(tense music) - You are looking at an ancient mystery.

Several miles east of downtown St. Louis, Missouri, a giant grassy mound rises from the earth.

Nearby, a constellation of smaller mounds dot the landscape.

These are not natural formations.

These are the ruins of Cahokia, the largest and most important ancient American city north of Mexico, one of the only surviving clues about a rich, ancient civilization that few today have ever heard of.

Who built Cahokia and why?

And why was it abandoned before Columbus ever set foot in the Americas?

Today, researchers, along with the descendants of this civilization are using cutting-edge science and technology to uncover the secrets of this lost city.

What they're discovering could forever change our understanding of this ancient civilization and paint a new picture of North America before Europeans arrived.

(pensive music) (dramatic music) The people who built Cahokia were part of an ancient civilization that's known today as the Mississippian cultures, Native Americans who lived around 800 to 1600 AD across what is today the American Midwest and Southeast.

Unlike the popular image of Native Americans as small, nomadic groups, the Mississippians lived in planned cities centered around massive earth and mounds, ceremonial plazas, and a sophisticated agricultural system focused on maize, squash, and beans.

And no city in the Mississippian world was bigger than Cahokia.

- [Dr. Edward] Cahokia was the largest Indigenous Native American settlement north of the valley of Mexico.

- We believe that Cahokia probably covered something like five or six square kilometers, which makes it orders of magnitude larger than any other place in North America prior to European contact.

- [Joe] At its height around the year 1100 AD, Cahokia was a bustling metropolis, home to at least 15,000 people, rivaling London in size.

Researchers have uncovered evidence of massive monument construction, complex social hierarchy, and advanced urban planning, demonstrating a level of complexity unrivaled in any North American city of its time.

- [Dr. Robin] Right now, when you're here, it's a nice, green field.

This is what Cahokia looks like for us today.

When we were here a thousand years ago, it would've been teeming with crowds of people, and there may have been places where people were exchanging goods, different religious ceremonies would've been going on.

- [Joe] The most striking feature of this ancient city was the more than 100 large earthen structures that tower over the Mississippi River floodplains.

At the center of it all stood Monks Mound, the largest earthen monument in North America, perhaps even in all of the Americas.

It's estimated that Cahokian builders carried nearly 15 million loads of dirt to build Monks Mound.

- The labor that goes into constructing the monuments at and around Cahokia is really something that's hard to wrap your head around.

Not only are people using stone and wooden tools to extract that earth from one part of the site, they're then moving it in basket loads to add to different layers of mounds.

- [Dr. Robin] The base of Monks Mound is actually larger than the Great Pyramid of Cheops in Egypt.

It's 100 feet high.

On the top of Monks Mound would've been an enormous temple that peaked to 50 to 75 feet high or more, so it would just have been an amazing site.

It must have been overwhelming for people.

- [Joe] Researchers are trying to understand how and why this place evolved from a simple settlement into such an awe-inspiring metropolis.

- [Dr. Robin] People have been living here in this general area for at least 12 to 14,000 years.

For most of that time, people lived here as hunters, gatherers, and fishers.

- Sometime in the 800 AD, we see the formation of what we would call villages, like permanent settlements for people constructing houses and living there year round.

- [Joe] And then around 1050, something happens here where Cahokia explodes.

We know that around 1000 AD, many indigenous societies in North America were establishing complex social hierarchies centered around religious and political leadership.

Around the same time, Cahokia became a major center of trade, worship, and governance in the Mississippian world.

But to unlock the secrets of exactly how Cahokian lived, archeologists are peering beneath the surface of these ancient ruins.

- We're conducting one of the largest geophysical surveys that's ever been done in the world.

We're trying to look at one of the largest pre-contact cities in the world and understand how that's organized, how people were arranging themselves into neighborhoods without ever disturbing the ground.

The instruments we're towing over the ground here at Cahokia are allowing us to map a lot of the subsurface architecture that still remains in the ground from Cahokia.

- [Joe] It's like an X-ray, peering beneath the ground without doing any destructive or invasive excavation.

- Magnetometers measure very subtle disturbances in the Earth's magnetic field.

Areas of burning show up very clearly.

The burning actually makes the soil more magnetic, so we can quite often see that.

But anytime soil been moved around from one place to another, whether it's construction of mounds, embankments, trenches, so we can identify those through our magnetometer survey.

- [Joe] These varying magnetic signatures when assembled together construct a blueprint of how Cahokia was built and organized.

- So at a site like Cahokia, house basins are probably the most useful archeological feature that we can detect using magnetometry.

We see those very clearly in magnetometer results.

- [Dr. Robin] For the very first time, we're gonna be able to say, "Where are the neighborhoods?

How many people lived here?

How big was this place?"

- [Joe] Many anthropology textbooks paint a picture of early cities as dense, tightly-packed spaces with little organization.

But archeological work from several continents is teaching us that there was more than one way to build a city.

What the magnetometry reveals is that Cahokia had urban planning that included neighborhoods with small plazas and buildings grouped together for different purposes.

One of the most striking areas of the city is this circular arrangement of wooden posts.

This so-called Woodhenge was positioned to align with the sunrise at key times of year, like solstices and equinoxes, giving us clues about the astronomical knowledge and ritual practices of its builders.

- [Dr. Edward] Perhaps not unlike a modern city today where you might have an administrative downtown area with a few small buildings, but then out from that city center, you have more populated parts of the town or city.

- [Joe] These views of the buried city suggest that Cahokia might have been even more complex than we imagined.

But by the mid 13th century, things in the big city began to change.

(pensive music) By 1250, Cahokia was in decline.

No new mounds were built, and the population began to dwindle.

- [Dr. Robin] It appears that over a span of 50 to 100 years, things just started trickling away, but we don't know why.

- [Joe] Evidence suggests there may have been rival tribes in the region.

Some archeologists believe that this conflict or political strife may have triggered Cahokia's decline, but others think that nature itself played a role.

'Cause around this time, a period known as the Little Ice Age brought colder drier weather to this region.

With corn harvests failing, Cahokia's massive population may have become impossible to feed.

But because Mississippians left no written records, the exact reasons for its fall still remain a mystery.

But the Mississippian culture didn't disappear.

It evolved.

Today, the Osage and other Indigenous Nations trace their heritage back to Cahokia, carrying forward its deep knowledge and traditions.

- One of the most important stories for us to always be telling about places like Cahokia is about the Indigenous sciences that were there to understand our place on earth, to understand our relationship with the plants and animals, to understand how to best inhabit this space.

- [Joe] Yet too often, this sophisticated understanding is overlooked, replaced by harmful stereotypes.

- Cahokia is such a powerful story.

When we can understand it as this feat of engineering, when we can understand it as this feat of Indigenous science, it's this really powerful place that can help us to understand that we weren't primitive nomads, wandering the desert in search of one drop of water or one drop of food.

- [Joe] Another misconception is that the people of Cahokia simply vanished.

- I hope the story that is told about Cahokia enables people to see that Indigenous Peoples did not just mysteriously disappear back into the earth from which they came, but in fact moved on for intentional and conscious reasons, but they continued to find ways to thrive into the future.

The Osage Nation today is this powerful, thriving nation.

So often that's not the story that gets told about who and what Indigenous Peoples are.

- [Joe] For the Osage Nation, Cahokia's legacy isn't just about the past.

It's a living heritage that guides their vision for the future.

- When you go back up on Monks Mound, for the Aboriginal-grounded person for the Osage-grounded person, you'll feel that emotion.

Our people felt that.

They practiced that.

We come from people that built that.

What people would spend decades by hand and accumulate the mass for Monks Mound?

Who would do that?

It's a prayer, and that's what Osage woke up with every day, and they went to retire at the end of the day in prayer.

Everything was about prayer.

- The spaces that we inhabit, the words that we speak, all of these things have lessons for us.

And they have important lessons for us, not just about who we were as a people, but who we should be into the future.

That there's values there, and that a huge part of the work that we're doing as an Osage Nation is rebuilding those relationships about who we are, not so that we can go back in time and not so that we can inhabit and bring those exact practices forward.

It's so that we can take the values and the many, many lessons that our communities learned through time, with place to help us build the strongest future possible for ourselves.

- For many, when they think of advanced civilizations in the Americas, their minds turned to the Inca, Maya, or Aztec.

But Cahokia stands as a reminder that sophisticated societies existed across the continent before European contact.

As oversimplified ideas about Indigenous societies continue to be proven wrong, we are learning more about the complexity and resilience of a civilization that shaped the American landscape in profound ways.