The sprawling 2,200-acre Cahokia Mounds complex in Collinsville, Ill., are some of the most impressive Native American mounds in the country. While settlement in the area may date to roughly 1200 BC (during the Late Archaic period), the mounds as they are today were settled circa 600 AD (during the Late Woodland period). The mounds were probably built during the 9th century during the Emergent Mississippian cultural. The settlement has the distinction of being the largest, most influential urban settlement of the Mississippian culture.