Archaeologists believe that this complex and sophisticated prehistoric "chiefdom", today part of the Cahokia Mounds State Historic Site and a UNESCO World Heritage Site, was the centre of a vast trading network supported by the rich agricultural bounty of the Mississippi floodplain. The site may have been abandoned in the 1400s, but the giant earth mounds built for ceremonies and burials remain. The tallest, at 30m with a base of more than 5ha, is Monks Mound.