The book’s cover is a painting by Stanley Donwood, apparently of a vast sun rising beyond twining branches at the end of a sunken lane. I took it at first as a suggestion of an emergence; a return from dark into light. Robert Macfarlane was similarly misled: ‘When I spoke to Stanley about it he said, ‘No – that’s in fact a nuclear explosion. That solar glow is the last thing you will ever see as the blast wave rushes towards you’.’

Which makes it singularly appropriate to the atmospheres Macfarlane conjures in his latest – and darkest – book, Underland: A Deep Time Journey. In it, he travels into the world beneath our feet and what we have made of it – physically, with mines and tombs, and metaphorically, with myths and legends. It takes him from Bronze Age funeral chambers in Somerset, via the catacombs of Paris to a nuclear bunker in Finland.

It’s a book that expands our notions of what constitutes landscape. It’s one full of wonders – in Kulusuk, Greenland, he celebrates ‘the wildest land I have ever seen’ – but also of warnings about the harm we are doing in this overheated age of the Anthropocene, when ‘permafrost’ has been rendered nonsense and ‘glacial pace’ can mean rapid melting.

At 42, Macfarlane still has a frame fit for potholing, but he has had to spend a lot of time recently in dark, poky places. ‘I didn’t just have to learn to see in the dark,’ he told me. ‘I had to learn to write about darkness, both the darkness of human actions and the darkness of matter.

‘My natural inclination is to celebrate and towards joy. There is, I hope, a lot of joy in this book, but there are also chapters about atrocities and reprisal killings, about nuclear waste, about trauma, how landscapes hold memories of trauma. But it feels like the time that I’m writing into, really.’

He couldn’t have picked a better moment to surface, either, following the demonstrations by Extinction Rebellion in London, Sir David Attenborough’s prime-time exposition of climate change on BBC One, and the popularity of Greta Thunberg, the 16-year-old Swedish activist.

[The uneasy link between a writer’s life and work]

Some of that writing comes out of schools, often inspired by The Lost Words, his collaboration with the artist Jackie Morris, in which they have helped to ‘rewild’ the language of a generation that was more familiar with attachment, bullet-point and chat room than with acorn, bluebell and conker.

In Underland, Macfarlane writes of how dark matter scientists, working more than half a mile below the earth’s surface, study the birth of the universe; of how glaciologists drill down into the extraordinary archive that ice holds, partly so they can foretell the future of climate. He says he often imagines a ‘spectral, post-human palaeo-geologist’ examining the rock record of what we are leaving behind, the marks of an addiction to fossil fuel.

In Underland, Robert Macfarlane writes of how dark matter scientists, working more than half a mile below the earth’s surface, study the birth of the universe

He adds: ‘I think we will look back at this anomalous 40 to 50 years of plane travel – extensive plane travel really only began in the seventies – in astonishment at the profligacy of it.’ I mentioned the American writer Amy Irvine’s suggestion that lovers of the great outdoors consider taking ‘a vow of environmental chastity’ and foregoing their weekend fling with the wild to reduce their carbon footprint: ‘Can we love it enough to... gulp... stay home?’

Macfarlane says he has taken vows of his own: ‘I gave up eating meat a long time ago. My six-year-old hasn’t got a passport. I didn’t fly long-haul for eight years... I will be travelling around America to talk about [the Anthropocene], and I thought really hard about that. So, the answer is: I’m a sinner like the rest, but this doesn’t go unexamined.’

Can he imagine a time when he would feel it necessary to deny himself an escape to his beloved Cairngorms? ‘No. In the economies of carbon, that’s very low down the list. It may all come to that in 20, 30, 40 years’ time, but I think a sort of utter, battened-down puritanism is not going to be successful either.’

From peaks to pits

Over five books and 2,000 pages, Robert Macfarlane has travelled from the heights to the depths, with the aim of mapping the relationship ‘between landscape and the human heart’. The other four books are:

Mountains of the Mind: A meditation on Macfarlane’s own fascination with high ground, and a cultural history of how our perception of mountains has changed ‘from superstitious fear to secular worship’.

The Wild Places: Macfarlane undertakes a series of journeys through Britain and Ireland, making visible the wilderness left, and recording what may soon vanish.

The Old Ways: The author walks ancient routes everywhere, from the chalk downs of England to the bird islands of north-west Scotland, to the sacred landscapes of Spain and the Himalayas.

Landmarks: Our dialect words to do with place are disappearing in an increasingly denatured world. Landmarks is partly a gathering of such terms, partly a demonstration, through the work of some of Macfarlane’s favourite writers, of the spells they can weave.