Cahokia was a Native American city that existed between roughly 950 and 1400 CE along the bank of the Mississippi River opposite the site of present-day St. Louis. It's the biggest Native American city that was ever built in North America both in terms of physical size and population, which at its height around 1100 CE may have broken 40,000 people, which Wikipedia points out is higher than the population of London at that period in time.
Cahokia was like a little bottle-study of the rise and decline of "civilization" in general. The archaeological evidence shows that with Cahokia's rise came the development of new and increasingly "complex" culture and even what seems like a new religion that eventually spread to dominate the region; the specialization of labor, industrialized agriculture to support its growing population, trade with far regions, feeder suburbs, and the development of organized leisure. But it also suffered all of the downsides of urbanization too - sanitation challenges leading to increasing disease, damage to the surrounding environment from large-scale resource gathering, the rise of upper classes that thrived on the backs of lowers, possibly a degree of institutionalized slavery, and leaders that held such a sway over the people that it seems like one or more of them were able to get away with some pretty dark things, at least for a time. Toward the end came a definite fear of attack by outsiders and there may have been some serious class conflict too.
After a relatively quick zenith, Cahokia declined - and I don't mean like deflated into some minor settlement, I mean it declined all the way to zero. It took a couple hundred years, but the place bled people until there was literally no one left - and not just in Cahokia; the entire vicinity for miles around was just totally abandoned and stayed that way for another couple hundred years. The people didn't just vanish into thin air - we know where they migrated to, and they took little bits of Cahokia culture with them like art and crafting styles and games and so on. But what they DIDN'T take with them, was any kind of oral history of Cahokia or its leaders. Which is a little spooky, when we're talking about people for whom oral history is bread-and butter and they're just like "okay well, let's just all agree that whole thing never happened."