Even though its more than 168.25 mi of surveyed passages make this cave the world's third-largest (Kentucky's Mammoth Cave is the longest), it isn't the size of Jewel Cave that draws visitors, it's the rare crystalline formations that abound in the cave's vast passages. Wander the dark passageways and you'll be rewarded with the sight of tiny crystal Christmas trees, hydromagnesite balloons that would pop if you touched them, and delicate calcite deposits dubbed "cave popcorn." Year-round, you can take ranger-led tours for a fee, from a simple half-hour walk to a lantern-light tour. Surface trails and facilities are free.
U.S. 16, 15 mi west of Custer, Custer, South Dakota, 57730, United States