Wind Cave National Park was our country's 7th national park in 1902 and the first to protect a cave. It has just over 165 miles of explored passageways, making it the 3rd largest in the country and 6th largest in the world. All 165 miles are compacted into one square mile and go up to 527 feet below the surface. They suspect that there are up to 1,500 miles of passageways, although many are too small for a human to fit and therefore may never be explored. Unlike most caves that have stalagmites and stalactites, this cave has neither because there is no water dripping from the surface. The entire valley used to be a warm ocean and the cave's limestone was carved out by acid in rainwater as the ocean receded. It gets its name due to the air that is released or sucked in to its natural entrance to balance barometric pressure.
Exploration of the caves occurs via various ranger-led tours, and I chose the Natural Entrance Tour on my visit (advance reservations on www.recreation.gov are highly recommended). Tours range from an hour and half to more than 4 hours and have levels of difficulty. At the end of my tour, we were 20 stories underground, directly under the visitors center, and took an elevator back to the top. One of the other tour options was the Wild Cave Tour, which was 4 hours long and let you belly-crawl through the smaller tunnels. In the visitor center, you had to be able to fit through the 'squeeze box', a narrow wooden box with two open ends that was designed to mimic the narrowest tunnel you would encounter on the tour. It was pretty entertaining to watch people try to fit through and get stuck. Several fathers were suckered into trying it by their small children and could rarely fit (most women didn't have a problem due to narrower shoulders). I totally would have done this tour if I had more time and have added it to my list for the next time I'm in the area. You can find cave tour information here.
Note that GPS directions for the visitor center are often inaccurate, but you can find the correct directions here. There aren't many amenities or any food options available within the park, so stop for snacks and water in the town of Pringle if you're coming from the north or Hot Springs if you're coming from the south. Watch for bison as you enter the park boundaries and be mindful that they may be crossing the road.
Once your tour and exploration of the visitor center is complete, be sure to check out some of the places listed in the sidebar on the right. You'll pass through at least one of them on the way to your next destination and all are worth a visit.