From Machu Picchu to Angkor Wat, the ruins of once-powerful ancient civilizations are among the most popular tourist attractions around the globe. But what about the United States? Though they don’t have the fame of the Egyptian pyramids, pre-Columbian ruins exist in America. 

The largest ancient American city north of modern-day Mexico is still relatively unknown. Sitting in the countryside near the Illinois-Missouri border not far from St. Louis, the site known as Cahokia consists of gigantic mounds, most of which were built about 1,000 years ago. Though lacking the stone structures that characterize other ancient settlements, this was an important city in its time, inhabited by as many as 20,000 people until the early 1400s.

Today, Cahokia is one of only 22 UNESCO World Heritage Sites in the U.S. and is a National Historic Landmark, which gives it protection under law.

Scholars estimate that the city was made up of about 120 mounds, which covered almost 4,000 acres. The tallest of the 80 remaining human-made hills stands more than 100 feet above the surrounding Illinois prairie. 

Monks Mound

Today, visitors climb stairs to reach the top of Monks Mound. (Photo: Bryce Edwards/flickr)

Like so many ancient ruins around the world, little is known about why Cahokia was abandoned. Theories include an invasion by a hostile tribe or a surprise migration of local bison herds, perhaps because of some sort of climate change event. One of the most interesting theories suggests that the city simply grew too large and local resources could not sustain the population.   

When French traders first arrived in the area, the city had already been abandoned, but the Cahokia people, part of the Illini tribe, inhabited the lands around the mounds. Though they were the source of the name by which the site is now known, the Cahokia people were likely not the group that built and inhabited the mounds. The Illini were part of the Mississippian culture, pre-Columbian peoples who lived in what is now the central U.S. Some of these tribes were known for building large mounds, and any of these could have been responsible for constructing Cahokia. 

The mounds were most likely built by hand, with laborers carrying earth and stones to the construction site in woven baskets. The largest, a 100-foot-tall hill known as Monks Mound, had a 50-foot-wide, 100-foot-long wooden building on top. Because wood and earth were the main construction materials, these buildings did not remain intact for long after they were abandoned.

Cahokia Mound

Cahokia Mounds in Illinois is the site of the largest pre-Columbian city in the U.S. (Photo: Michael Dolan/flickr)

Though the city’s buildings did not last indefinitely, 50 years of careful excavations have unearthed interesting discoveries that led scholars to believe that this was a very advanced civilization for its time.

One area, dubbed Woodhenge, consists of a series of holes that once held wooden poles that measured the angle of the sun to tell the time and date. Excavations have also unearthed a workshop where metals were partially melted and reformed in a method similar to that used by blacksmiths. Evidence of agriculture exists in both small-scale neighborhood gardens and larger fields outside Cahokia.

The city’s mounds had natural plazas between them, with an area known by archeologists as the Grand Plaza at the center of the city. Evidence indicates the 50-acre field was originally covered with small hills but was purposely leveled to be used as a gathering space or athletics field. 

Woodhenge area of Cahokia

Part of the area known as Woodhenge was reconstructed in 1985. Monks Mound is in the background. (Photo: John W. Schulze/flickr)

The different heights of the mounds suggest some sort of hierarchy among the inhabitants. Some people suggest that the large building on top of Monks Mound was a sort of palace for the tribe’s leaders.

Some mounds were obviously used for burials. Skeletons have been found in multiple places, including some with wounds that suggest ritual killing or sacrifice. The position of other bodies suggests that they may have been buried alive. This evidence points to a darker side to life in Cahokia but also links the people of the city with other Mississippian tribes. Many of these groups made ritual human sacrifices when elite members of their tribe died. 

To truly appreciate Cahokia's place in the history of North America, you have to put its size into perspective. Even if the most modest peak population estimates are true — about 10,000 residents — the land that is now the U.S. wouldn’t have a city larger than Cahokia until the 17th century.

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