. The sensor constantly monitors the pressure of oxygen, and if you need more, it replaces it. The filter can work for about 10 hours.

The alternative is diving with a regular tank, where every time you exhale, gas comes out. There are two problems with that. One is volume, because if you're going deep, you have to take a lot of gas if you're exhaling every breath. That's more tanks and heavier equipment. The second is cost. To be able to dive to the depths we go, we remove nitrogen and add helium, but helium is an expensive gas. If we're exhaling, we're spending about $10 to $20 per breath.

The Earth's atmosphere is 79 percent nitrogen, 20 percent oxygen, and the remaining 1 percent is other gases. That's what we normally put in our tanks. But nitrogen at that high pressure, the deeper you go, the more narcotic it gets. Nitrogen makes you drunk at that depth, and we don't want to be impaired. At 100 feet, if you're on a tank that has 79 percent nitrogen, it's equivalent to consuming two or three glasses of wine. At 200 feet, it would be like drinking a whole bottle. At 300 feet, it would be five bottles. That's why we remove some of the nitrogen and use helium instead.

When we get to our dive site, we spend about an hour setting up the rebreather, testing, and everything else. We go through a very long checklist. Usually, the team is three or four total divers. We plan how deep we're going, how long, but always have a contingency if we find something deeper or shallower and have to change on the fly. We use hand signals for the most part. Sometimes we talk through the regulators, but that's not easily understandable. Five-hundred feet is our maximum depth.

A lot of people ask if there's more pressure on the body at that depth, but no. Water doesn't compress. I could sink to the bottom of Mariana Trench, and it would feel the same. It wouldn't crush me. It's the gas that compresses, and all of the problems come from that.


When we breathe, there's gas that gets into our bloodstream. Basically, the molecules of gas get closer the deeper you go. This isn't a problem going down, but it creates challenges when you come up. If you come up too fast, the gas turns into bubbles. It's almost as if you're opening a bottle of soda. If you open it really fast, the CO2 turns into bubbles and comes out very quickly. But if you open that bottle slowly, no bubbles will formed. This is serious business in your body because bubbles in your bloodstream will stop blood flow. And depending on where it stops flow, it can be life threatening. If it's to the heart, your heart stops. If it's to your brain, your brain stops working. This is what's known as the "bends" and is the reason we take hours coming up from the depths.

When we're going down, it is a straight shot. We go as fast as we can clear our ears. It takes five minutes at the most. Then we try to find a place where fish or other things are that we want to catch. We have to find a place within five minutes. If we don't, we turn around and come back. If we do find a place, we stay down there between 10 and 15 minutes, that's as long as we can, mainly because it takes so long coming back up.

It's controlled chaos, I guess. We can't be far away from one another, but can't be bumping into ourselves, either. We try to catch anything we're interested in bringing back to the surface. For fish, since they've spent their whole lives down there, we have decompression chambers that we put them in and then seal at that depth. Then we send them back to the surface. Support divers collect the chambers, and they slowly decompress the fish over days, not hours. For us, after the 10 to 15 minutes of catching specimens, we begin to slowly swim back up to the surface. That process takes us seven hours.

On the way up, we stop every 10 feet, with each stop lasting longer. The first would be one or two minutes, the last at ten feet below the surface is like two-and-a-half hours. When we get to 100 feet, support divers usually bring cameras and gear we can use to research those depths. We take pictures, survey bleached corral, conduct fish surveys where we just count the fish—anything to keep us occupied. For 95 to 98 percent of our dives, we swim down along the wall of a reef until we reach the depth we want, so there are things to explore going back up.

But sometimes, the reef starts at 100 or 150 feet, so when we get past that level on the way up, there's no more reef. In that case, it's really boring. There's nothing to do. We keep our minds occupied in different ways, like rock, paper, scissors. We have those underwater writing slates, so we play tic-tac-toe, too. Sometimes I bring paperbacks underwater. If it's a five-hour decompression, I'll read a book. If I can't finish the book on that dive, I'll put it in a Ziploc with water inside of it. If you keep it wet and you're not very rough, the book stays OK, and I can read it on the next day's dive. I've read some of Hemingway's books that way.


After the dive, I'm exhausted and hungry. Some divers eat underwater, but I don't like it because there's always salt that comes in and that makes me more thirsty. Those divers eat bananas or apples, anything that doesn't dissolve underwater. To do so, they turn their head down. If you open your mouth with your head vertically, water comes in. But if you turn your head down, there's an air pocket in your mouth. There will be some water, but it won't be completely wet.

Of all my trips down to the Twilight Zone, the one fish that has really blown my mind is sacura speciosa. The common name people are calling it now is the "mind-blowing super freaky rainbow fish." It's really rare. CAS is the only aquarium in the world that has one. No private collectors have it, because no one goes to those depths. We collected that particular fish at around 487 feet underwater.

When you're diving at that depth, you have to keep track of a lot of different things. People compare using the rebreather to flying an airplane. Some say you don't dive a rebreather, you fly it. It's not as complicated as an airplane, but it is just as dangerous. If you do one or two things wrong, you're done.

See Dr. Luiz Rocha's work on his website.

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