The most important excursion from Cahokia is to Cahokia Mounds State Historic Site, about 8km north in the Collinsville area. This is the site of the largest pre-Columbian city north of Mexico — at its peak around 1100 CE, it had a population of 10,000 to 20,000 people and covered nearly 16 square kilometres. The great Monks Mound, the largest earthwork in the Americas, rises 30 metres from the surrounding plain. There is an excellent interpretive museum on site. The mounds are a UNESCO World Heritage Site.
Across the Mississippi, St. Louis is visible from Cahokia and easily reached in 15 minutes. The Gateway Arch and its museum, the City Museum, and the many neighborhoods of St. Louis are all within easy reach.