The Underworld by Susan Casey is a wonderful book on deep sea exploration, following other books Casey has written about the ocean, including The Devil’s Teeth and The Wave.
She notes that the deep ocean is the waters below 200 meters, approximately six hundred feet, and has a fascination with the deepest parts of the ocean, including the twilight zone region described as being from six hundred to thirty-three hundred feet deep, midnight zone from thirty-three hundred to ten thousand, abyssal zone from ten thousand to twenty thousand, and hadal zone from twenty thousand to thirty-six thousand feet deep. The deepest part of the ocean is Challenger Deep, at 35,786 feet within the Mariana Trench.
Rather than being barren as was once thought, the deep ocean is alive with fantastic creatures, bacteria, and microbes that are the source of life, whose understanding yields new medicines and treatments for disease. Also, the deep ocean buffers carbon and regulates conditions on earth.
Covered in the book are deep sea submersibles, the ships that a very few people have taken to the bottoms of the ocean. In 2012, James Cameron touched down at the bottom of Challenger Deep, the third person to ever reach that point, and Casey writes heavily about billionaire explorer Victor Vescovo. He in 2018 commissioned a submersible, the Limiting Factor, that he could pilot to the depths of the ocean and set out to dive to the deepest points in all five ocean basins. The effort was known as the Five Deeps and carried a roughly 50-million-dollar price tag. Vescovo has also been to the top of the seven summits, to both poles, and to space.
It’s a great story of eclectic and fascinating people along with amazing geography. Casey was to go in a submersible down to explore Lo’ihi, the next Hawaiian volcano, thirteen thousand feet tall, and a mile below the surface, which should poke out of the water in perhaps a hundred thousand years. She details how we have to understand the deep ocean, its geology, and the life there to fully understand our world, including its plate tectonics and accompanying earthquakes. Casey covers hydrothermal vents, the giant squid discovered in 2012, and the Lost City, an underwater area of more than thirty white pinnacles and spectacular formations, formed by chemical reactions between mantle rock and seawater.
Not unexpectedly, there is detail about people wanting to effectively destroy the ecosystem of the ocean, by deep-sea mining nodules containing metals. Mining is controlled by the International Seabed Authority (ISA), an organization noted as akin to oil executives deciding on forest ecology. Petitioning the ISA on behalf of energy companies is the country of Nauru, containing some 11,000 people, with leaders who have stripped their land bare and now seem to want to do the same to the ocean. There is also a chapter about shipwrecks and treasure, specifically the San Jose, as written about in The Wager by David Grann. She includes mention of the nonprofit OceanX started by Ray Dalio, the marine research group Inkfish, and the BBC series Blue Planet II. The book is a great narrative about Casey's love of the deep ocean and desire to see it.