THE PROVENANCE OF THAT hundred thousand dollars, which was assembled through small and large donations from every corner of the U.S., was as diverse as America itself. It had come from Muslims and Hindus, Christians and Jews, Buddhists and atheists, and maybe even a Wiccan or two. Some of it was sent by a Baptist youth group in Alabama; some came from a North Carolina chapter of Future Teachers of America; and some was a gift from veterans of the 82nd Airborne Division. From Massachusetts, a 12-year-old girl named Gabby contributed $50 that she'd earned babysitting. Lorraine, a single mother from Simi Valley, California, sent in a box of pennies worth $7.47, followed later by a check for $25. From San Diego, an 82-year-old woman named Hannah, retired and living on a shoestring, wrote to say she wanted to help with what little she could spare. She gave $100.

Regardless of where the money originated, all of it had been funneled through a three-story office building at the east end of Main Street in Bozeman, Montana, the headquarters of the Central Asia Institute (CAI), an operation created to honor a promise made in 1993. That was the year Greg Mortenson—the fellow now catching his breath inside the converted shipping container that serves as our departure lounge inside the Kabul airport—attempted to climb K2, the world's second-highest mountain. He did so while wearing a shalwar kameez, taking on this killer peak with a group of 12 underequipped climbers who were known as the Rejects.

Mortenson had launched the climb in memory of his younger sister Christa, who'd suffered from epilepsy and had died of a seizure on her 23rd birthday. After exhausting himself by taking part in the successful high-altitude rescue of another climber, he was forced to turn back 2,000 feet shy of the summit. Then, while trekking down the 39-mile Baltoro Glacier, Mortenson got separated from his Balti porter and wound up staggering into a village called Korphe, a place so destitute that roughly one in every three children perished before the age of one. It was in Kor­phe that he was offered shelter, food, tea, and a bed. And it was in Korphe, one afternoon during his recuperation, that he came across 82 children sitting outside writing their school lessons with sticks in the dirt—with no teacher in sight. One of them was a girl named Chocho, who made Mortenson promise to come back someday and build them a school.

The fulfillment of that pledge—the subsequent chain of events through which a lost mountaineer found his life's calling by promoting literacy in the impoverished Muslim villages of the eastern Himalayas—is a story that, by now, a lot of people have heard. Two years ago, with the help of co-author David Oliver Relin, Mortenson, now 50, laid out his unusual saga in Three Cups of Tea, a book that tells the tale of how he sold his car and his climbing gear and started raising money for schools—literally one cent at a time, beginning with a school in Wisconsin where the kids filled two 40-gallon trash cans with 62,345 pennies.

When the book was published, in March 2006, it debuted at number 14 on The New York Times' nonfiction bestseller list before dropping off. Then, starting last year, sales began to explode. Driven by a surge of interest from women's book clubs and churches, the book climbed to the top of the paperback list, where at press time it had spent more than 27 weeks at number one. As of October, Three Cups of Tea had sold 2.1 million copies, been printed in 19 languages, and spawned a sequel—due out next fall—that will complete the dirtbag-to-rock-star chronicle of a mountaineer who, by all reasonable expectations, could easily still be living out of the back of his car.

In the 15 years since his Korphe experience, Mortenson has made 37 trips to Pakistan and Afghanistan, logging nearly 250,000 air miles a year while organizing the construction of 78 schools that currently serve some 28,000 students. He's also put together graduate-scholarship and teacher-training programs, paid to build women's vocational centers, and underwritten water-purification projects. He has probably spent more time, traveled more miles, and come to know the region more deeply than any diplomat, soldier, academic, or aid worker in America. "Greg is my hero," says newsman Tom Brokaw, who donated $100 in 1994, in response to a hand-typed letter from Mortenson that hundreds of other prominent recipients ignored. "This gentle but determined young man is winning hearts and minds one school at a time, in a part of the world where we rely too much on the military response and not enough on the humanitarian and social approach."

As Three Cups of Tea's success continues, the Mortenson juggernaut shows no sign of letting up. In the past year, he's made roughly 150 appearances in more than 100 U.S. cities, sharing his story with crowds of up to 20,000—a big change for a man whose slide shows, until just a few years ago, drew only half a dozen bored shoppers at REI and Patagonia stores. The CAI's gross intake has grown from $480,000 in 2002 to nearly $4 million last year. These days, tickets to Mortenson's presentations sell out within hours, hosts often have to book a second venue, and most speaking requests must be made a year in advance.

"The irony is that we're actually trying to slow down our growth," says Mortenson, who has the weary look of a bear in serious need of hibernation. "Yet the harder we try, the faster it seems to grow."

Through it all, Mortenson has struck a chord with groups that don't usually see eye to eye. In addition to speaking before Rotary clubs, in public libraries, and at college commencements, he has appeared at Islamic schools in Chicago, at a synagogue in New York City, and before a group of lesbian activists in Marin County.

And then there's the U.S. military, currently in its seventh year of deployment in Af­ghanistan, where coalition casualties started exceeding those suffered in Iraq last May. In November, Mortenson—who served as an Army medic from 1975 to 1977—was asked to share his views about Pakistan and Afghanistan with General David Petraeus, whose focus on building relationships with local communities dovetails with the CAI's approach. Sometime this winter, Mortenson is slated to do the same with the office of Admiral Mike Mullen, chairman of the Joint Chiefs of Staff. He has lectured at Annapolis, West Point, and the Air Force Academy. Three Cups of Tea is now recommended reading for officers enrolled in graduate-level counterterrorism courses in the Army, Navy, and Marines.

Mortenson is also widely respected overseas—rare for an American these days. He has given presentations in several countries, including Sweden, Tanzania, and the United Arab Emirates. Pennies for Peace, an offshoot of the original Wisconsin piggy-bank drive, now has programs running in Europe, Asia, and South America. Last year, it was picked up in South Korea, China, and Rwanda.

Mortenson's visibility, however, has put him in the crosshairs of extremists and crackpots. In Pakistan, two religious clerics issued fatwas (since rescinded) calling for his expulsion. In August 1996, he was kidnapped in Waziristan and held for eight days before being released. Earlier this year, a CAI school in Afghanistan was attacked by the Taliban.

There have been some nasty reactions at home, too. "In the U.S., I get quite a bit of hate mail and criticism," Mortenson says. "People have attacked me for working with Muslims, for acting like a colonialist, for being a traitor to America. But we continue to operate, because we have fierce local support. The communities we work in will do anything for education, and as long as they want schools, our mission is to provide them."