Guided tours of parts of the 3rd largest cave in the world.
Established in 1908, Jewel Cave National Monument is home to over 179 miles of mapped cave passages. The cave features larger chambers and more interesting formations then nearby Wind Cave NP, and much of it is covered in spar crystals.
A guided tour is the only way to see the cave system unless you are in the park's limited volunteer cave exploring group. Depending on the time of year, up to 4 different tours are offered with the potential for the tickets to be sold out during the summer and holidays; tickets are only available the same day on a first-come-first-serve basis: http://www.nps.gov/jeca/planyourvisit/hours.htm. The cave is 49°F/9°C year-round but access can be closed in the winter due to severe weather.