In close quarters to the modern-day city of St. Louis, Missouri, Cahokia Mounds State Historic Site — also a World Heritage Site — is one of the most prominent Native American settlements ever discovered. The site itself was once home to a bustling Native American city, covering 3.5 square miles, and had an estimated population of 40,000, making it the largest known urban center in America at the time. The city probably peaked around the year 1200, and it represents just how complex and advanced Native societies actually became.

Source: 5 Stunning Places Where You Can See Native American Ruins

I’ve lived in proximity to St. Louis most of my life and hadn’t heard of this until recently.