If I were a shrink, I’d worry about Robert Macfarlane – his dicing with eschatology, his claustrophilia, his recklessness, some of the company he keeps (sewer punks, cavist ultras, grotto mystics). But I’m not: I’m merely a repeatedly delighted fan of a true original. Macfarlane is a poet with the instincts of a thriller writer, an autodidact in botany, mycology, geology and palaeontology, an ambulatory encyclopedia – save that much of the time (a dodgy word in this context) Macfarlane does not ambulate but hauls himself feet first through tunnels the circumference of a child’s bicycle wheel in absolute darkness where day, night, maps and GPS do not exist. That’s when he is not being driven at absurdly high speed through potash mines beneath the North Sea’s shipping lanes by a gung-ho security specialist or lifting a rust-flaked manhole cover to gain admittance to some nether world or trespassing in a government’s subterranean chambers. When this orphic mole comes up for air he relaxes by climbing the transporter bridge high across the Usk at Newport.

His obsessive pursuits and explorations are all potential killers. That, evidently, is part of their appeal. Risk is like a drug. Maybe ‘like’ is redundant. Macfarlane does not delude himself. He lists cavers and divers who have died. Some have disappeared only to be found years later, while some have got trapped in places from which neither they nor their bodies can be removed: burial in slow motion. He writes, ‘I could only understand these pursuits … as fierce versions of the death drive … But over time I saw that … there was another aspect to the thanatos at work. Divers and cave divers often describe their experiences in terms of ecstasy and transcendence.’

That polarisation or separation is not, however, apparent in his own subterranean jags, which are described like unusually scholarly adventures and which grip the reader. The sense of mortality is more potent than that of wonder or sublimity. How will he get out of this fix? We know that he will get out, just as we know that, say, Richard Hannay and Allan Quatermain will survive whatever scrapes they have got themselves into.

That knowledge does not preclude the primitive, powerful urge to want to discover what happens next, even as Macfarlane is listing terms from the lexicon of extreme caving: ‘terminal sump’, ‘chokes’, ‘dead out’, ‘the dead zone’. These pitiless words could equally describe terranean loci; he quotes Anselm Kiefer, ‘There is no innocent landscape, that doesn’t exist.’ Well, if you’re Kiefer it obviously doesn’t because that great artist creates his own landscapes with an emphatic bias towards the horrors he was born into. Macfarlane, a great artist in an unclassifiable synthesis of disciplines, is a generation younger and, to put it baldly, has to go looking for trouble. It’s not an all-enveloping mantle of guilt and shame and having to face up to the abominable. Nonetheless it’s there all right, in forms as lethal as nuclear waste (potentially deadly for 100,000 years) buried in southern Finland, as magnetically enchanting as a labyrinth that pulls you into its depths while whispering sweet promises of death, and as banal as an abandoned Welsh slate mine that is now a de facto wrecker’s yard for old smokers (he spots a blue Cortina estate and a moss-green Triumph Herald).

It’s this sort of site that prompts Macfarlane to counter Philip Larkin’s overquoted ‘What will survive of us is love’. ‘Wrong,’ he writes. ‘What will survive of us is plastic, swine bones, and lead-207.’ In other words, a souvenir gift pack of the Anthropocene, which will endure along with what is already down there mutating very slowly indeed.

The book’s settings are various. But in all of them darkness, discomfort and danger provide the decor. Such is the intensity of Macfarlane’s prose that the negative becomes the positive, the subterranean turns into the quotidian, the creatures of the blackness just go about their routines and the exceptional is the rule.

The result is that when Macfarlane writes about his fellow explorers in their homes on the surface of the earth they acquire a peculiar exoticism. Drinking tea with Sean and Jane Borodale in their Mendips cottage takes on the quality of a bizarre rite. A retired Norwegian fisherman talking about how he built a stucco mini-palace for his Indonesian girlfriend’s nail parlour business has the feel of someone describing a kind of arcane mythology. These terrestrial diversions adhere to a different time scale from the nether world, where the rules of chronometry are exposed as a Swiss con job.

The imaginative young scientist Merlin Sheldrake, a scholar of flowerless plants, talks about mutualism and the subterranean non-parasitical symbiosis of mycorrhizal fungi and certain trees. This is initially familiar stuff, but he then refers to a ‘wood wide web’, a mycelial network in which plants exchange nutrients and other compounds with each other. Sheldrake’s qualified animism connects with evidence-based Western scientific orthodoxy. He has an attractive second string: he makes mead, cider and a coca drink. He also has friends who bring guitars, harmonicas, drums and bones to Epping Forest, where he and Macfarlane sleep under the stars. All this campfire excitement, like some moot of the Order of Woodcraft Chivalry, pushes Macfarlane, ‘despite a learned wariness’, towards anthropomorphism. But by the next chapter he appears cured.

On occasions he writes with a campaigning spirit, for instance against the oil drilling taking place around the Lofoten Islands, six hundred miles north of Oslo. The situation is all too familiar: powerful industrial interests ranged against an alliance of fishermen, environmentalists and those who see tourist opportunities. What lifts Macfarlane’s account far above the routine is his relation of a meeting with a remarkable fisherman called Bjørnar Nicolaisen: ‘We face death every morning,’ he says, ‘to bring food to those idiots … politician-idiots.’

Nicolaisen is contemptuous, as any sentient person should be, of the so-called ‘consultation’ process – the usual PR mendacity. In this instance the Norwegian Petroleum Directorate gave itself permission to carry out seismic mapping. But Nicolaisen is an inspired organiser who eventually not only prevented oil extraction but also helped to bring this assault on the planet to the forefront of the nation’s consciousness. Macfarlane shares Nicolaisen’s passion and probity. Underland is a moral hymn to the strangeness of existence and a sharp warning not to take anything for granted.