Cahokia Mounds is an archaeological site located near present-day Collinsville, Illinois. It was once a thriving urban center of the Mississippian culture from around 600 to 1400 CE. The site features over 100 mounds, with the largest, known as Monks Mound, standing about 100 feet tall. These mounds were used for various purposes, including ceremonial, residential, and burial activities. At its peak, Cahokia was one of the largest cities in the world, with a population estimated between 10,000 and 20,000 people. The site is recognized as a UNESCO World Heritage Site, highlighting its significance in understanding pre-Columbian North American history.