"I tell you," wrote Marco Polo, "that this palace is of... unmeasured wealth." Its roof is sheathed in gold "in such a way as we cover our house with lead." Even the floors are gold, "more indeed than two fingers thick. And all the other parts of the palace and the halls and windows are likewise adorned with gold." In this gilded domain, he declared, lived the ruler of an island kingdom called Cipangu (that is, Japan), whose waters yielded red pearls "very beautiful and round and large."

Scholars believe Europeans had never heard of Cipangu before Polo told them about it in The Description of the World, which he started writing about 1298, a few years after he returned home to Venice from a 24-year Asian odyssey. Though fascinated, Polo's readers, according to one account, concluded that his tales were "fabulous...mere dreams." But as decades passed, some began to take Polo seriously. In Christopher Columbus' copy of The Description, which survives, "gold in the greatest abundance" and "red pearls" are written in the margin beside the Cipangu reports. Although the handwriting may not be Columbus', he is said to have sought Cipangu among the Caribbean isles on his 1492 voyage.

Columbus never came anywhere near Japan, of course, but what would he have found? Red pearls? Experts say the oysters that produce them don't inhabit Japanese waters. A golden palace? Japan's Golden Pavilion, the gold-leaf-covered Kinkakuji, was built in 1397, a century after Polo published.

Truth to tell, many of Marco Polo's tales of treasure were just that—tales. Tall tales. Readers who persevere in Polo's often confusing, disjointed text will encounter preposterous supernatural events and an astonishing bestiary, including men with the features of dogs. Some readers have even concluded that the book is a total fake. If Marco Polo went to China, British Sinologist Frances Wood asked some years ago in a book titled, appropriately enough, Did Marco Polo Go to China?, why did he fail to mention chopsticks, tea and the binding of girls' feet?

At the British Library, where Wood curates the Chinese collections, the switchboard lit up with calls from journalists and scholars. After all, Polo's book has ornamented libraries the world over for centuries and is regarded, despite its flaws, as one of the world's greatest travel accounts. Wood had taken on a global icon. "I knew that Marco Polo was a household name," she told an interviewer, "but I was unaware that millions of people all over the world felt passionately about him and would be baying for blood."

Polo's fellow Italians have long assumed that he was a fibber; both he and his text are known in his homeland by the name Il Milione, and many think it's because the book includes a million tall tales. But didn't Polo enrich Italy by bringing home pasta and ice cream? Nope, those are myths. Still, Italians weren't about to tolerate a challenge to Polo's integrity by a foreigner, and many other people in the world are likewise invested in him. In China, historians staunchly defend the man who helped put their country on the map.

About ten years ago, as a staff writer for National Geographic, I followed Polo's journeys across Asia, from Iraq to China and homeward via Sumatra, India and Sri Lanka, using his book as my guide. (There are about 120 versions of his narrative; the one I carried, generally considered the most authentic, is translated from a 14th-century copy in the French National Library.) Like others who have examined his writings closely, I am dismayed by his omissions and floored by his whoppers. But I am ultimately convinced of his essential truthfulness. Why? For one thing, his itineraries, as laid out by the sequence of book chapters, are fundamentally accurate, whether he's crossing Central Asia or central China. Where did he acquire that geographical information if he didn't make these journeys himself? No skeptical investigator has ever proved that he copied from some Arab or Chinese source. And while it's true that Polo is guilty of curious omissions (those chopsticks, for example), he expanded medieval Europe's meager knowledge of Asia with such hitherto-unknown names as Cipangu, Java, Zanzibar and Ceylon (Sri Lanka), besides identifying China's great cities and describing such features as the Takla Makan Desert and the Yangtze River. Having followed Polo's tracks, I know firsthand that he also got many things right, such as: both lapis lazuli and rubies are found in the Badakhshan region of Afghanistan; in China's southwest a minority people eat raw flesh; people in Sumatra and Sri Lanka make a joy juice from fermented palm tree sap.

Polo also produced an extensive report on Hindu customs in India, a country that clearly fascinated him. But his great love was Catai, as he called China. No kingdom ever had a better PR person. Time and again Polo wrote of Catai's wealth in silk and spices (no exaggeration) and declared that people had "all things in great abundance." So far, so good. But soon he was claiming that Hangzhou had 12,000 bridges arcing over its canals, a ludicrous inflation, even though Hangzhou was the world's largest city at the time; he even accorded the much smaller Suzhou 6,000 bridges. "Take that, Venice!" he seemed to be saying to his canal-rich hometown. (A later traveler could find only 347 bridges in Hangzhou, including those in its suburbs, and just 290 in Suzhou.)

Polo practically bubbled with enthusiasm as he described the palace of Kublai Khan, the Mongol ruler of Catai, in what is today Beijing. (He called the capital Cambaluc, a corruption of its Turkish name, Khanbalikh, "Khan's city.") The palace was "the greatest that ever was seen," with a hall large enough to accommodate 6,000 diners, and was encompassed by a wall four miles around. In some versions of his book, the wall grew even longer, in one case to 32 miles. Mangling his claims according to their own whims, The Description's translators, scribes and finally printers (beginning in 1477) often took his inconstant veracity down a further peg or two.

Whenever Polo mentioned Kublai, he laid it on thick. His hunting retinue, we are told, included 20,000 dog handlers; 10,000 falconers carrying gyrfalcons, peregrines, saker falcons and goshawks (Polo showed himself to be an avid birder); and unstated numbers of lions, leopards and lynxes to go after wild boars and other big animals. Still extolling his overlord—he claimed to have been a trusted servant of Kublai's regime—Polo wrote that the new year was celebrated in Cambaluc with a parade of Kublai's elephants, "which are quite five thousand, all covered with beautiful cloths," and with gifts to the ruler of "more than 100,000 white horses very beautiful and fine."

It's true that Mongol lords reveled in the royal hunt, a huge spectacle, and that they celebrated holidays grandly. And no doubt Kublai, like many Asian potentates, kept stables of elephants as a mark of power—but nothing like 5,000. And historians are confident that he didn't hunt with any 20,000 dog handlers or 10,000 falconers. "The numbers are staggering—they're obviously exaggerated," says Professor Morris Rossabi of the City University of New York, author of the definitive study of Kublai's reign. It is difficult to imagine his people maintaining, for example, a royal herd of 100,000 steeds in the region of Beijing. "People in the north didn't grow enough food to sustain themselves," Rossabi says. "Most of it had to be brought from the south. I can't believe they devoted tremendous amounts of pastureland to having 100,000 horses." Some scribes who copied Polo's text shrank the elephant herd to 500 or omitted it altogether, probably smelling excess, while one version raised it to 105,000.

Still, Polo had plenty of authentic marvels with which to astonish his countrymen—black stones that burned better than wood; money made of paper, porcelain, asbestos; huge oceangoing ships. And he documented China's wealth in silk and spices as well as its commerce with India, Java and other parts of Asia—valuable information for a trading state like Venice.

So why all the hyperbole? We'll never know for sure, but exaggeration is sometimes a character defect in adventurers—Walter Raleigh's gold-strewn El Dorado comes to mind. And in 13th-century Europe, even outright lies were a literary conceit. Grotesque beasts and magical doings were routine in the modest libraries available to even the most educated Europeans. The Histories of Herodotus, for example, told of gold-digging ants in India and winged snakes in Egypt.

I believe Polo kept a journal during his travels; if not, how did he manage, when at last home in Venice, to set down the wealth of detail that he had accumulated during his two dozen years of travel? Polo's diary: what a sensational discovery that would be! He doesn't say he kept one, but a version of The Description that appeared in Venice in the 1500s, supposedly based on authentic manuscripts, declares that he brought home "writings and memoranda." And these, it is said, were shared with a writer who helped him produce his book. That person is identified at the beginning of the text as Rustichello of Pisa, who'd been reworking some of the romantic stories of King Arthur, and whose writings had found their way into European libraries. According to Polo, he met Rustichello in a Genoa prison, into which Polo had been thrown after being captured in a sea battle between Venice and rival Genoa about 1298. Sounds like another tall Polo tale, but so far as scholars know, it's true.

Scholars see the hand of Rustichello in the book's account of a battle between Genghis Khan and Prester John, a Christian ruler in Asia, early in the 13th century. With its huge loss of life—although no body count was recorded—the engagement made a good story. Too bad there was no such person as Prester John; as historians know today, he was entirely a European invention. The legend was no doubt well known to Rustichello, while less so to Polo.

I also suspect Rustichello of concocting the tale of robbers able to "make the whole day become dark" as they swept down upon travelers. Polo described such an attack on his caravan in the desert of Iran. The passage continues, suspiciously, in the third person: "Moreover I tell you that Master Marc himself was as good as taken by that people in that darkness."

Then there are the mangonels, or catapults, Polo writes about. According to the author, Polo, his father and uncle helped build huge rock-hurling machines that inflicted terrible damage on the city of Xiangyang as Kublai pressed his conquest of the southern Chinese dynasty, the Song. Chinese as well as Persian sources describe the destruction, but credit Syrians employed in Kublai's army for the catapults. In any case, the siege occurred in 1273, and almost all authorities believe the Polos didn't reach China until two years later. Polo probably heard of the siege and took note of it. It may be that Rustichello, always attracted to stories of battle, came across it somewhere in his reading and decided to make the Polos military engineers.

Starting home by ship in 1291 or 1292, Polo was forced to spend five months on "Java the Less"—Sumatra—waiting for monsoon winds to shift so that he and his shipmates could sail northwestward toward Ceylon and India. Polo reported, accurately, that cannibals dwelled on Sumatra and, less accurately, that the island was home to some strange beasts, including enormous unicorns, in size "not at all by any means less than an elephant."

"I tell you quite truly," Polo continued about Sumatra, "that there are men who have tails more than a palm in size." And on an island that he called Angaman—probably referring to the Andaman Islands in the Bay of Bengal—"all the men...have the crown of the head like a dog and teeth and eyes like dogs." Tales of strange creatures abounded in Asia as well, and Polo (who apparently never set foot on the Andamans) may have heard about them from sailors. It's also possible that he—or Rustichello—simply drew on the elaborate mythical bestiary of Europe's Middle Ages. (Or perhaps, as John Larner argues in Marco Polo and the Discovery of the World, Polo was simply describing the islanders metaphorically.)

Even as he served up these wild reports, Polo methodically cataloged a South Asian cornucopia, about which Europe knew almost nothing: the nutmeg and aromatic roots of Java, the camphor and coconuts of Sumatra, the pearls, diamonds and pepper of India, ivory from several places—these and many other goods, all tantalizing to European merchants, were commingled with the beasts and fantasies. It's as if the world, as Europeans viewed it, were a mix of real and unreal.

Some readers took notice of things "which are reckoned past all credence," as a Dominican friar recorded. When Polo was dying, in 1324, friends urged him to remove "everything that went beyond the facts," presumably to cleanse his soul.

Polo refused, saying he had not written half of what he had seen. He might have added: "And only half of what Rustichello and I invented."

Mike Edwards covered 6,000 miles in Marco Polo's footsteps.