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In 1960, when the U.S. Navy sent Don Walsh and Jacques Piccard on the world’s first extreme deep dive — nearly 11 kilometres underwater into the Marianas Trench — they chose their precise dive location by spending days patrolling the area, dropping sticks of dynamite into the ocean, and measuring the number of seconds each blast took to echo back from the deep. 

The story is just one of many in Laura Trethewey’s new book, The Deepest Map: The High-Stakes Race to Chart the World’s Oceans, which illustrates humanity’s problematic relationship with the salt water world that covers 71 per cent of the planet. The recurring theme in many of those stories: we know embarrassingly little about the oceans, and yet we exert extreme influence on them, with near disregard. Even our best efforts to learn more about the oceans are fraught, like sticks of dynamite thrown overboard into an unknown habitat. 

In 2019, the global initiative Seabed 2030 estimated that just 15 per cent of the seafloor was mapped to a useful resolution.(Satellites have given us rough maps of the whole ocean floor, but can only document features 1.5 kilometres or bigger.) By the time The Deepest Map was published in 2023, that number was much closer to 25 per cent. Trethewey introduces readers to many of the people who helped make that progress. 

We meet Marie Tharpe, whose determined map-making skills created the first scientific map of the Atlantic seafloor, in turn helping bring the then-controversial theory of continental drift into mainstream acceptance.  We meet Cassie Bongiovanni, Tharp’s freshly graduated modern day counterpart, who ends up mapping over one million square kilometres of seafloor while working on the Five Deeps project. We meet Victor Vescovo, the wealthy adventurer whose disposable income and drive to set records funds the Five Deeps (and who hires Bongiovanni to do her work.) We meet Andrew Muckpah, an Inuit hunter who learns to use a basic sonar in order to contribute to mapping the ocean floor around his home of Arviat, Nunavut. And there’s a host of others including deep-sea biologists, tech startup CEOs and undersea archaeologists. 

Trethewey gives readers a real glimpse into the exciting world of exploration that is helping to map the ocean floor, taking us on board the vessel Pressure Drop while Victor Vescovo makes some of his remarkable dives into the deepest parts of the ocean. But there’s also more sobering chapters, like when Trethewey finds her way into the library of the International Seabed Authority in Jamaica, which is packed with highly detailed seafloor maps submitted by mining companies claiming tracts of seabed they think could be profitable. 

Not many landlubbers will have heard of the Clarion-Clipperton Zone (CCZ), “a deep abyssal plain halfway between Hawaii and Mexico, that is roughly the size of Europe.” But the prospectors are lining up, and the technologies being developed and tested that will eventually be able to strip the CCZ of its manganese-rich seafloor. Commercial deep sea mining has yet to begin in the CCZ, writes Trethewey, but already it promises the creation of Dust Bowl-esque sediment plumes that have the potential to wreak havoc for hundreds of miles.   

Trethewey doesn’t gloss over the potential for catastrophic consequences from mapping and exploring the deep ocean. “We will very likely end up damaging the seafloor before we finish exploring it,” she writes. And she spends some time grappling with the role of science in cracking open the deep sea world to industrial development. More than one of her interview subjects points out that much of humanity is still healing from the exploration and mapping of the world above sea level. 

But the questions Trethewey encourages us to ponder in The Deepest Map are not centred around whether to map and explore the deep sea, they’re about how that exploration happens, who controls it, and what it leaves behind.  The deep sea could become our next Amazon, warns Trethewey, heavily plundered and degraded. Or it could become our next Antarctica, governed by international treaties that protect it in the name of science. Laura Trethewey’s thorough accounting of our knowledge of and relationship to this “last truly mysterious place on Earth” can only help us along the right path. 

The Deepest Map is published by Goose Lane Editions.