Six Flags St. Louis, originally known as Six Flags Over Mid-America, is an amusement park located in Eureka, Missouri, a suburb of St. Louis. The park opened in 1971 as the third theme park of the Six Flags chain and was the last park built under the Six Flags name. Unlike its sister parks, Six Flags Over Texas and Six Flags Over Georgia, which are owned by limited partnerships and managed by Six Flags, the St.