Wind Cave National Park is just south of Custer State Park. In fact, I inadvertently took a gravel road into Wind Cave NP off of the State Park's Wildlife Loop Road one day. Wind Cave's claim to fame are rare cave decorations called "box work." It kind of reminds me of lace hanging down from the ceiling (pic: http://www.nps.gov/mwr/customcf/apps/CMS_HandF/Pictures/WICA_Boxwork-for-DYK.jpg). It also has some beautiful frostwork that look like suspended sparkly white snowflakes in the light (pic: http://www.nps.gov/wica/naturescience/images/Frostwork-35.jpg).