Archaeological excavations along river valleys have shown continuous habitation since about 9000 BCE. Beginning before 1000 CE, the people of the Mississippian culture created regional political centers at present-day St. Louis and across the Mississippi River at Cahokia, near present-day Collinsville, Illinois. Their large cities included thousands of individual residences. Still, they are known for their surviving massive earthwork mounds, built for religious, political and social reasons, in platform, ridgetop and conical shapes. Cahokia was the center of a regional trading network that reached from the Great Lakes to the Gulf of Mexico. The civilization declined by 1400 CE, and most descendants left the area long before the arrival of Europeans. St. Louis was at one time known as Mound City by the European Americans because of the numerous surviving prehistoric mounds since lost to urban development. The Mississippian culture left mounds throughout the middle Mississippi and Ohio river valleys, extending into the southeast and the upper river.