Cahokia Mounds, a UNESCO World Heritage Site about 12 kilometres east of downtown across the Mississippi in Illinois, was the largest Pre-Columbian city north of Mexico — the centre of Mississippian culture around 1050 to 1350 CE. At its peak the city covered six square miles and had a population estimated at up to 20,000.
About 80 earthen mounds survive of the original 120 or more. Monks Mound, the largest, rises 30 metres over three terraces and was the ceremonial centre. A small museum interprets the site well: dioramas, recovered artefacts, an excellent introductory film. Allow half a day with the climbing of Monks Mound and the longer interpretive walk. Free admission; donation requested.