Though there are some debates over the nature of Cahokia — we'll get to the leading theory in a second — we know enough about the city to place it in its cultural context. The Mississippian culture stretched from the Great Lakes to what is now the Southeastern United States, and there's ample evidence to suggest its trade networks brought goods from Mexico and even farther.

At the center of it all was Cahokia — but that wasn't its real name. It was Europeans who called it that, after the Cahokia people who dwelled there at the time (even though they actively denied that they had any connection to the city). In fact, much of the Mississippian culture had faded by then, so much of what we know about them is from oral tradition and archaeological discoveries. Taken together, the evidence seems to indicate that there might have been a religious movement behind the city.

By the 12th century, Cahokia had swelled to a population as large as 30,000. For comparison, London at the time was hovering around 18,000. Residents dwelled around a central plaza at the base of Cahokia's most prominent feature — Monk's Mound. This massive structure rises 30 meters from the ground, and a speaker at the top could likely be heard throughout the entire 50-acre Grand Plaza. The other mounds of Cahokia number more than 100, and they played a variety of roles. Some featured community buildings in their upper terraces, and others contained the bodies of human sacrifice victims. (Religion back then was a bit more serious than it is nowadays.)

So if Cahokia was so huge and influential, why was it all but abandoned by the time Europeans arrived? There are frustratingly few clear answers to that question. By 1350, the city was nearly empty, and scholars disagree if it was disease, environmental factors, or cultural conflicts that led to its demise. One thing is for certain — it wasn't the usual suspects, since Hernando De Soto wouldn't become the first European to reach the Mississippi for another 200 years. Though the demise of Cahokia is a mystery, the site has much to tell us about pre-Columbian America. Valuables originating from hundreds of miles away show that the societies of the time were not the primitive, isolated tribes we often imagine them to have been, and structures such as Woodhenge prove the Mississippians had a sophisticated understanding of astronomy. Basically, it's the lost city of Atlantis, right smack in the middle of the American Midwest.