Underground is also beautifully written. Hunt is attuned to the smells and textures of subterranean places (in the dark, visual comparisons don’t get you very far), to the “earthy, almost pastoral aroma, like rain-soaked chalk” of the Paris catacombs, and the “old farm shed full of fertiliser” he smells in the New York sewer system. Below Manhattan on summer nights, he says, “you can almost smell the city’s granite bedrock”.

Hunt’s instincts are journalistic rather than scholarly, however, and if I have one frustration with this book it is that it contains no notes or bibliography. 

Hunt’s journey through time and the human psyche is so sweeping (from micro-organisms to the roots of shamanism) that it can be quite an exhilarating experience. But even if that sounds too esoteric, chapters like the aforementioned Paris expedition, and how Hunt’s quest for the NYC graffiti artist REVS connects with 14,000-year-old clay bisons in a cave in the Pyrenees, are thrilling pieces of travel writing. One way or another, Underground will permanently change the way you see the ground beneath your feet.

The connections Hunt makes — for instance, between his own experience as a teenager exploring an abandoned train tunnel in Providence, R.I., and a young Mexican man who discovered the walled sanctuary within the ancient Mayan Balankanche cave — sometimes feel strained. Similarly, some of his sweeping conclusions (for instance, that “our connection to caves may well be our most universal, most deeply inscribed, perhaps our original religious tradition”) seem overblown. That said, if not taken too seriously, Hunt’s musings on our relationship to the underground world, drawing on literary, academic and mythological sources, are both provocative and satisfying.

Throughout this fascinating book he ties together the ancient and the modern, the familiar and the distant with remarkable deftness and sympathy. In the end, examining his own compulsion to go deep, he seeks total isolation in a natural cave, in ‘heavy, ancient dark, Book-of-Genesis dark’. Deprived of sight, he writes, ‘my thoughts earthwormed down inside my body, chewing through my inner architecture. It was the feeling of being peeled open, turned inside out.’ Sensory deprivation of this kind has been used to torture and to brainwash – and, as in the case of Pythagoras, as a way to go on a spiritual odyssey. We go deep to get high.