Why would a mountaineer risk an icy tomb to conquer one more peak? Why run across Death Valley 19 times? Why traverse the Atlantic in a kayak? The answer, learns Mike Higgins, lies in spiritual bliss. Plus our intrepid reporters reveal just what it's like to brave the elements in search of a trip to remember

Joe Tasker, one of the world's most daring and accomplished mountaineers, disappeared in 1982. His death, on Mount Everest, posed a tragic question for Maria Coffey, the young girlfriend he, once and for all, left behind: why do certain people time and again leave their partners and children, families and friends to undertake extremely high-risk expeditions, putting themselves through eye-watering deprivations to do so? Despite the wealth of adventure literature, Coffey realised that most accounts – including Tasker's own classic mountaineering book Savage Arena – preferred to deal with the nuts and bolts of their achievements, rather than dwell on anything more, well, airy-fairy.

So Coffey managed to get some well-qualified mountain types to stop talking about crux pitches and summit pushes for a minute and, in her 2003 book Where the Mountain Casts its Shadow, extracted the decidedly sheepish admission that mountaineering provided a sense of spiritual transcendence these men and women could find nowhere else. Their frankness intrigued her; the result was her next book Explorers of the Infinite, an attempt to extend this question to snowboarders, ultra-runners, free-divers, round-the-world yachtsmen... anyone who pushes themselves to extremes in the outdoors.

She wondered what she was letting herself in for. "I was particularly worried about talking to some of my old climbing mates from England," Coffey tells me down a crackly line from South-east Asia. "In North America people are more open to expressing these experiences. But, with a few exceptions, people were actually quite relieved to tell me [their stories], particularly with the paranormal stuff, because they thought anyone they told would think they were crazy."

Delving into her book (as yet unpublished in the UK), you can see why they might have been reticent. There's the Mexican mountaineer who believes he was guided down an 8,000-metre-high mountain by the ghost of an old friend; the ultra-runner who has run across Death Valley 19 times and sees the experience as a "portal" into another realm. Coffey herself touchingly admits to "a premonition of my lover's death on Everest and 'visitations' from him after the news was confirmed". But before you wonder whether Coffey and her interviewees are one paddle short of a kayak, the writer is quick to reassure the more rationally minded. These adventurers, she says, aren't just globetrotting navel-gazers; they're able to offer raw, anecdotal insights on topics from neuroscience, theology and anthropology to precognition and telepathy.

But first, says Coffey, we need to find a language to be able to talk about these experiences. She has a point. In her book, Coffey quotes the French extreme skier Patrick Vallençant: "To ski a very steep slope is completely beautiful... I become part of this cosmic dimension... all the beauty of the world is within the mad rhythm of my blood." You might have experienced what he is talking about for yourself, but you probably don't want to end up sounding like him in trying to express it.

Reverend Neil Elliot, an Anglican vicar who wrote his PhD on the spirituality of snowboarding, approaches the problem from another direction. God, according to Elliot, can be found boarding in deep powder snow, or "soul-riding" as it's called. "Suddenly, everything was flowing," recalls the Rev Elliot from his snowboard. "I was both in and out of time. There and not there... I was very close to God at that point." But while stomping powder is a short cut to the Almighty for Elliot, he acknowledges that these "raw spiritual experiences" can lead to "non-institutional spirituality". As the neuroscientist Susan Greenfield has said, in these moments "our brains can... revert back to the state where our carefully controlled life fades... where we literally let ourselves go... A higher plane of pleasure can often be identified with joy and in turn with religious experiences."

All of which sounds like fun. What happened to Alan Burgess most certainly doesn't. Twenty thousand feet up a Himalayan mountain, he and the two fellow mountaineers he was roped to were caught in a massive avalanche: "That's when Burgess did the impossible," relates Coffey. "He freed his hips and hurled himself upward. Like a Hollywood martial artist, he did a backflip out of thousands of tons of avalanching snow. And then he held the two other climbers." Burgess is modest about his unlikely feat, claiming he just reacted and "did what my training had given me". Yet many of those undertaking arduous journeys must wish they could conjure up such superhuman performance at will.

One case in point is a German doctor named Hans Lindemann. Lindemann had already powered himself across the Atlantic in an open canoe when he decided to repeat the feat – in a kayak, in which he couldn't even lie down. His psychological preparation was formidable. Lindemann believed fear was his biggest enemy, and that he could dispel the cold, the damp and his own doubts through meditation and "auto- suggestions", such as "I shall succeed." He set off from the Canary Islands in October 1956, ate raw fish, turned down food from a passing freighter, and was capsized by storms, but: "I felt enormously content, as though I were in another world in which I had no discomfort... where I no longer heard the incessant howling of the wind and knew nothing of my desperate situation." Seventy-two days later he stepped ashore on the Caribbean island of St Martin, four stone lighter, but alive.

Lindemann had been inspired by his encounters with voodoo and its capacity for deep, powerful concentration. And what emerges in Coffey's study is the greater sensitivity for the natural world in non-organised, non-Western belief systems. Take Karsten Heuer and Leanne Allison's experience following a caribou herd on foot for hundreds of miles across Alaska. They soon found themselves accepted by the herd, walking like caribou, and able to hear better than ever. In among these smelly, sweaty, snuffling beasts, says Heuer, "my mental clutter had been shed". The pair's experiences were met with surprise and approval by the native Gwich'in people, to whom that particular caribou herd was sacred. What is perhaps most interesting about Heuer and Allison's months with the caribou is that Heuer was a healthily sceptical scientist before their journey. "He emerged transformed by his bizarre experiences," says Coffey. "And Karsten said he hopes that these things are never explained – he'd rather the mystery remains."

Repeatedly in Explorers of the Infinite there is the sense that humanity has lost a sense of communion it once had with the natural world. A loss which is a fair exchange for hot showers and central heating, you might say. But Coffey reminds us that we have sacrificed a lot. She mentions the "primitive" Indian Ocean tribes who sensed the tsunami of 2004 and retreated to high grounds. And what has happened to the phenomenal skills of the Polynesian islanders who, more than 2,000 years ago, successfully navigated the 16,000,000 square miles of the Pacific to colonise its islands? All but gone.

And so Coffey's book goes on, wrestling with psychic powers, "precognition" and phantom climbers, more than willing to lend a sympathetic ear to these fantastic accounts. "I've always tried to avoid flaky science," she says. "[But] I began to realise that consciousness studies are very exciting – does our mind extend beyond our brain? I believe the paranormal experiences I write about will eventually be explained."

But are these weird and wonderful experiences available to we mortals? Well, ultra-marathons are booming, attracting tens of thousands every year. But there is an easier route to enlightenment: with her husband, Coffey runs an adventure-travel company. I ask if she's able to provide her clients a transcendental experience on their trips, but she shies from the question. "I think it's better described as putting our guests on a 'path of magic'. We lead them to special places that we know well, such as Halong Bay in north Vietnam. It's a huge maze of strangely shaped limestone islets rising sheer from calm jade waters, and paddling there is like stepping through the looking glass. Kayaking slowly, we hug the islands, suggesting at times that we all be very quiet and introducing elements of surprise – such as the long cave opening into a hidden lagoon inside a hollowed-out island." Does it work? "I've watched hard-bitten corporate lawyers become entranced and transformed out there, admitting to a new sense of awe, wonder and peace."

For more information, visit hiddenplaces.net. 'Explorers of the Infinite' is published by Tarcher/Penguin USA