Situated eight miles northeast of St. Louis, this 2,200-acre Illinois state park is located on land once home to the largest settlement of the pre-Columbian Mississippian culture. This central gathering site originally covered 4,000 acres and boasted a population of 10,000 to 20,000 in its center, with 40,000 to 50,000 individuals residing in Greater Cahokia, composed of additional sites in modern-day St. Louis and East St. Louis. Ridgetop mounds defined the limits of the city, with other earthworks featuring plateaued and conical peaks that designated their residential, civic, agricultural, religious, and commercial usages. The present-day site includes 51 mounds, while historical records indicate that there were more than 100 earthworks in use between AD 1000 and AD 1350. The 100-foot-high Monks Mound is Cahokia’s central earthwork. Constructed in fourteen stages to ultimately encompass fifteen acres, it is the largest indigenous earthen structure north of central Mexico. Some other features were constructed with wood, including an astronomical observatory for solar tracking. A portion of the site contains a reconstruction of a defensive palisade, which is estimated to have encircled much of the site during its population peak from 1000-1350. Supplementing the visitor experience, an Interpretive Center was designed by Booker Associates and opened in 1989. Sited in the eastern side of the park, this contemporary structure hosts archeological exhibitions of artifacts found in the mounds and historical recreations based on these findings. The state of Illinois created a state park to protect and interpret the mounds in 1923. The site came under threat from federal highway development in the 1950s, when an interstate highway was constructed through the Cahokia Creek floodplain. To protect the site from further encroachment, the mounds were listed in the National Register of Historic Places in 1966 and designated a National Historic Landmark in 1964. The Cahokia Mounds were named a UNESCO World Heritage Site in 1982.