Wind Cave – located in Hot Springs, S.D. – was originally sought out to be a mining ground by several explorers, including J.D. McDonald in 1890, before being established as the sixth largest cave in the world. Although McDonald’s attempts to mine were unsuccessful, he and his family realized they could make money by giving cave tours and selling formations from the cave, and they set out to improve the entrance and expand the passageways for tours. (U.S. Air Force photo by Airman 1st Class Hrair H. Palyan/Released)