Get a taste of what awaits you in print from this compelling excerpt.

"Off your bums, you pillocks," the one-armed man said. "We've got two more mountains to climb." Partly out of curiosity, but mainly because I was much  more inclined just then to sit on my bum than to pull my battered body up another mountain, I stalled for time with a question: "What's a pillock?"

"You don't know the word 'pillock?' " laughed Pete Crow, the comedian on our team of climbers. "Don't you Yanks speak English?"

"A pillock is a bloke who's always faffing around," piped up Phil Barnes, a powerful man with legs like tree trunks stuffed into black British Army boots. When I looked mystified at that, Kelvin Highmore, the kindly father figure of our team, came to my rescue. "He means, you know, a stupid berk," Kelvin said. "A skiver. A wally."

By now other members of the group were throwing out their own definitions. But my lesson in British slang came to an abrupt end when Tom Perkins, our one-armed captain, pulled a glove onto his left hand with his teeth and started trotting up the rocky slope of Scafell Pike, England's highest mountain. We all scrambled to catch up. In a flash, the seven of us were off on the second leg of Great Britain's notorious Three Peaks Challenge.

At that point I did feel like a pillock, a faffer, and a genuinely stupid berk just for taking on the Three Peaks, one of the more grueling endeavors yet devised by the fiendish minds of people who climb mountains for fun. The Challenge is a demanding test of stamina, logistics, and sheer willpower, but it stems from a simple geographic fact: While Britain has produced some of the world's greatest mountaineers, the country has no great mountains of its own.

British climbers regularly train on Ben Nevis, a flinty bald peak in the Scottish highlands. By official measure of the national Ordnance Survey, its summit is the highest point in Great Britain. But that's like being the longest hole on a miniature golf course. Ben Nevis rises an underwhelming 4,408 feet (1,344 meters) above the silver-gray lochs of central Scotland.

The trek from sea level to summit is not exactly a walk in the woods (for one thing, there are no trees on the Ben's stony escarpment), but sending a veteran mountaineer up Ben Nevis is the equivalent of asking a four-star chef to order in pizza from Domino's.

To overcome this altitudinal shortfall, serious British climbers sometimes cram two or three different summits into a single day's outing. The mother of all these multi-mountain climbs is a trek with pan-Britannic sweep, taking in the highest peaks in each of the three countries that make up the island of Great Britain. That's the Three Peaks Challenge: You have to climb the tallest mountains in Scotland (Ben Nevis, 4,408 feet [1,344 meters]), in England (Scafell Pike, 3,205 feet [977 meters]), and in Wales (Snowdon, 3,560 feet [1,090 meters]).

And you have to do them all in 24 hours.

Get the whole story in the pages of National Geographic magazine.

This June the British Fire Service celebrates its 20th Three Peaks Challenge. Fire Service Three Peaker "historian" Ludo Macauley of the Salisbury Fire Station first began organizing teams in 1983 after a colleague's wife died of cancer. "The firefighters wanted to do something for the charities that had helped the family," he notes, "and at the same time catch the imagination of the public that would be sponsoring them."  The first team of 19 firefighters completed the Challenge in 23 hours, 43 minutes and raised over £2,200 ($3,400) for a Salisbury hospice organization. Today over 120 teams totaling 900 people will raise as much as £150,000 ($240,000) for charity. A third of those teams will complete the Challenge in under 20 hours. Why do it? "Charity is a powerful motivation for some," says Macauley, as is competition and speed. But some do it for the mere enjoyment of "the tough experience ... one that participants are willing to return to year after year."

—Mary Jennings