Long before the arrival of European explorers, soldiers and settlers in North America, the portion of the continent north of Mexico was inhabited by as many as 18 million native people. And contrary to the popular perception of American Indians living a nomadic existence, many of the continent’s aboriginal inhabitants lived in thriving urban centers.

One settlement, Cahokia in modern-day Illinois, had a population of 20,000 at its peak around 1100-1150 A.D. Around that same period in time, New Mexico’s Chaco Canyon was the center of a sophisticated culture that erected what were the most massive buildings on the continent, until the rise of skyscrapers built from steel girders in the late 1800s.

Those urban centers were part of what historians Lisa Krissoff Boehm and Steven Hunt Corey have described as “a landscape rich with its own history—a land shaped by diverse peoples living in varying patterns of settlement.”

Like cities in other parts of the world, Cahokia, which sprawled over an area of about five square miles, developed in a highly desirable spot. The settlement was situated along a flood plain that provided fertile soil for agriculture, with nearby hickory forests to provide wood and other raw materials as well as wildlife to hunt, according to Lori Belknap, site manager for the Cahokia Mounds State Historic Site.

Cahokia also had convenient access to the nearby Mississippi River, which its residents—a people known as the Mississippian culture—navigated in large dugout canoes. “It likely was a trading center,” Belknap says.

Like a modern city with suburbs, Cahokia’s outer edge was a residential area, consisting of houses made from sapling frames lined with clay walls and covered by prairie grass roofs. Further inside was a log palisade wall and guard towers, which protected a central ceremonial precinct of the site, including Monks Mound, the Grand Plaza and 17 other mounds. More than 100 mounds extended more than a mile outside the wall in all directions. Some served as bases for what probably were important community buildings, while other cone-shaped mounds functioned as burial sites. Still others apparently were markers that delineated the city’s boundaries, according to Belknap.

At the center was the 100-foot-tall Monks Mound, the largest earthen mound in North America, which had four terraces and a ramp or stairway leading up from the ground. From the top of the mound, one could take in a panoramic view of Cahokia and its surrounding realm.

One of the most remarkable things about Cahokia is that it appears to have been carefully planned around 1000 A.D., with a rectangular-shaped Grand Plaza whose core design mirrors the native vision of the cosmos, according to archaeologist Thomas Emerson. From the beginning, the city’s builders had “grandiose visions of what Cahokia would be,” Emerson explains. “It did not grow by slow accretion through time.”

The events that led to the deliberate building of Cahokia and the rapid growth of its population remain unclear. “A religious prophet? The immigration of a foreign elite group? The introduction of maize?” Emerson says. “The options seem endless, but we have few answers right now.”

Cahokia’s decline, which began around 1250 or 1300, and culminated in the site’s abandonment by 1350, are similarly mysterious. A recent study suggests the settlement’s demise was linked to climate change since a decrease in rainfall would have affected the Mississippians’ ability to grow their staple crop of maize. Others think that the sheer size and diversity of the Cahokian population may have led to irreconcilable rifts.

“It was a large population, composed of immigrants from the midcontinent who brought very different practices and beliefs to the city,” Emerson says. “The management of differences requires a strong social and political consensus within a group. If that consensus collapses, societies will fragment into their smaller groups that existed based on kinship, ethnicity, religious beliefs, residential propinquity, shared economic goals, etc.”

In New Mexico, the Chaco Canyon settlement flourished between 850 and 1250 A.D. Over the years, researchers have come up with wildly varying estimates of the center’s peak population, from around 2,000 to as many as 25,000, according to a 2005 National Park Service report.

Chaco Canyon appears to have been the ceremonial, trade and administrative hub of a network of neighboring communities, some as far as 60 miles away. A 2016 study by University of Colorado Boulder researcher Larry Benson found that Chaco Canyon’s salty soil wasn’t good for growing corn and beans, so the settlement had to import food and other resources from those places. Those communities were connected by an extensive network of roads and an irrigation system, according to Boehm and Corey.

Builders in Chaco Canyon developed sophisticated stone masonry construction techniques that allowed them to erect 150 multi-story structures, some as tall as five to six stories in height, with hundreds of rooms. In addition to stone, the builders used about 240,000 trees, some harvested from the Chuska Mountains about 50 miles to the west, according to a 2015 study by University of Arizona scientists.

The great houses, as these massive structures were called, probably weren’t dwellings, but rather public buildings used when people of the region gathered for ceremonies or to engage in commerce, according to NPS.

“Elite chiefs constructed the great houses to demonstrate their authority,” Benson, an adjoint curator of anthropology at the University of Colorado Boulder’s Museum of Natural History, says. “However, they did not live in the canyon. Instead, they lived in wetter more productive regions at the periphery of the San Juan Basin where they oversaw the production of foodstuffs and the harvesting of mammals.”

In 2017, DNA analysis of remains suggested that the settlement may have been founded and ruled over a period of more than 300 years by dynastic elite that controlled the ritual practices at Pueblo Bonito, the 600-room structure that was the settlement’s most important building.

Like Cahokia, the Chaco Canyon settlement was abandoned eventually. Some have suggested that people in the area cut down too much of the forests, leading to erosion and destruction of farming. But a 2014 study by University of New Mexico researchers concluded that there wasn’t evidence to support that scenario.