Badlands National Park, in southwest South Dakota, preserves 244,000 acres (98,740 ha) of sharply eroded buttes, pinnacles and spires blended with the largest protected mixed grass prairie in the United States. The Badlands Wilderness protects 64,144 acres (25,958 ha) of the park as a designated wilderness area and is the site of the reintroduction of the black-footed ferret, the most endangered land mammal in North America. Containing the world’s richest Oligocene epoch fossil beds, dating 37-28 million years old, the evolutionary stories of mammals such as the horse and rhinoceros arise from the 244,000 acres of sharply eroded buttes, pinnacles, and spires. Bison, bighorn sheep, endangered black-footed ferrets, and swift fox roam one of the largest, protected mixed-grass prairies in the United States.