A couple of years ago, if you were to go in search of the noted British explorer and nature writer Robert Macfarlane, you would have done well to look in two places: in the wildest mountain ranges on the planet, where he can often be found wandering, or in the deepest stacks of the library at Cambridge University, where he teaches. The former venue has given him the stuff of numerous books and essays, the latter a rich trove of poetic language, much of which he has been pleased to rescue from oblivion: Words such as nailbourne, which is what a Kentish farmer might call an intermittent stream, and swang, what a Yorkshire orchard keeper might call a low-lying, flood-prone piece of land, dot his pages like crystalline mountain lakes.

But his new book finds him in a different milieu entirely: the dark, dank spaces of the underworld, where humans have feared to tread but have trod nevertheless for thousands of years, hiding treasure, burying the dead, making shrines. The subtitle of Macfarlane’s new book, Underland: A Deep Time Journey, suggests the ancientness of our fascination with the hidden worlds beneath our feet—worlds to which he transports readers with the most elegant of prose surrounding the most imponderable of mysteries, such as the staggering diversity of subterranean fungi, about which he writes, “All taxonomies crumble, but fungi leave many of our fundamental categories in ruins.”

Underland Macfarlane roams the world to find unusual places, such as a vast network of caves in southern China that “was found to possess its own weather system” and the Iron Age mines of southern England. The place that perhaps made the biggest impression on him, he says from his English home, is what he calls “the humming, roaring blue tube of a glacial moulin on the vast Knud Rasmussen glacier in East Greenland.” He rappelled into the vertical shaft left by melting ice to experience what the world is like from the underside of a slowly moving river of ice, and not without fear. “I remember thinking,” he continues, “as I lowered down into the glowing blue of the moulin meltshaft, that I felt as if I were dropping into a pore in the skin of an immense creature, possessed of a pliant, patient liveliness.”

Underland is a book that has been nearly 10 years in the making, inspired by three subterranean events that occurred at roughly the same time: the underground imprisonment of the 33 Chilean miners deep below the Atacama Desert, the eruption of Iceland’s Eyjafjallajökull volcano, and the explosion of the Deepwater Horizon in the Gulf of Mexico. That the three events happened in such quick succession seemed a sign to get into the bowels of the world for a good look. As he was finishing his text, he recalls, a team of Thai soccer players was rescued from a cave after having been lost in the underworld for two weeks, a sign that it was time to leave such venues behind. “I had no idea when I began it almost a decade ago,” he says, “that Underland would become the longest, darkest, strangest, most wondrous and ‘deepest’ book I’ve written, and possibly ever will write.” Gregory McNamee is a contributing editor.