Susan Casey is a champion of the deep ocean—revealing its natural wonders, its importance to the planet’s health, and its vulnerability to potential exploiters. The bestselling author of three previous books about the marine world, Casey worked her many contacts to gain entree to the most hostile environment on Earth for humans, one that can only be reached with cutting-edge technology and unlimited funds.

After years of research, Casey was able to dive in a manned submersible to one of the deepest spots on Earth. But her connections also gave her a front-row seat to the destruction of the Titan, the OceanGate submersible craft that imploded during a June 18 dive to explore the wreck of the Titanic. All five people onboard were killed.

Her new book, The Underworld: Journeys to the Depths of the Ocean (Doubleday, Aug. 1), shows off Casey’s superior writing and reporting skills. A high-energy, mile-a-minute talker with a passion for her subject, Casey spoke with Kirkus about the book by telephone from her home in New York. Our conversation has been edited for length and clarity.

You knew people involved in the Titan tragedy. Give us your take on it.

I wrote the book proposal for The Underworld in 2018, and at the time I lived in Hawaii and was looking for a way to get into this environment. OceanGate was putting out a lot of statements saying [they were] going to do cutting-edge submersibles and allow people to come into the deep ocean environment. I reached out to the company and spoke to a PR person and some of the operations team.

I didn’t know too much about submersibles at that time. But I had another person that I was spending time with during my reporting, a submersible pilot for two of these subs, the Pisces IV and the Pisces V—they’re at the University of Hawaii’s undersea research lab. He was an experienced pilot; he’s a character in the book. I said, “I’m thinking about going out with these OceanGate people,” and he just stopped and said, “No, you aren’t.”

Once I found out about OceanGate, I backed away fast. There was a whistleblower, David Lochridge, the director of marine operations. He had produced a 10-page report, which I read. It was exactly the kind of detailed report that you would want your director of marine operations to make, 14 pages of very specific problems. [OceanGate owner] Stockton Rush’s response was to fire him on the spot, to give him 10 minutes to clear out his desk. Then, four days later, there comes a letter from OceanGate’s lawyers, basically threatening Lochridge. I won’t go into all the details, but suffice it to say, I don’t know how we could have actually come up and said, this guy’s going to kill somebody, before it actually happened.

You’re saying that you don’t know how it could have been stopped?

I don’t know how it could have been stopped. He had his sub registered in the Bahamas, he had it leaving from Canada, sailing directly into international waters. One of the men that I knew and admired and write about in The Underworld, Paul-Henri Nargeolet, was killed. Nobody could figure out what he was doing there; all I can say is he was a man who loved the ocean, and he loved that wreck [the Titanic]. I think that, unfortunately, he gave a sense of legitimacy to a rogue operation. This is a fledgling industry; I think it’s really important that we do go down there and explore the deep. To the extent this gives people the idea that this can’t be done safely or that it’s always a crapshoot, that is really unfortunate, because it’s not true.

What kind of person is attracted to this kind of exploration?

To quote Don Walsh, one of the best submersible pilots: It’s curious people. It’s curious people who act on their curiosity.

It struck me that another key element is the willingness to tolerate risk.

Every submersible operation I observed or was part of [involved] machines that have been rigorously tested, checked, built, and adhere to the rules of physics, and so they’re overbuilt. It’s far more dangerous to get in your car than it is to dive in a submersible. The people that I spent time with in the ocean are risk analysts, not risk junkies. And there’s a certain amount of humility in that mindset, because the deep ocean doesn’t negotiate. If you want to go down there, there’s a very clear protocol of things that have to happen before you can do it safely, and everybody does those things. Except for OceanGate.

I want to touch on the sheer wonder of what you experienced, which is vividly conveyed in the book. What was your favorite weird creature?

More than a single weird creature, it was more like an overall experience of this realm with glowing, shimmering, twinkling creatures that live entirely in the dark and communicate with light. There’s a moment that really sticks in my mind, when we flashed the lights of the sub and all the creatures flashed back, and you saw that the water was just absolutely filled with life. And by the way, they’re also the biological carbon pump, one of the main mechanisms to absorb our excess heat and carbon dioxide. They move up the water column at night, they eat phytoplankton that has carbon from the sunlight, and they swim back down and excrete it. It’s a way of getting carbon from above and sequestering it below. It’s like the world’s largest animal migration, every single day.

You describe several dire threats to the health of the deep ocean. Which one worries you the most at this point?

Deep sea mining will be the most destructive thing that humans could ever do, on an order of magnitude. I don’t like the unsustainable and cruel practices of industrial fishing, I don’t like the fact that there’s plastic embedded in the creatures of the ocean, but the deep sea mining freaks me out.

You’re addressing the proposal to mine manganese-rich nodules from the deep ocean floor.

There’s no sense of how that ecosystem could ever repair itself. Those manganese nodules take a million years to grow a millimeter or something. They’re not rocks, they’re more like trees in a forest. There are animals living in them, on them, and under them in this particular area they’re going to mine first. They’ve found that 50% of the sea creatures that are there are there because of the nodules. It’s like a reservoir of genomic innovation, a DNA database for the Earth going back hundreds of millions of years.

And they’re just going to rip up those sediments and all the microbial life within them. They’re going to be sucked up a pipe. It’s like something out of Avatar, you know? Scientists have been racing to find out what’s down there, and even after several years of studying it, they’ve only managed to sample about 0.01% of it. But approximately 90% of the animals they have found are new species. I felt like I was almost going to have a nervous breakdown when I was writing that chapter.

I really hope that wisdom will prevail. A lot of countries have come out and said that at the least we should have a moratorium [until] we understand what we’re about to do. Scientists have recommended 10 to 30 years of further study before we even consider it. So far, those voices have not prevailed.

Hopefully your book will get the word out.

If I could hope for one thing, it would be that. I really hope my book will introduce this sort of magical realm and enchant people so that they want to care and that they want to protect it. It’s just so magnificent.

Mary Ann Gwinn is a Pulitzer Prize–winning journalist in Seattle.