“The Underworld: Journeys to the Depths of the Ocean,” by Susan Casey. New York: Doubleday, 2023. 352 pages. $32.00 (Hardback).

For every dollar the United States spends on oceanography, we put $150 into exploring space. Susan Casey, a lifelong aquaphile and the author of award-winning books on rogue waves, great white sharks and dolphins, thinks we’ve got our priorities reversed. “Our survival depends on the ocean,” she writes. “[N]ature runs as a massively interconnected system, with the deep sea as its motherboard,” and yet the lower reaches of the sea are still woefully misunderstood and increasingly endangered.

For most of human history, knowledge of the sea was confined to its upper story, the 600 feet light can penetrate. But that’s merely a 5% sliver of the ocean’s mass, while the 95% lower down – home to mountains taller than the Alps and chasms six times deeper than the Grand Canyon – teems with life, most of it wholly unexamined.

Far below, the deepest-diving whales and sharks are creatures with glass skeletons, two mouths or three hearts. Some emit light. Some are colonies of hundreds of semi-autonomous beings, all thriving under pressures “that would crush a Mack truck.”

Just as odd, and possibly more important, at least to us, are the deep’s single-celled bacteria and nucleus-less archaea and viruses. These minute ancestors of all later living things represent a vast reservoir of new knowledge. Deep ocean discoveries have already led to new antibiotics, biomaterials and cancer treatments, and we’ve scarcely begun to learn about all the gifts the deep sea has yet to offer.

It wasn’t until the 12th century that anything much was known of the ocean’s lightless, deeper regions. Casey tells the gripping story of William Beebe and Otis Barton, who in 1930 explored the depths off Bermuda in their tethered Bathysphere, a hollow 4.75-foot ball of steel outfitted with a searchlight and observation ports.

Though nothing was thought to live at that depth, they found the ocean was full of life and luminescence more than a half a mile down. As Beebe watched through three inches of fused quartz, “troupes of winged pelagic snails fluttered by like pixies, and 6-inch dragonfish with dagger teeth and luminous barbels swinging from their chins emerged as tiny frights in the inky water.” All this while, “green-glowing lanternfish twinkled past the Bathysphere in a chorus line.”

Casey has a sure-handed way of dealing with the fascinating details of seafloor geology – from the subduction zones where one undersea tectonic plate forms a trench as it bends to slide under another, melts in the earth’s underlying mantle, and resurfaces as a chain of belching volcanos, to the mysterious vents through which the mantle itself breathes 600 degree mineral-laden gasses in which one-celled animalcules somehow bask and prosper.

She’s equally captivating when it comes to larger sea creatures: giant groupers that peer back at you through a diving vessel’s ports and Greenland sharks that can live 400 years and grow large enough to swallow polar bears. Farther down, everything pulses with light, including glowing jellyfish, looking like “a donut in a fright wig,” and iron-eating bacteria that, when roiled, puff up in luminous red clouds.

Some readers will focus more on Casey’s human stories, her vivid portraits of the oddball designers and death-defying pilots who risk everything to uncover the earth’s deepest secrets.

Much of the book draws on the “Five Deeps” project, a series of dives in the free-ranging undersea vessel Limiting Factor to explore the deepest destinations in the world’s five ocean basins – the Pacific, Atlantic, Indian, Arctic and Southern seas. These include the lowest spot on earth, the Challenger Deep, 6.6 miles down in the Pacific’s Mariana Trench, where the Limiting Factor creaked and groaned under eight tons per square inch of pressure, equivalent, Casey says, to “292 fully fueled 747s stacked on top of you.”

Funded by adventure-happy billionaire Victor Vescovo (in addition to reaching all Five Deeps, he’s skied to both poles and climbed the highest peaks on each continent), the Limiting Factor has survived electrical fires, communication failures and encounters with abandoned fishing gear. These near disasters make exciting reading as Casey brings each dive to life, including the time she piloted the Limiting Factor herself, diving 17,000 feet to explore Lô‘ihi, an underwater volcano larger than Mount Etna and still growing as it rises from the seabed to become Hawaii’s next island.

Still, Casey’s ultimate subject is the sea itself and her fears for it as global warming threatens undetermined oceanic calamities and corporations scheme to monetize the seabed. The exploiters’ current plans involve a 1.7 million square mile zone in the Pacific between Hawaii and Mexico whose bottom is rich in metals needed for today’s batteries.

Prodigious mining machines are poised to scoop out vast reaches of the ocean floor, disrupting everything that lives there and raising suffocating silt plumes with the potential to kill whole species of creatures whose properties are entirely unknown. Casey is painfully aware of how much more we have yet to learn about the deep and how quickly it will be lost if, as she says, “we trash the book before we even crack the cover.”

Regrettably, while the text cries out for pictures of the people, creatures, and diving vessels Casey describes, the publishers decided to do without illustrations. The publishers opted out of an index, too, a serious flaw in a book so rich in data and details. Despite those omissions, however, The Underworld is a wonderful treatment of a vital but sadly under-covered topic.

— Reviewed by Joe Glaser, WKU English Department