Collinsville is a modest suburb east of St. Louis, but it sits at the edge of one of the most remarkable archaeological sites in North America: Cahokia Mounds State Historic Site, a UNESCO World Heritage Site and the most important remnant of the Mississippian mound-building culture.
At its peak in the 11th and 12th centuries, more than 20,000 people lived here — making it larger than London at the time. About 80 mounds survive across the site, ranging from small platform mounds to the immense Monks Mound, the largest prehistoric earthen structure in North America. At 100 feet high and covering more ground than the Great Pyramid of Giza at its base, it takes a few minutes of climbing to reach the top, where the scale of the ancient city becomes clear.
The visitor center has a well-made introductory film and a museum covering Mississippian life, trade networks, and the still-mysterious collapse of the city around 1350. Allow two to three hours. If you are passing through the St. Louis area, this is worth a stop.