When the weather is good, or when I'm tired of having seven million neighbors, I drive north from downtown Beijing. It takes an hour and a half to reach Sancha, a quiet village where I rent a farmhouse. The road dead-ends at the village, but a footpath continues into the mountains. The trail forks twice, climbs for a steep mile through a forest of walnut and oak, and terminates at the Great Wall of China.

Once, I packed a tent, hiked up from the village, and walked eastward atop the wall for two days without seeing another person. Tourists rarely visit this area, where the wall is perched high along a ridgeline, magnificent in its isolation. The structure is made of stone, brick, and mortar; there are crenellations and archer slits, and guard towers that rise more than twenty feet high. The tallest one is known locally as the Great Eastern Tower, and just before it an inscribed marble tablet sits on the wall. Originally, there were many such tablets, but this is one of fewer than ten that are known to remain on the wall in the Beijing region. The inscription notes that in 1615 A.D. a crew of two thousand four hundred soldiers built a section of the wall which was fifty-eight zhang and five cun long. That's about six hundred and fifty feet, and the bureaucratic precision of the inscription, in this forgotten place, seems as lonely as words can be.

In November of 2005, I hiked to the Great Eastern Tower with two friends who were visiting from New York. After reaching the tower, we began the long descent to the south. This stretch can be treacherous: many of the brick ramparts have collapsed. I was picking my way downhill when something in the rubble caught my eye. It was white--too white to be brick, too big to be mortar. I dug it out and saw four neat rows of carved characters.

It was a fragment of another marble tablet. I could make out some of the words: something was six chi high, and something else was two zhang. But the writing was in classical Chinese, which I've never studied, and the surface was badly scarred.

"How long do you think it's been buried here?" one of my friends asked.

"I have no idea," I said. But I knew one person who might be able to tell me. My friends and I covered the artifact with loose bricks, and I memorized the location. A month later, I returned with David Spindler.

David Spindler stands six feet seven, and he is reserved in a way that is characteristic of many men who are very tall and thin. For years, I'd seen him occasionally at social gatherings in Beijing, where he always seemed to make sure that he was sitting down. He is thirty-nine years old, with short sandy hair, a long face, and gentle eyes. He chooses his words very carefully. Since 2002, he has worked full time as an independent scholar of the Great Wall, and although he has no academic affiliation or outside funding, he has become one of its leading researchers.

On a cold December morning, Spindler and I set off to find the marble tablet. In the city, everything about his appearance had seemed chosen to avoid attention. But in the mountains he wore a red-checked wool hunting shirt, a floppy white Tilley safari hat, high-end La Sportiva mountaineering boots, and large elk-leather gloves designed for utility-line workers by J. Edwards of Chicago. He looked like a scarecrow of specialty gear--some limbs equipped for hard labor, others for intense recreation. Over the years, Spindler had determined that this was precisely the right ensemble for the Great Wall, where thorns and branches are common. For a face mask, he'd cut a leg off a pair of sweatpants, scissored a round hole, and pulled it over his head. ("It covers your neck.") He wore polyurethane-coated L. L. Bean hunting trousers that had been reinforced by his neighborhood tailor. Denim patches covered the pants, like a friendship quilt linking Freeport and Beijing.

We followed the wall east. Every hundred yards or so, it connected to a tower. These structures were crumbling but still impressive, with high vaulted ceilings and arched windows. Periodically, Spindler pointed out details: a place where a door used to be barred, a brick frame that had once held an inscribed tablet.

"The towers and the wall were totally different projects," he said. "First, you had the brick towers, and the wall was just local stone. And then they came and improved the wall. That's why these towers look a little funny."

He pointed out a place where the wall's crenellation ran into the open window of a tower--the kind of thing that happens when you use two different contractors. Near the Great Eastern Tower, one section of wall had fallen down entirely. Spindler believed that the construction project of 1615 had ended right there, at the edge of a short precipice. He had measured it once, using the details found on the marble tablet near the tower. "These guys really hosed the next construction crew," he said. "What could they do? It's really hard to build from that point."

I had hiked this section perhaps fifty times, but I had never noticed the details of construction. In my mind, it was simply the Great Wall--complete and virtually timeless. For Spindler, though, it was a work of pieces and seasons. Construction generally took place in the spring, when the weather was good but Mongol raiders weren't active. "Energy in the Mongol world was fat on the horses," Spindler said. "They didn't have that after the winter, so the spring was not a good season for raiding. Summer was too hot. They didn't like the heat; they didn't like the insects. The Mongol bowstrings were made of hide, and with the humidity they supposedly went flat--this is described in Ming texts. Most raids took place in the fall."

We came to the smashed tablet, and he crouched in the cold, running a finger along the carved characters. He recognized it immediately as a piece of a tablet that dated to 1614. The county antiquities bureau had recorded its inscription in 1988, but not its original location on the wall, and since then it had disappeared. Some looter had probably broken it.

"It's saying the height of the wall, including the crenellations," he explained. "And then it starts in with all the names of the officials. God, it's good that somebody got this down before it was destroyed."

Spindler took a tape measure to the fragment, calculated the space between lines, and quickly computed the original dimensions. Slowly, he walked back along the wall, looking for a place where it could have been mounted. He measured an empty brick-bordered ledge: perfect fit. For this small section of the wall, he now knew the basic story of two construction campaigns in the sixteen-tens. Before leaving, we returned the fragment to the spot where I had found it.

While we were there, a peasant hiked up from the south. He was trapping game; a dozen wire snares were looped over his shoulder. If the presence of a six-foot-seven foreigner in a floppy hat and elk-leather gauntlets surprised the man, he didn't show it. He asked if we had extra water, and Spindler gave him a bottle. During the following year, Spindler and I hiked through several villages together, and each time locals hardly seemed to distinguish between the two of us. Andrew Field, a friend of Spindler's who teaches Chinese history at the University of New South Wales, once told me that an unusually tall person might feel more comfortable here than in America. "In China, sure, he's a monster," Field said. "But aren't we all?"

In 221 B.C., Qin Shihuang became the first ruler in Chinese history to declare himself emperor. After consolidating power, he commanded the construction across the north of roughly three thousand miles of changcheng. The term translates as either "long wall" or "long walls"--Chinese doesn't differentiate between singular and plural--and the barriers consisted of hard-packed earth. Over the centuries, many other dynasties faced the same basic problem as the Qin: the wide-open frontier of the northern plains made them vulnerable to the nomadic Mongol and Turkic tribes that inhabited these lands. The nomadic threat was more intense in some periods than in others, and Chinese dynasties responded with different strategies. The Tang, who ruled from 618 to 907 A.D., built virtually no walls, because the imperial family was part Turkic and skilled in Central Asian warfare and diplomacy. Even when dynasties constructed walls, they didn't necessarily call them changcheng; more than ten terms were used to describe the fortifications.

The Ming usually called theirs bianqiang--"border wall(s)"--and they became the greatest wall builders in Chinese history. They came to power in 1368, after the collapse of the Yuan, a short-lived Mongol dynasty that had been founded by Kublai Khan. The Ming constructed large fortifications of quarried stone and brick in the Beijing region--these are the iconic structures (some of them rebuilt and restored) that seem to continue endlessly in tourist photographs. They were the only dynasty to build extensively with such durable materials, and many sections of Ming wall ran for miles. But the bianqiang was a network rather than a single structure, and some regions had as many as four distinct lines of fortifications.

In 1644, domestic rebels stormed the capital, and the Ming emperor committed suicide. In desperation, a military commander in the northeast opened a major bianqiang gate to the Manchus, a northern tribe, in the hope that they would restore the ruling family. Instead, the Manchus founded their own dynasty, the Qing, which lasted until 1912. The Qing had little use for the walls, which were abandoned to the elements.

As Western explorers and missionaries began to penetrate China in the eighteenth century, they toured the Ming ruins and confused them with stories of Qin Shihuang's three-thousand-mile wall. Foreigners assumed that the Beijing region's trellised brick fortifications were part of an unbroken line that had stretched across the north for two thousand years. In 1793, an Englishman named Sir John Barrow saw a section of wall near Beijing and, extrapolating from its measurements, declared that the entire structure must have contained enough stone to build two smaller walls around the equator. (Westerners rarely visited China's west, where most walls were made of tamped earth.) At that time, foreigners usually called it "the Chinese wall," but by the end of the nineteenth century, as the exaggerations accumulated, it had become the Great Wall of China. In February of 1923, a National Geographic article began, "According to astronomers, the only work of man's hands which would be visible to the human eye from the moon is the Great Wall of China." (It wasn't visible from the moon in 1923, and it still isn't.)

Eventually, the misconceptions made their way back to China. Under threat of foreign domination, leaders like Sun Yat-sen and Mao Zedong realized the propaganda value of a unified barrier. Changcheng became the equivalent of "the Great Wall," a term that encompassed all northern fortifications, regardless of location or dynastic origin. It delineated what was, essentially, an imaginary structure--a single, millennia-old wall.

Today, the concept of the Great Wall is so broad that it resists formal definition. When I met with scholars and preservationists in Beijing, I asked how changcheng should be defined, and I never heard the same thing twice. Some said that in order for a structure to qualify as part of the Great Wall it had to be at least a hundred kilometres long; others believed that any border fortification should qualify. Some emphasized that it had to have been built by ethnic Chinese, whereas others included walls built by non-Chinese tribes. Nobody could give an accurate length estimate, because there has never been a systematic survey. Last year, various articles in China Daily described the Great Wall as thirty-nine hundred miles, forty-five hundred miles, and thirty-one thousand miles long.

There isn't a scholar at any university in the world who specializes in the Great Wall. In China, historians typically focus on political institutions, while archeologists excavate tombs. The Great Wall fits into neither tradition, and even within a more discretely defined topic--say, the Ming wall--there's almost no scholarship. The fortifications have been poorly preserved, and in the past many sections of low-lying wall were plundered for building materials, especially during the Cultural Revolution. In the nineteen-eighties, a Harvard Ph.D. student named Arthur Waldron became interested in the relationship between Chinese and nomadic groups. "So I went to the library and thought I would find a big book in Chinese or maybe Japanese that would have everything about the Great Wall," he told me recently. "But I didn't. I thought that was strange. I began to compile a bibliography, and after a while I said, 'This does not add up to the image that we have.' "

In 1990, Waldron published "The Great Wall of China: From History to Myth." Drawing on Ming texts--he didn't conduct significant field research--Waldron described key aspects of wall building during that dynasty. He also identified many modern misconceptions about the wall, including the notion that it's a single structure. It was a breakthrough book, and one that should have provided a foundation for further scholarship. But since then there hasn't been another work of significant new archeological or historical research, apart from one Chinese book by a surveying team that describes a six-hundred-mile series of Ming fortifications in the east. (Another book, published last year, "The Great Wall: China Against the World, 1000 B.C.-A.D. 2000," by Julia Lovell, a fellow at Cambridge, is primarily concerned with exploring the wall as a symbol for the Chinese world view. She draws a parallel, for example, between the ancient wall and the current government's Internet firewalls.)

In China, one of the best-known experts, Cheng Dalin, is not an academic but a retired photographer. For more than twenty years, Cheng specialized in taking pictures of the wall for the Xinhua News Service. In his spare time, he studied history, and he has published eight books, combining photographs and research. "The Great Wall touches on so many subjects--politics, military affairs, architecture, archeology, history," he told me. "Within each specialty, it's too small. And taken as a whole it's too big. You have to find little bits in so many different books; it's not concentrated in one place. And nobody will pay you! How will you eat? How can a person spend ten years reading all these books?"

David Spindler first started hiking the Great Wall in 1994, when he was the only American studying for a master's degree in history at Peking University. He had always been athletic--as a student at Dartmouth, he'd rowed varsity crew and was on the cross-country ski team--and he saw hiking as the perfect break from city life. At Peking University, he wrote a master's thesis in Chinese about Dong Zhongshu, a philosopher in the Western Han dynasty, in the second century B.C. After receiving his degree, Spindler decided against pursuing a career in academia. For a spell, he worked as an assistant in CNN's Beijing bureau, and then he became a China market analyst for Turner Broadcasting. But neither journalism nor business felt right, and the only constant in those years was hiking the Great Wall.

In 1997, he entered Harvard Law School. It was a homecoming--he'd grown up in Lincoln, Massachusetts--but he missed Beijing and found himself searching for distractions. ("I split a lot of wood," he told me.) During his first vacation, Spindler returned to China to hike. By then, he had the idea that in his spare time he could write a book about the Ming-dynasty Great Wall, and he began reading histories. After graduation, he accepted a consulting job in the Beijing office of McKinsey & Company; every free weekend, he hiked or studied Ming texts. Finally, after two years at McKinsey, he quit in order to pursue his research full time. His goals were ambitious: to hike every section of the Great Wall in the Beijing region and to read every word about the structure that was written during the Ming dynasty.

Spindler had paid off his law-school loans, and he had sixty thousand dollars in savings. He expected that it would take him a year or two to complete his field work. He hiked to wall sections, took notes, and recorded details on a spreadsheet. Often, he saw more wall in the distance, and he marked these sitings on another database, which identified future research trips. The to-do list seemed to get longer with every journey. In 1985, a Chinese satellite survey had identified three hundred and ninety miles of wall in the Beijing region, but Spindler found that the fortifications were actually much longer.

He became a fixture in the National Library of China. He read from the Ming Veritable Records, a day-to-day history of the dynasty, and he tracked down the reports of various Ming officials. Sometimes he found a specialized work dedicated to wall defense. Some books could only be located elsewhere, so he spent weeks on the road. In a freezing library in Guangzhou, he read a detailed Ming guide to key wall fortifications; as far as he could tell, the book had not been quoted since 1688. He flew to Japan in order to read an obscure Chinese history written by Yin Geng, an official who served in the Ministry of Defense during the mid-sixteenth century. Spindler spent three weeks in Tokyo, and during that time he ate dinner in a restaurant twice. The other nights, he cooked pasta, cabbage, and tomato sauce and poured yogurt on top. ("It's cheaper than cheese.") In Beijing, he rented a small apartment in a run-down building for two hundred and twenty-five dollars a month. At the Miyun bus station, which is close to many sections of wall, the minivan drivers began greeting him by shouting, "Beidianzi, six yuan!" Beidianzi is a village, and six yuan is the deal Spindler struck after an epic bargaining session that has become part of Miyun minivan lore.

Over four years, he earned a total of six thousand two hundred dollars from the occasional lecture or consulting job. In 2003, he applied for a grant from the National Endowment for the Humanities, which occasionally funds projects by independent scholars. A panel of anonymous academics assessed the proposal, and they were withering. One wrote, "The applicant has no track record as an interpreter of the humanities." Another remarked, "Likelihood of completion: Not clear." The following year, with guidance from former classmates who had become professors, Spindler applied again. This time, the evaluations were positive, at least in the terms of a jargon that is nearly as formalized as classical Chinese. (Panelist 1: "I feel that [the proposed book] would be a quality interpreter of the humanities.") But the application was still rejected.

In Beijing, Spindler dated a former law-school classmate who had become an executive at Siemens. "She was very supportive," he told me. "I couldn't have asked for anything more." But he kept finding more Great Wall and more Ming texts; finally, in 2005, they parted ways. "It's certainly the reason we broke up," he said. "She couldn't see an end to it."

At the time of our first hike, Spindler had been researching for nine years, with the past four devoted entirely to the project. But he had yet to publish one word about the wall, and he had no formal contact with academia. He was extremely cautious, in part because he had grown accustomed to working in isolation. Nobody had ever combined field and textual research in this depth--it would have been impossible for any academic based in the United States or Europe--and his methodology had become as demanding and idiosyncratic as his hiking gear. In his mind, it was pointless to begin writing while his to-do spreadsheet still listed more than a hundred days of hiking.

The numbers consumed him. During our trip to the Great Eastern Tower, he commented that it was the eightieth day he'd spent on the wall that year. Since 2005, he has dated K. C. Swanson, an American freelance journalist who lives in Beijing. "David tends to remember days in relation to his wall hikes," she told me. "One day he told me, 'This is our one-year anniversary,' and he said that we had started dating two days after a certain trip he took. It's like primitive cultures where people date things by when the volcano erupted."

In 2006, Spindler began giving more lectures--his main client was Abercrombie & Kent, a high-end travel service--and his income rose to twenty-nine thousand dollars. He has no hobbies to speak of, and his bookshelf is devoted almost entirely to wall research. He owns five CDs. He has always had many close friends in the city, Chinese and foreign, but few understand his obsession. He never romanticizes the wall, telling its history with the precision one would expect of a lawyer and a consultant. (In describing a series of attacks on the wall in the sixteenth century, Spindler told me that a Mongol leader adopted "what people at McKinsey would call a 'me-too strategy.' ") "A couple of times I've tried to ask why it really gets him," Swanson said. "Maybe it could be easily explained in an emotional way, but that's not how David is. He's an eminently rational person doing what is basically an irrational thing."

In October, I accompanied Spindler on his three-hundred-and-thirty-first journey into the field. By public bus and hired minivan, we travelled to a remote village called Shuitou. In 2003, while visiting the wall here, Spindler had seen some high ridges that he thought might contain more fortifications. In the village, he had also studied a Ming wall tablet that was now kept in a peasant's home. Last year, the Chinese government passed the first national law protecting the Great Wall, and Ming artifacts cannot be bought or sold, but the remoteness of many sections has made enforcement difficult.

In Shuitou, we asked about the tablet, and a woman told us that the owner was out of town.

"Do you want to buy it?" she asked.

Spindler declined, and then told me, "They offered to sell it last time, too."

The harvest was nearly finished, and the wind rustled stalks of corn that stood dead in the fields. Beyond the village, we climbed a steep section of wall, where thousands of Mongols had attacked in 1555. Spindler said that the typical Chinese defense relied on crude cannons, arrows, cudgels, and even rocks. "There were regulations about how many stones you were supposed to have, and how you were supposed to bring them to the second floor of the tower if there was an attack," he said. Later, he pointed out a circle of loose stones that had been carefully arranged atop the wall. Four and a half centuries later, they were still waiting for the next attack.

The Mongols liked to come at night. They travelled on horseback, usually in small groups. Near enemy territory, they followed ridgelines, because they feared ambushes. They were not occupiers. They penetrated Chinese lands, gathered booty, and returned home as quickly as possible. They liked to steal livestock, valuables, household goods, and Chinese people. They carried the men and women back to the steppes and forced them to form families. Then they sent the men south to gather information on Chinese defenses, using their wives and children as hostages.

The most vivid accounts of the Mongols were provided by Chinese officers who served in the north. Yin Geng, the author of the book that Spindler read in Japan, had particularly intimate contact with Mongols during the mid-fifteen-hundreds. ("They like to fornicate, paying little attention to whether it's day or night or whether there's anyone watching.") Like most Ming writers, he calls them lu--barbarians. ("Every barbarian family brews alcohol and all of them like to drink; the barbarians drink like cattle, not even stopping to breathe in the process.") His account is a dark sort of anthropology, written in the hope that the reader will come to both know and hate the enemy. ("Barbarians like to spear babies for sport.")

Arthur Waldron identified three basic Ming strategies for dealing with the northerners. In the early Ming, the Chinese often took the offensive, pushing Mongol settlements away from the frontier. The second approach was buying off key Mongol leaders with gifts, official titles, or opportunities for trade. But some Ming emperors refused to negotiate with people they believed were savages. The third option was building defensive walls--an ineffective tactic, in Waldron's view, and one that he compares to the Maginot Line. Wall building became the trademark of the later Ming, he writes, because the dynasty had become too weak to fight and too proud to conduct diplomacy.

Spindler believes that the late-Ming response was less rigid than that. In his reading, he has found that the Chinese tactics varied locally, depending upon specific threats. And wall building was often coordinated with offensive and accommodationist strategies. In any case, he is convinced that no Chinese policy could completely resolve their problems with the Mongols, whose internal power struggles contributed to the raiding. In Mongol culture, legitimate leadership was supposed to be confined to the direct heir of Genghis Khan, and to pass only to the firstborn son of each generation. Outside that narrow line, ambitious contenders often found that the easiest opportunities to gain status lay to the south.

In the fifteen-forties, Altan Khan was frustrated by his genealogical standing--he was the second son of a third son--and so he attempted to improve his lot by establishing trade relations with the Chinese. The reigning Ming emperor, Jiajing, refused. On September 26, 1550, the night of the mid-autumn festival, Altan Khan led tens of thousands of Mongols on a surprise attack northeast of Beijing. They breached the crude stone wall there and pillaged for two weeks, killing and capturing thousands of Chinese. After that, the Ming began using mortar on a large scale to improve the fortifications.

The oldest son of Altan Khan, known as the Yellow Prince, tried a different strategy. He married dozens of women from important Mongol families as a means of solidifying his alliances with them. But he began to have financial problems, which he solved by sending the women back. Accompanied by their families, the ex-wives began visiting Chinese wall garrisons, demanding support. In 1576, after one such appeal was rejected, a raiding party penetrated a gap in a remote part of the defense network. The region was so rugged that the Ming believed that no wall was necessary, but the Mongols got through, killing twenty-nine Chinese. The Ming responded with another major wall-building campaign, this time using brick, which allowed construction on even the steepest terrain. Spindler calls the incident of 1576 "the raid of the scorned Mongol women"--a failed harem that helped to inspire the stunning brick fortifications of Beijing.

Historians generally portray the Great Wall as a military failure and a waste of resources. Spindler disagrees, noting that the improved wall held back major attacks in the sixteenth century. At Shuitou, where we hiked, the Chinese defeated thousands of Mongols. For the Ming, the wall was only part of a complex foreign policy, but, because it's the most obvious relic, it receives disproportionate blame for their fall.

"People say, was it worth it?" Spindler said. "But I don't think that's how they thought at the time. You don't get a nation-state saying, 'We're going to give up this terrain' or 'We're going to sacrifice x number of citizens and soldiers.' That's not a calculus they used. An empire is always going to try to protect itself."

In the afternoon, we bushwhacked. On his hikes, Spindler sometimes followed game trails, and often he walked atop wall sections, where the brush is less dense. But occasionally there was no option other than to pursue a ridge straight through the brambles. He called this "hiking like a Mongol," and I hated it. I hated the thorns, and I hated the bad footing. I hated how my clothes got torn, and I hated the superiority of Spindler's bizarre wall regalia. I hated how branches that were chest-high for him hit me in the face. Mostly, I hated the Mongols for hiking this way.

When we reached the ruins of an old stone fort atop a ridge, it felt as if we had emerged from a long swim underwater. To the east, the view opened for twenty miles. Only a single settlement was visible--the village of Zhenbiancheng, still surrounded by the high stone walls of a Ming garrison. Looking down on the walled settlement, Spindler remarked that it had been a hardship post, where commanders had requested that soldiers be paid in grain rather than in silver. "There was bad inflation during the Ming," he said. "It was connected to the discovery of silver in the New World."

The next morning, after camping for the night, we discovered faint traces of another stone fort with a sight line to the ridge. Spindler theorized that they had both been used to send signals to Zhenbiancheng. (Chinese used gunpowder blasts to communicate along the fortifications; Mongols used smoke signals.)

Spindler was skilled at deconstructing the wall, but eventually he would have to create something out of all the pieces. The facts were scattered in remote places, on high ridges, and in lost books, and they could be as distracting as a thorn in the face. "Because David is not ensconced in academia, he's got a lot more freedom to develop his own line of inquiry," his friend Andrew Field told me. But there was a risk to the isolation. "I'm trying to urge him to seek closure," Field said. "But, the way David's mind works, he has an amazing ability for detail."

The bushwhacking was intensely time-consuming, and it could also be dangerous. During our hike above Zhenbiancheng, Spindler said that in 1998 a friend had fallen off a tower and broken his wrist. They worried about descending late in the day, so they spent the night on the wall. In retrospect, Spindler regretted not descending immediately.

"Was he in pain?" I asked.

"Yeah," Spindler said. "He was in a lot of pain."

Spindler always told friends where he was hiking, and he almost never made overnight trips alone. His most frequent companion is Li Jian, a classmate from Peking University who now works in the rare-books division of the National Library. Her first expedition with Spindler, in 2000, was a three-day hike. "I had always had problems with insomnia," she told me. "But when I got back from that hike I slept really well!" Since then, she has spent a hundred and eighty-five days on the wall with Spindler.

In time, Li Jian acquired an L. L. Bean wool hunting shirt, a white Tilley hat, La Sportiva mountaineering boots, and elk-leather line-worker gloves from J. Edwards of Chicago. She cut off a pair of long underwear at the knee and scissored a round hole for her face. In the field, she's a five-foot-two Chinese double of Spindler, following him through the brush.

In June of 2003, they set off for a three-day hike in the wilds of Mentougou, in the western Beijing region. The mountains there are weirdly shaped; the high peaks are easy to negotiate, but the lower flanks deteriorate into unexpected cliffs. Hiking like a Mongol, Spindler got lost, and every attempt to go down ended at a sheer drop. They ran out of food after three days. Fortunately, it had rained, so they drank water that had settled in the hollows of rocks. Friends organized a search party and drove out from Beijing. Five days after Spindler and Li Jian had set off, they finally found a trail and made it back, meeting the search party en route. Today, Li Jian continues to hike the wall as therapy for insomnia.

Chinese universities may not have produced Great Wall specialists, but a small community of wall enthusiasts has developed outside academia. They tend to be athletic--a rare quality among the Chinese intelligentsia. And the Great Wall attracts obsessives. Dong Yaohui, a former utility-line worker, left his job in 1984 and doggedly followed wall sections on foot for thousands of miles across China. After writing a book about the experience, he helped found the Great Wall Society of China, which publishes two journals and advocates preservation of the fortifications. Cheng Dalin, the retired Xinhua photographer, graduated from a sports academy. William Lindesay, a British geologist and marathoner, came to China in 1986 and spent nine months running and hiking along the wall. He settled in Beijing, published four wall-related books, and founded International Friends of the Great Wall, a small organization that also focusses on conservation.

The most active Great Wall researcher at Peking University is a policeman named Hong Feng. As a child, he enrolled in a sports school--he became a sprinter and a long jumper--but he always enjoyed reading history. After barely missing the cutoff for admission to college, he entered the police academy, and was eventually assigned to the city's unit at Peking University. In the mid-nineteen-nineties, he began hiking recreationally and was disappointed with contemporary books about the wall. "They make too many mistakes," he told me. "So I started reading the original texts."

I met Hong Feng in the Peking University police station, where he was working a twenty-four-hour shift. He is the station's supervisor, and uses his days off for hiking trips. At forty-five, Hong is tall and extremely fit, although he suffers from a chronically sore right elbow, which was injured when he fell while researching. He often visits the university library, but he has never discussed his research with professors. "Scholars in the archeology and history departments just aren't interested in the Great Wall," he told me.

During his hikes, Hong Feng noticed a puzzling fifteen-mile gap in fortifications to the northwest of Beijing. Modern writers had claimed that the landscape was so rugged that it didn't require defenses, which made no sense to Hong. He had visited other areas that were much steeper yet heavily fortified, so he turned to the Ming Veritable Records. He discovered that the Ming believed the region contained an important longmai, or dragon vein, just north of their ancestral tombs. A dragon vein is a ridgeline critical to feng-shui, so the Ming went to the trouble of building elaborate walls farther north, on terrain that was naturally less defensible.

Hong Feng published an article about his findings on www.thegreatwall.com.cn, which has become home to the most vibrant community of Chinese wall enthusiasts. The site was launched by Zhang Jun, a software engineer, on May 8, 1999--the day that the Chinese Embassy in Belgrade was bombed by NATO. (NATO said that the attack was a mistake.) Members of the Web site have regular dinners in Beijing, and at one of the events I asked Zhang Jun why he had been inspired to found the Web site on that particular date. "You can say that the Great Wall was built to protect China," he said, choosing his words carefully.

The Web site has five thousand members, many of whom are interested in the wall for a combination of patriotic and recreational reasons, although there's also a small community of serious researchers. David Spindler joined in 2000. Like everybody else, he adopted an online name--Spindler's is Ah Lun, a derivative of the Chinese name that he was given by a language teacher--and he frequently corresponds with others in Chinese. But he doesn't attend functions, and he has never identified himself as a foreigner. Starting last fall, he posted two long Chinese articles on the Web site, describing the construction history of specific sections of wall. He told me that he would eventually write his book in English, but for initial articles it made sense to write in Chinese, because the Web site is the only community that cares about such discrete topics.

Spindler had asked me not to identify him to the other members of the Great Wall Web site, and I didn't, but they quickly brought up Ah Lun on their own. Hong Feng, the policeman, spoke admiringly of Ah Lun's research, assuming that he was Chinese. "He doesn't write very much, but what he writes is deep," Hong said. "He must be some kind of graduate student or scholar. I don't ask, and he doesn't tell."

Eventually, Spindler planned to "come out" as a foreigner, but he had always been wary of the site's nationalism. And he remembered the way he felt after defending his thesis at Peking University. "My professor said, 'In the rules for foreigners, we usually give them a little more latitude,' " Spindler told me. "If I had had more presence of mind, I would have said, 'Well, I've been here for the experience, and I'll be happy to walk away without a degree.' " He continued, "I want my work to be evaluated on these stand-alone terms. Who it's written by, whether he's Chinese or foreign, shouldn't matter."

Because Spindler was worried that he wouldn't get the credit he deserved for his work, he published under a pseudonym--it seemed contradictory, like many of his actions. He was extremely cautious, but somehow he had risked everything--financial stability, relationships, personal safety--for his research. He had confidence in his ideas about the wall, and described them with perfect clarity, but he refused to start writing his book before the spreadsheets satisfied him. At times, it seemed quixotic--the single-minded pursuit of a strangely ambitious structure--but beneath it lay a deep commitment to rationality. He believed that the wall had been built for a military reason, and that he was researching it in the best way possible. He hated any symbolic use of the Great Wall, especially for something as complex as Chinese culture. For Chinese, the wall usually represents national glory, whereas foreigners often see it as evidence of xenophobia. Spindler believed that neither interpretation was useful. "It's just one manifestation of what China has done," he said. "It's just a way they defended themselves."

Of all the people I met, Hong Feng had a viewpoint that reminded me the most of Spindler's. Hong's online name is Shi Shu, which means "to reach the end of the books." "People in China always describe the Great Wall as a symbol of ethnic pride," Hong told me. "But that's an exaggeration. It wasn't supposed to be a great monument like the Pyramids. It was built in response to attacks."

At the end of December, I accompanied Spindler on his three-hundred-and-fortieth trip along the wall. During a previous visit to Miyun, north of the city, he'd seen some high ridges that he believed might contain towers of piled stone. Slowly, we climbed to the ridges: nothing. But it was another day to be checked off on the to-do list.

Although I had never liked the bushwhacking, during the past year I had come to appreciate the distinctive rhythm of the trips. Every journey had it all: good trails, bad trails, hellish thorns, spectacular views. No matter the landscape, I could always see Spindler up ahead, his white hat bobbing above the thickets.

On the way down, we found a dead roe deer in a trap. The loop snare had caught the animal around the neck; it must have strangled itself. Just beyond that, we reached a long section of wall where most ramparts had crumbled away. As I walked atop the structure, my boot got caught in a hole. I tripped and fell down a short ledge, pitching head first toward a ten-foot drop. Somehow--things happened very fast--I threw myself down against the wall. I slammed to a stop with my head peering over the edge.

"Nice save," Spindler said, after he had rushed over. I rose slowly, and tried to walk, and knew that my left knee was badly hurt. But we were miles from help, and the temperature was well below freezing; the only option was to keep moving.

During the descent, I leaned on Spindler whenever possible. It took three hours, and I remember every minute. The next morning, I went to the hospital for X-rays. The doctor told me that I'd broken my kneecap and I'd be on crutches for six weeks; and that was the last time I walked on the Great Wall of China.

The day after the accident, Spindler stopped by my apartment. He asked if there was anything I needed, and I could tell that he felt bad about what had happened. He mentioned that he had made a quick analysis of the spreadsheets, which showed that mine was only the second casualty to be sustained in approximately twelve hundred and fifty person-days of hiking. Later, he confirmed that the exact figure was twelve hundred and forty-five.

In February, before leaving on a research trip to Taiwan, he visited me again. He planned to study some Ming maps and memorials that were held in Taipei's National Palace Museum. He still hadn't written anything in English, but additional Chinese articles were in the works, and he seemed to be thinking more about the future. He planned to start writing the book within a year or so; after it was finished, he'd find a way to continue researching the wall. Maybe he'd start a Ph.D. program, or perhaps he'd remain independent, supporting himself with lectures and books. "I'll need to learn other languages in order to get academics to give me the time of day," he said. "You really get written off if you don't know Japanese, if you don't know Mongolian. There are others that would be helpful. Probably the next one would be Russian, and possibly German. I guess it would be helpful to learn some Manchurian. A little Tibetan. But those are further down the list."

I hopped on crutches to the door and said goodbye. He had an early flight; in order to save money, he'd booked a ticket with a seven-hour layover in the Macao airport. When I'd asked how many Beijing hiking days were left, he didn't hesitate. "Eighty-six," he said.

COPYRIGHT 2007 All rights reserved. Reproduced by permission of The Cond¨¦ Nast Publications Inc. ( Edited by Carmen Apr.10.2008 )