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Public Radio's Environmental News Magazine (follow us on Google News)

Journeys to the Depths of the Ocean

Air Date: Week of

The Underworld: Journeys to the Depths of the Ocean is Susan Casey’s latest book. (Image: Courtesy of Susan Casey)

The oceans cover 70 percent of our “blue planet” yet remain largely unexplored because of the intense pressures at depth. But there are some intrepid few who have descended into this “underworld” and lived to tell of its marvels, and journalist Susan Casey profiles them in her latest book. She joins Host Steve Curwood to talk about The Underworld: Journeys to the Depths of the Ocean.


DOERING: It’s Living on Earth, I’m Jenni Doering.

CURWOOD: And I’m Steve Curwood.

Seen from space, ours is a blue planet thanks to the oceans that cover some 70 percent of its surface. Yet despite their dominance we really know very little about them. Our soft human bodies just aren’t designed to withstand the intense pressures of the deep oceans, let alone the lack of air for lungs to breathe. But there are some intrepid few who have descended into this vast and seemingly forbidden underworld and lived to tell of its marvels. Journalist Susan Casey is among them, and in her latest book, The Underworld: Journeys to the Depths of the Ocean, she illuminates the immense world miles beneath the surface and the people committed to exploring it, and she joins me now. Welcome back to Living on Earth, Susan!

CASEY: Thanks, Steve.

CURWOOD: So when you step into the formal text of your book, you start talking about the history of the ocean and how humans thought of it as filled with monsters hundreds of years ago, their drawings are some famous ones. There are myths that Atlantis is down there. This is supposed to be a fairly spooky place. Of course, nowadays, we know better. But I have to say some of the photos in your books of some of the deep-sea creatures, well, maybe they are kind of monstrous. How do you think about both how far we have come in understanding the ocean and the mysteries that remain?

The Limiting Factor landing on the seafloor of the Mariana Trench’s Challenger Deep, the ocean’s deepest spot (35,876 feet). (Photo: Courtesy of Susan Casey)

CASEY: Well, it's really exciting to me that basically, we're the first generations of people that will actually be able to know, what are these creatures, what is down there. Technology is helping us, but it wasn't all that long ago, only about 150 years, that nobody knew at all what the bottom of the ocean looked like, what it was made of, you know, what lived down there, did anything live down there? So there were, preceding that, a lot of superstitions and myths and lack of knowledge, which led to confusion, I think. And one of the things I say in the book is, imagine you're a medieval farmer, and you're walking along the shore in Scandinavia sometime during the 16th century. And you come across the stranded, beached body of a sperm whale, like what are you thinking that is? You don't really know. But it looks pretty monstrous. And we've been told, throughout sort of the deep history, that, you know, there were monsters in the Bible, there was Leviathan, there were monsters in Greek myths. They lived underwater in this hidden underworld that we weren't privy to. And the thing is, they really aren't monstrous, but they're very different from us. They have a very different set of evolutionary needs. They've adapted to incredible pressures. They've adapted to living full time in the darkness. And this is not something that we relate to, life that really doesn't look like us. It may not even obey the same basic laws of life that we do, that, you know, say we thrive on photosynthesis. There are creatures at the bottom of the ocean that will never see the sun. And so their lifeforce is basically coming from the internal heat and minerals and gases and microbes from the planet in a process known as chemosynthesis. We only discovered this in 1977. So life had this entirely different trick up its sleeve that we didn't even know about. And we're still finding out all sorts of things. But I do think it's the otherness, sometimes, that makes people think ‘monster.’ But a lot of the times when you see these creatures, I mean, I tend to think of them as just adorable. A lot of the times they're quite small, the fish with the really big teeth and the big eyes, that's what you get when you adapt to living in the darkness 100% of the time, and food doesn't come along very often, and when you catch it, you have to have jail bars on the front of your mouth to make sure it doesn't get away.

CURWOOD: Now, of course, you point out in your book that yeah, maybe they don't have the visual acuity all the way down, but they use sound in intricate and exquisite ways.

The elongated bristlemouth, a tiny, glittery predator that is likely the most abundant vertebrate on earth. Trillions of these fish live in the uppermost layer of the deep ocean, known as the twilight zone (from 200–1,000 meters). More creatures live in the twilight zone than in all the other regions of the ocean combined. (Photo: Paul Caiger, Woods Hole Oceanographic Institution)

CASEY: Oh, absolutely. I mean, sound travels far and fast in the ocean, more so than it does in air. And when you think about it, you know, we're primarily visual creatures. But that's not the greatest sense on a planet that's 95% in darkness 100% of the time. So there are other means in the ocean of communicating, and sound is one of them. There's also bioluminescence, which is the ability to create light. Not very many creatures have this on land. But in the ocean, it's a major part of the arsenal of tools that they have to be able to hunt, to mate, to get away from danger, to do all sorts of things. So when you go down in a submersible, you see these bursts of light, like fireworks kind of all around you. Because 80% of the animals in the uppermost parts of the deep ocean can illuminate themselves.

CURWOOD: Talk to me about the layers of the deep ocean. I mean, I've gotten to go, I don't know, maybe 70 or 80 feet down. And you know, with a tank strapped to my back at one point, there's a lot more ocean there. And you in fact, were able to go on a few deep-sea dives. What was that like? And talk to me about the layers that you went through.

A Sloan’s viperfish, also found in the twilight zone. Eighty percent of the twilight zone’s creatures use bioluminescence for hunting, mating, hiding, and other purposes. Deep-sea fishes like this one look fearsome, but most are quite small. (Photo: Paul Caiger, Woods Hole Oceanographic Institution)

CASEY: When people think of the ocean, they typically think of the very uppermost layer, which is the sunlight or photic zone, where there's light through the water. And even at the deepest you can scuba dive, you're still seeing sunlight, at least there is sunlight in the water. But below that is the uppermost layer of the deep ocean, and that's the twilight zone or the mesopelagic zone. And that goes from 200 meters, so about 600 feet, down to 1000 meters, so about 3300 feet. And then below that you have another vast midwater area called the midnight zone that goes from 1000 meters to 3000 meters. Below that you have the abyssal zone, or the abyss, which goes - it's the largest ecosystem on Earth, and it's 3000 meters to 6000 meters. And then below that, in certain parts of the ocean, there's the deepest part. It's where the tectonic plates collide and one plate is being subducted beneath another, and that's from 6000 meters almost down to 11,000 meters in the deepest spot in the ocean. That's called the hadal zone. And those deep areas are called hadal trenches, where these tectonic plates are colliding. The Mariana Trench is one that people tend to know. But there are dozens of others. So the deepest spot that we know of is called the Challenger Deep in the Mariana Trench, and that's 35,867 feet by our latest calculations. It's actually very hard to get an exact depth calculation. And I went as far down as the abyssal zone. I went about 17,000 feet, about 5200 meters. I also went through the twilight zone, which is really amazing, because I call it the Manhattan of the deep, because there are more creatures living in the twilight zone than in all the other regions of the ocean combined. So it's a really active place and very scenic because all these creatures are illuminating themselves and flashing and blinking and wafting in front of you and sparkling.

A Triton three-person submersible with a transparent pressure sphere at 3,300 feet in the Bahamas. Transparent pressure spheres offer passengers a kaleidoscopic view of the deep, but they can’t be used below 6,000 feet. At greater depths, a titanium or steel pressure sphere is required because of the immense pressures. (Photo: Courtesy of Susan Casey)

The abyss is... it took us two and a half hours of free falling to get down into that realm and get the sense that the deeper you go, the more profound it feels. You're definitely aware of the weight, the pressure. I mean, that's 500 atmospheres on your head. There's a gravitas to it. But there's also a serenity, a real serenity, you realize that it is not space, because it's alive. I never forgot where I was. But I was in such awe of where I was that the time goes very elastic, like you could be down there for an hour and you would think you were down there for 15 minutes. There's no signposts. There's none of the things that we tend to look for above, you know, that give us a sense of our surroundings. It's all fluid. It's a spectacular experience that really changes your perspective of your place on Earth. This vast, vast, vast region that we never even think of - it is the vast majority of the Earth. So it's amazing to meet it.

CURWOOD: It's interesting the nomenclature, though, we use culturally. All the way at the bottom, the Hades zone. Hades, the god of the underworld. Your book, in fact, is called The Underworld. And in a colloquial way, we are thinking of, you know, crime, the wrong side of life when we talk about the underworld. Why did you pick this title? Why did you pick this name? And why do people who look at the oceans use such names to describe what's there?

Leptothecata jellyfish are one of the many creatures living in the twilight zone. (Photo: Paul Caiger, Woods Hole Oceanographic Institution)

CASEY: Well, I think we have it baked into our language, you know, enlightenment, you're being raised up. And anything that takes us upward toward the light, like, up is good, down as bad. And we're kind of scared of bad, you know, it's dark, and we don't know what's there and we can't breathe. But to go into the underworld in Greek mythology is, it's the hero's journey. It's, you’re going down there, and it's not an easy journey. It takes humility, actually, it's the opposite of conquest. When we go into the deep ocean, we're using the best technology that we have, the best engineering that we, we have, and we know exactly what kind of forces we're going to be meeting. At the bottom of the Mariana Trench, for example, it's, you're gonna have 16,000 pounds of pressure per square inch. So there are rules that you have to adhere to, to be able to make this journey and come back home. And we're not necessarily the best at that. We're good at, you know, we love to the idea of blasting our rockets into space, and maybe we'll get a Mars colony, and maybe we'll expand our reach. But the inward journey, the journey into darkness, it's a different kind of journey. And the underworld in other cultures, it - that's where they kept the treasure. And I mean, I think that's one of my major theses in this book, is that this is a realm that is magical beyond measure, fascinating beyond measure. And we've neglected it, even though it really behooves us to understand more about it, because we're a little bit scared of it. And some people are a lot scared of it. But it is where the treasure is.

A species of cusk eels known as robust assfish, found in the waters as deep as 27,000 feet. These assfish live in the ocean’s deepest realm, known as the hadal zone (named after Hades, the underworld). (Photo: Alan Jamieson and Thomas Linley)

CURWOOD: So your book describes a series of some kind of amazing characters, people who, in fact, decide to go down there. Talk to me about some of those people and their love of this underworld.

CASEY: When I report my books, I try to embed myself with people that are sort of doing the most extreme things in the environment that I want to write about and doing them well, so that there's a sense of being on the cutting edge as I'm following them through, say, in this book, it was embedding with the Five Deeps Expedition, which took place in 2019, and it was sort of the debut of the first full ocean depth manned submersible, which was owned by an individual named Victor Vescovo, businessman from Texas, who's also a very committed adventurer. And by the time I met up with him, Victor had already been to the top of the tallest peaks on every continent and skied to the north and south poles, and had looked around and thought, "Okay, what else can I do?" and was kind of shocked to find out that we hadn't been to the bottom of the hadal trenches, for the most part. And only three people in history had been to the bottom of the Challenger Deep, which is the deepest spot in the Mariana Trench. And in the time that it had taken three of us to get down there, you know, like 200 people had been to the International Space Station and thousands of people had been to the top of Mount Everest. So he thought, "Well, wait a minute, let's do that." I tend to like very obsessive, very colorful characters, and he is certainly one. He commissioned, I think the best submersible company in the world, this company Triton Submarines based in Florida (I call them the Apple of submarine design) to create a two-person sub that could repeatedly go safely to the bottom of the deepest spots in the ocean every day of the week. And we had never had such a vehicle. It was not necessarily agreed upon that it was even possible because of the pressures to do this. And so Five Deeps was this sort of historical expedition that I was fortunate enough to be able to embed myself with. You know, in nonfiction, I make room for luck. And this was really a stroke of luck, timing wise, because I think in the history of deep-sea exploration in the future, will be divided before this submersible and after this submersible. It really opened up the hadal zone. And along with taking me down there, Victor also took a whole lot of hadal scientists who had been studying this realm, and it's not easy to study the deepest parts of the ocean, but they had never seen it. So he took them down in person. It just changed everything, this submersible, so I really wanted to share all kinds of details of that with readers.

These hadal snailfish, photographed in the Mariana Trench, are the world’s deepest fish. They’ve been found as deep as 29,300 feet and have adapted to the hadal zone’s crushing pressures. Hadal snailfish have no swim bladders or other air cavities in their bodies; their innards are encased in a buoyant, transparent gel. (Photo: Alan Jamieson)

CURWOOD: You have the curiosity for what's down there, then you get to go. What did you discover?

CASEY: I love the question because you discover things about yourself. You discover this perspective, our place on Earth, like I really am a fan of this humility in the face of nature that's bigger than we are, so much bigger than we are. And I think it's very hard for people to wrap their heads around how vast the deep ocean is. Everything we know, we see on land, is 2% of Earth's biosphere. The deep ocean is 95. So you get the sense of yourself as exquisite, but insignificant piece of the cosmos. You know, we're just here as part of it. And I love that. I love that feeling of humility, of being part of nature and being able to sort of revel in it. I think we lose sight of the fact that we are part of nature, and we think our job is to run it and own it, and, you know, use it. That's a delusion, and the ocean will set you very straight about this very fast.

CURWOOD: So, Susan, before you go, what do you hope for the future of underworld exploration?

Susan Casey, the author of The Underworld, entered the hatch of Victor Vescovo’s full ocean depth sub, the Limiting Factor. (Photo: Joe MacInnis, Courtesy of Susan Casey)

CASEY: You know, if you're not claustrophobic, I think going deep, like going below 600 feet if the possibility arises, and I think there will be more opportunities in the future to do this safely, it really is something that I think everybody would revel in, except for possibly claustrophobic people. But I just think that it is, the ocean is the greatest teacher, and the deep ocean in particular, for the sense of humility. I think it's a superpower. And I wish we could avail ourselves of it more above the surface, because I think that we would have a smoother ride. If we realize that this is all this massively interconnected, intricate system, and we're just beginning to understand that, and we should move with respect and caution. And at the same time, just delight in the beauty of it. Because everybody thinks they know that the deep ocean is spooky and scary, but I don't think people realize at all how beautiful it is.

CURWOOD: Susan Casey's book is called The Underworld: Journeys to the Depths of the Ocean. Thanks so much for taking the time with us today.

CASEY: It's my pleasure.



Explore more of Susan Casey’s work.

Learn more about the Five Deeps Expedition

Differentiate the ocean’s zones.

Purchase The Underworld and support both local bookstores and Living on Earth.


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