Susan Casey has a thing for oceans. The award-winning journalist and onetime editor of O, The Oprah Magazine and Sports Illustrated Women has, for the last three decades, dedicated herself to exploring and chronicling the vast waters that surround and sustain us, despite our often-scant regard for them or their miraculous inhabitants.

Yet none of the Toronto-born writer’s multiple, very thrilling ocean-themed bestsellers – Voices in the Ocean: A Journey into the Wild and Haunting World of Dolphins (2015), The Wave: In the Pursuit of the Rogues, Freaks and Giants of the Ocean (2010), and The Devil’s Teeth: A True Story of Obsession and Survival among America’s Great White Sharks (2005) – approached the scope of her latest, The Underworld: Journeys to the Depths of the Ocean.

In it, Casey, 60, documents a genuinely remarkable, frequently mythical place that, while riveting to behold, is hostile towards humans, owing to the spectacular pressure it exerts at its depths, reaching some 6,000 pounds per square inch at the site of the Titanic, for example, which lays 3,800 metres below the surface.

And yet, as Casey writes, that depth – defined as the abyssal zone (3,000 to 6,000 metres) – is an area where vibrant life almost entirely unknown to us is being gravely impacted by our pollution, pleasure cruising, overfishing and so on.

Navigable only by rare, very expensive and exhaustively tested submersibles, the depths reveal wondrous, previously unseen sea life, much of it bioluminescent. There are volcanic and hydrothermal vents, mountaintops with heights surpassing Mt. Everest, upside-down reflecting pools and bacterial life that could be game-changing for this planet and the creatures on it.

“The need for us to have eyes and ears and cameras and smart instruments and DNA analyzers and sensors and robots in the deep ocean — so that we can protect it and understand it and survive in a time of harrowing climate change — will only grow,” Casey writes in her epilogue. “Ignoring the aquatic realm that dominates this planet is no longer an option.”

Even as The Underworld makes that searing point, Casey’s vivid writing ensures the book is animated and engaging, introducing readers to a fabulous cast of scientists, researchers and explorers, as well as mind-blowing creatures “with eyes like headlights, flat, triangular heads, translucent blue skin, and white hearts that pump gin-clear antifreeze blood.”

That’s just one observed species: the icefish of Antarctica’s Weddell Sea. The Underworld, to borrow from Casey, brims with “constant astonishment,” not least when the author — alongside independently wealthy and insanely intrepid, record-setting deep-sea explorer Victor Vescovo — achieved her lifelong dream by visiting the abyssal zone aboard the Limiting Factor submersible in Hawaii in January 2021.

As Casey tells Zoomer from her part-time home in New York’s Hudson Valley (she also lives part-time in Maui), nothing in fiction rivals the real wonders within our planet’s seas.

Kim Hughes: Of all the amazing discoveries you made while writing this book – previously unknown sea creatures, their sometimes-staggering populations (in the quadrillions), the vivid blues of the water as one descends – what was the most beguiling?

Susan Casey: It’s an overarching observation, but in the beginning, I basically laid out the thesis that, the deeper you go, the more astonishing everything becomes. The most beguiling thing is that thesis is true. When you go down, there is a sense of gravitas as you go deeper. Even though there are a lot of beautiful, showy things to look at in the zones where most of us hang out, there is something in the deeper layers that gives you a sense of your place on Earth and in the scheme of life. There is a spiritual revelation. I had expected it to be like that, but I had no way of knowing for sure.

KH: In the book, you talk about how descent is typically equated with negative things: descent into madness, descent into illness and so on, and that influences many people’s perceptions about the deep ocean. Does that concept cut across cultural and language lines?

SC: No. And in fact, I had looked for an opportunity to write about that, but couldn’t find quite enough information to make a section of it. In Sanskrit, the Indian Ocean’s name is Ratnakara, which is “repository of jewels.” There have been different mythologies for what happens in the underworld. Even within western civilizations, there are positive myths. Gilgamesh goes into the depths seeking immortality. What has given us this weird, downward-towards-doom mentality is relatively recent. I also think it dovetails with all our worst fears: darkness, not being in control, being underwater, monsters that can eat us. These are what we fear about the deep ocean. So, historically, getting us to think it was the world’s worst place wasn’t a hard sell. And as the Carta Marina [a 16^th-century Swedish marine map depicting sea creatures that dominated folklore for millennia] shows, people really did believe the oceans were filled with monsters. There was a lot of superstition giving those beliefs legs that lasted for centuries.

KH: A recurring theme is how grievously underfunded deep-sea research and exploration is compared to what governments spend on space exploration. You write that, “For every dollar the [U.S.] National Oceanic and Atmospheric Administration spends on ocean exploration and research, NASA gets a hundred and fifty.” If you could wave a magic wand and be sure one very rich or very influential person read this book and upended that inequality, who would it be?

SC: I don’t know if one person can change the trajectory of what we fund or don’t fund. Jeff Bezos wouldn’t be bad if you had to pick one because he has the resources to back all kinds of research. But it’s not fair to put it on one person. There are some very wealthy people funding ocean research, like [former Google CEO] Eric Schmidt and Wendy Schmidt, and David and Lucille Packard at the Monterey Bay Aquarium Research Institute. Before he died, Microsoft’s Paul Allen funded a lot of research for deep sea exploration. Almost every week it seems, some very wealthy person wants to fund this science, which is great. I just wish the average person walking down the street would think of it as part of Earth and not something that doesn’t exist or doesn’t matter or is just this spooky void. Because it’s not.

KH: What was the hardest thing to get right with this book?

SC: The amount of material I had to digest was the biggest challenge. Structuring the narrative was up there, too. I have been working on this book concertedly for about six years, but I have been wanting to write it since after I wrote The Devil’s Teeth, so I’ve been collecting information about it since 2003. It’s been a long process, wrapping my mind around this. Everything that I learned about the ocean in my previous books gave me the confidence to tackle a story this big.

KH: You’re plainly obsessed with the ocean. Did you ever think about being an oceanographer rather than a journalist?

SC: No, because language was always what I wanted to pursue. It’s funny. My boss at Hearst when I was the editor of O was Ellen Levine. We used to call her Ellen Levine AAD for “almost a doctor,” because she was married to a doctor, and she was always the person we went to for medical advice. I am AAS — “almost a scientist.” Maybe more like a science groupie.

But my role is kind of important because I see myself as a proxy for the average curious person. And I want to take these amazing stories that come from scientists’ work and tell them. I want to be a bridge. The science world has its own lingo and way of presenting information. All that stuff is important in that realm, but it creates a barrier for the lay reader. That requires crunching the information on one end and using language to simplify it without making it inaccurate on the other.  That’s what I love doing more than anything.

KH: You knew Paul-Henri Nargeolet, the legendary French deep-sea explorer and Titanic expert who was among those who perished last June on the Titan submersible. He’s even pictured in your book. Everyone in the deep-sea community knew that submersible was unfit for the depths it attempted, as you outlined in a recent Vanity Fair piece. So why did someone as experienced as Nargeolet ignore the risk and tag along?

SC: It’s the question nobody can answer. I met PH on a trip to Tonga and everybody loved him. He was a superstar in this realm, an icon. Everyone knew in 2018 that the Titan was not safe. Surely PH knew it, too. All I can tell you is that he was very emotionally involved with the Titanic and knew everything about it. A friend of his speculated that it was hard for PH to imagine an expedition to the Titanic that he wasn’t a part of.

KH: What do you most hope readers will take away from The Underworld?

SC: I hope to convey to people that the ocean is 95 per cent of the Earth’s biosphere. We need to understand how this whole planet operates and we need to understand it quickly, because things are changing quickly. Without understanding the deep ocean – its mechanisms, chemistry, its character, its life forms, and microbial life – we don’t have a hope of understanding how it will help buffer our excess carbon, for example. Will it keep doing that? We urgently need to improve our relationship with it and understand it’s not part of the Earth. It is the Earth.