The Badlands Wilderness protects 64,144 acres (25,958 ha) of the park as a designated wilderness area[3] and is the site of the reintroduction of the black-footed ferret, the most endangered land mammal in North America.[4] Under the Mission 66 plan, the Ben Reifel Visitor Center was constructed for the monument in 1957–58. The park also administers the nearby Minuteman Missile National Historic Site.