Cahokia Mounds was first protected by the state of Illinois in 1923 when its legislature authorized purchase of a state park. Later designation as a state historic site offered additional protection, but the site came under significant threat from the federal highway building program in the 1950s. The highway program reduced the site's integrity; however, it increased funding for emergency archeological investigations. These investigations became intensive, and today continue. They have resulted in the present understanding of the national and international significance of the site. The site was designated a National Historic Landmark on July 19, 1964, and listed on the National Register of Historic Places on October 15, 1966.