When U.S. 40 reaches Collinsville, Illinois, the land is flat and open. Seedy storefronts line the highway: a pawnshop, a discount carpet warehouse, a taco joint, a bar. Only the Indian Mound Motel gives any hint that the road bisects something more than underdeveloped farmland.

This is the Cahokia Mounds State Historic Site, a United Nations World Heritage Site on a par with the Great Wall of China, the Egyptian pyramids, and the Taj Mahal. The 4,000-acre complex preserves the remnants of the largest prehistoric settlement north of Mexico, a walled city that flourished on the floodplain of the Mississippi River 10 centuries ago. Covering an area more than five miles square, Cahokia dwarfs the ancient pueblos of New Mexico’s Chaco Canyon and every other ruin left by the storied Anasazi of the American Southwest. Yet despite its size and importance, archaeologists still don’t understand how this vast, lost culture began, how it ended, and what went on in between.

A thousand years ago, no one could have missed Cahokia—a complex, sophisticated society with an urban center, satellite villages, and as many as 50,000 people in all. Thatched-roof houses lined the central plazas. Merchants swapped copper, mica, and seashells from as far away as the Great Lakes and the Gulf of Mexico. Thousands of cooking fires burned night and day. And between A.D. 1000 and 1300, Cahokians built more than 120 earthen mounds as landmarks, tombs, and ceremonial platforms.

The largest of these monuments, now called Monks Mound, still dominates the site. It is a flat-topped pyramid of dirt that covers more than 14 acres and once supported a 5,000-square-foot temple. Monks Mound is bigger than any of the three great pyramids at Giza outside Cairo. “This is the third or fourth biggest pyramid in the world, in terms of volume,” says archaeologist Tim Pauketat of the University of Illinois at Urbana-Champaign. It towers 100 feet over a 40-acre plaza that was surrounded by lesser mounds and a two-mile-long stockade. The monument was the crowning achievement of a mound-building culture that began thousands of years earlier and was never duplicated on this continent.

Why Cahokia crumbled and its people vanished is unknown. Malnutrition, overcrowding, a dwindling resource base, the raids of jealous trade partners—any or all of these reasons may have contributed to the city’s demise. No one knows whether the populace cleared out all at once or dispersed gradually, but by A.D. 1300 Cahokia was a ghost town. By the time Europeans arrived in the Mississippi bottomland, the region was only sparsely settled, and none of the native residents could recount what had happened there centuries before.

So far, archaeologists have uncovered no evidence of invasion, rampant disease, overpopulation, deforestation, or any of the other hallmarks of the decline and fall of civilization. Cahokia abounds in artifacts, but archaeologists have not yet made sense of them in a meaningful way. “It actually becomes quite scary,” says John Kelly of Washington University in St. Louis. “After a while you begin to realize that you’re dealing with rituals that had a great deal of meaning 800 years ago and that you’re kind of clueless.”

Intellectual frustration is not the only reason for Cahokia’s obscurity. Pauketat complains that the region is geographically challenged. It has the look and feel of a place “like Buffalo, except warmer,” he says. Cahokia doesn’t exactly lure others away from more exotic digs in Turkey, Mexico, or Peru, he says. “That’s the problem with this site.” Another reason for its lack of popularity is the ordinary, perishable building materials used by the residents. “Cahokians are discounted because they built with dirt—dirt and wood, things they valued,” says Pauketat. “I get tired of hearing people say, ‘We have civilization and you guys don’t.’ ”

Meanwhile, developers see Cahokia as ripe for expansion; strip malls and subdivisions threaten on every side. “It’s developing faster than we can survey,” Pauketat says. “We don’t know what we’re losing out there.” Although a good portion of the central city is now protected, archaeologists are discovering related sites throughout a six-county region on both sides of the nearby Mississippi—an area 3,600 miles square. Indeed, digs are under way in such unlikely places as a railroad yard eight miles west in East St. Louis, where a new bridge is scheduled. “If you want to find out the archaeology of an area,” says Brad Koldehoff of the Illinois Department of Transportation’s archaeology team, “build a road through it.”

Where in the world were the Cahokians? Map by Jeff Glendenning

One morning last September, a warm red sun rose behind Monks Mound, inching above the level terrace where a tribal palace once stood, burning the mist off the flat green expanses of former plazas. To the west of the mound, in a circle more than 400 feet in diameter, several dozen cedar posts rise to the height of telephone poles. The woodhenge, as the structure is known, is a reconstruction of a series of circles found in the 1960s and ’70s when excavations to build a mammoth cloverleaf joining three interstate highways unearthed the remains of several hundred houses and dozens of post pits. (The findings persuaded the Federal Highway Administration to relocate the cloverleaf a few miles north.)

At the autumnal equinox, the rising sun aligns exactly with one post when viewed from the center of the circle, just as it does at the spring equinox and the solstices. William Iseminger, assistant site manager for the Cahokia Mounds State Historic Society, takes these alignments as evidence that the posts may have functioned as a kind of calendar, marking the turn of the seasons. Other woodhenges may have been part of lesser mounds, but, says Iseminger, they are nearly impossible to find because the post pits are so far apart, and wood rarely survives centuries underground.

Many archaeologists point to the size and ambition of structures like the woodhenge as evidence of Cahokia’s sophistication. The construction of Monks Mound, for example, used between 15 billion and 20 billion pounds of soil, which were lugged to the site in woven baskets that held 50 to 60 pounds of dirt each. Grading and draining the 40-acre plaza in front of it meant moving just as much earth. The stockade walls consumed 20,000 trees. Subsidiary mounds in the city “grid” seem to be placed according to a rational design. These accomplishments imply organized feats of labor and planning enacted by a central authority.

In many excavations, the number of artifacts and the amount of refuse indicate the population spiked sharply around A.D. 1100, jumping from hundreds to perhaps tens of thousands of people. Large homes and mounds appeared where villages of small houses had existed just a generation before.

In the mid-1990s, excavations by Pauketat, Kelly, and others showed that the hills east of Cahokia were far more populous than anyone had suspected. A wooded rise among farmhouses in the city of O’Fallon marks the site of an ancient acropolis that probably served more than 500 people. At a site south of O’Fallon, Pauketat found remnants of 80 houses, three temples, clay pots, hoe blades, ax heads, and carved redstone statues. On a tree-lined street in Lebanon, a flagpole is planted in the center of a former platform mound marking another temple center.

Based on these findings, Pauketat estimates that as many as 50,000 people may have lived in Cahokia’s greater metropolitan area at the settlement’s peak. They seem to have appeared as if from nowhere. “Cahokia had to be created by large-scale migration from other places,” says Tom Emerson, director of the state transportation department’s archaeological program. “Nobody can breed that fast.”

Why did migrants come to Cahokia? Past theories suggested that the dual forces of nature and commerce drove the city’s rapid growth. The fertile bottomland was ripe for cultivation by farmers skilled in raising corn, squash, and sunflowers. The nearby confluence of the Illinois, Missouri, and Mississippi rivers could have put Cahokia at the nexus of trade networks that spanned much of the continent.

But American Indians had been building modest mounds in the Mississippi River valley since 3500 B.C.; they’d been growing corn with much the same tools for hundreds of years, and the rivers and floodplains had been there for thousands. Economic and geographic felicities alone cannot account for the sudden concentration of people in the area at a particular moment.

Pauketat has come to believe that charismatic leaders created a dynamic social movement with Cahokia at the epicenter, luring inhabitants of far-flung communities away from their homesteads to the fast urban action. Pauketat resists the term cult, but it evokes the phenomenon he envisions. “There were certainly individuals who were movers and shakers, but they weren’t consciously, deliberately exploiting people,” he asserts.

“Cahokia is a political construct,” Emerson adds. “It’s not due to some massive change in subsistence, it’s not archaeological, it’s not technological. It’s the kind of place that results from changes in how you conduct yourself socially and politically. What happened at Cahokia is politics, probably in the guise of religion.”

Not all scholars see a burgeoning statehood in Cahokia’s remains. Anthropologist George Milner of Pennsylvania State University believes there were at most 8,000 people at Cahokia, and he calculates that with even half that population one person per household working just a few weeks a year could have built Monks Mound. The construction would have proceeded at a desultory pace, he concedes; it may have required hundreds of years to complete. Only if the woodhenges and mounds were rapidly constructed would they require full-time laborers or engineers. And he is skeptical that the ecology of the region, abundant as it was, could have supported a community as vast as that supposed by Pauketat and others.

The trump card for Milner and other minimalists is the fact that, unlike the ancient Mesopotamians, Maya, Egyptians, and Chinese, Cahokians never developed a written counterpart to their spoken language. Writing is generally considered a prerequisite for the kind of record keeping typical of organized governments. (The names “Cahokia” and “Monks Mound” were applied long after the fact: Cahokia was the name of an Illini tribe that occupied the area in the 1600s, and Monks Mound was named for French Trappists who settled on one of its terraces in the 1800s.)

But champions of an advanced Cahokian civilization would rather make their case with numbers than with language anyway. Even Milner admits that if Cahokia was as populous or expansive as some claim, it would have exerted statelike control over its citizens.

To support his theory, Pauketat is looking for evidence that the settlements outside of Cahokia follow a planned pattern—a support network of communities allied with the power center, perhaps communicating with the capital using runners and smoke signals. He found traces of buildings at the intersection of Routes 159 and 64, now home to a Toys ‘R’ Us and a Ramada Inn, and he believes they may have faced Cahokia, a six-hour walk away. That orientation would bolster his contention that the outlying villages were all part of one big polity.

Early in his career at the state department of transportation, Tom Emerson found an eight-inch statuette at the site of a temple two or three miles from Cahokia. Five pounds of distinctive redstone called flint clay had been carved into a kneeling female figure sinking a hoe into the back of a serpent. The serpent’s tail climbs up the woman’s back, bearing squash and gourds like a vine.

The images echo familiar pre-Columbian themes of reproductive and agricultural fertility. As similar figures were discovered in the Cahokian environs, a pattern emerged. Around A.D. 1100, Emerson says, the elite of Cahokia seem to have co-opted or codified the fertility symbol, raising it to an unprecedented stature that became a kind of brand identity for the budding metropolis. “They’re taking a symbolism that exists across the entire hemisphere and selectively emphasizing parts of it to their own benefit,” Emerson says.

Some archaeologists have taken the emphasis on the bucolic feminine as a sign that Cahokian society was peaceful, egalitarian, and possibly matriarchal. There is, in fact, no evidence that the city was ever invaded, and no indication of bellicose tendencies other than the robust stockade surrounding the city center. But Emerson warns against this interpretation. For one thing, he says, war wasn’t necessary, because it would have been clear from the city’s size alone that it could mount raiding parties with more members than the total of men, women, and children in any of the surrounding villages. “Nobody could stand against Cahokia. I don’t know that they had to do much actual conflict. It was mostly intimidation.”

Cahokia’s downfall has been blamed on a variety of culprits. A corn-based, protein-poor diet might have sent urban dwellers west in search of buffalo. A centuries-long cold spell could have crippled the region’s agricultural productivity. Deforestation of the uplands would have choked downstream water supplies with silt and exacerbated flooding. Or the cause could have been those same intangibles invoked by latter-day theorists to describe Cahokia’s rise: a shift in belief systems or the balance of power. Certainly the sprawling pacts that Cahokian chiefs may have forged with nearby villages would have challenged any lasting centralization of power.

“The typical life history of a chiefdom is that it comes together, it has its heyday, and it falls apart, all within a couple of generations,” says Emerson. “The interesting thing about Cahokia is that it managed to hang together. The fact that it didn’t go on forever isn’t unusual at all.”

One of Cahokia’s chiefs appears to be buried in Mound 72, which lies a half mile south of Monks Mound. It is a modest hillock by comparison, but the site holds far grimmer implications about Cahokian society. During excavations there in the late 1960s, Melvin Fowler of the University of Wisconsin at Milwaukee uncovered the remains of more than 250 people. One middle-aged male had been laid on a shelf of 20,000 seashell beads arranged in the shape of a bird. Near him were the bones of six other people, a cache of more than 800 flint arrowheads, a rolled-up sheet of copper, and several bushels of unprocessed mica—all seemingly placed in tribute to the Beaded Birdman.

In other parts of the mound, skeletons of more than 100 young women clearly indicate human sacrifice, and another grouping of four men with no hands or heads denotes the same. Another 40 bodies seemed to have been tossed into a grave haphazardly. Other mass burials in Mound 72 show varying degrees of respect and carelessness—and seem to reflect some sort of social hierarchy as yet undeciphered. Human sacrifice, for example, can be a sign of a coercive society or of a cultlike mentality. “Mound 72 is an ancient text with its own set of Rosetta stones and is slow to give up its secrets,” Fowler wrote in Cahokia, a book he coauthored with Biloine Whiting Young.

The cause of Cahokia’s demise is no more certain, but at least one expert links it to the Toltec civilization of south-central Mexico some 1,400 miles away. Although no Mexican artifacts have ever been found at Cahokia, similarities in the monumental and ornamental styles are conspicuous—and far from accidental, according to anthropologist Stephen Lekson of the University of Colorado at Boulder. Lekson and anthropologist Peter Peregrine of Lawrence University in Wisconsin believe that the mound cultures of the American East, the pueblo cultures of the American Southwest, and the pyramid cultures of the Mexican highlands were not only familiar but possibly even integrated with one another.

There’s plenty of evidence for such an exchange at Chaco Canyon, where copper bells, macaw feathers, pyrite mirrors, and other Mexican goods turn up. But Chaco was a wannabe compared with Cahokia—much smaller, far less populous, and without a centuries-long tradition preceding its development. Cahokia, with its central location, entrenched culture, and extensive trade network, didn’t need Mexican trinkets to bolster its stature, Lekson says. “If someone from Cahokia showed up in any major town in Mexico, he’d be taken seriously,” says Lekson. “But if someone from Chaco wandered in, they’d ask him if he had an appointment.”

The Toltec, Chaco, and Cahokian societies all collapsed at very nearly the same moment, and Lekson believes that that, too, is no accident. Events in Mexico may have rippled up the Gulf Coast to the Mississippi and thence to Cahokia. “I’m not saying that Mexico is pulling everybody’s strings,” says Lekson. “But [the cultures] are more alike than not, and it’s interesting to ask why.”

Interesting as it might be, a continental perspective doesn’t yield an explanation, because no one’s sure what caused the Toltec regime to fall, either. It may be that if scientists ever determine why Cahokia fell, they may be able to help explain what happened elsewhere in the Americas. At present it’s still anyone’s guess. “We are telling stories that will fall apart in the future,” says Pauketat. “But we can’t ignore the evidence, either. You could make the mistake of saying this is a coercive society, based on Mound 72. Or you could look at the outlying villages and say, ‘This is a peaceful community.’ They must have wanted to build Cahokia. The truth may be somewhere in between. We don’t really know what happened here.”