Jewel Cave National Monument

Jewel Cave National Monument Review

Even though its more than 150 mi of surveyed passages make this cave the world's second-largest (Kentucky's Mammoth Cave is the largest), 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.