Collinsville is a city located mainly in Madison County, and partially in St. Clair County, both in Illinois. As of the 2010 census, the city had a population of 25,579, an increase from 24,707 in 2000. Collinsville is approximately 12 miles from St. Louis, Missouri and is considered part of that city's Metro-East area. It is the home of the Brooks Catsup Bottle Water Tower, the world's largest ketchup bottle, and is the world's horseradish capital.   Monks Mound, the largest man-made earthwork in North America, and now part of the Cahokia Mounds State Historic Site, is located here.