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 site of the Brooks Catsup Bottle Water Tower, the world's largest ketchup bottle, it is the world's horseradish capital, and according to legend is home to the Gates of hell.
Cahokia Mounds State Historic Site, a National Historic Landmark and UNESCO World Heritage Site, extends beyond the boundaries of the city toward the west. This prehistoric urban complex is estimated to have had a population of thousands at its peak, long before European exploration in the area. Monks Mound, the largest man-made earthwork in North America, is part of this complex.
Collinsville is located at 38°40′28″N 89°59′43″W (38.674398, -89.995402), approximately 12 miles due east of St Louis. The 90W longitude line passes through Collinsville.