Fodor's Expert Review Jewel Cave National Monument
Even though its more than 177 miles of surveyed passages make this cave the world's third largest (Kentucky's Mammoth Cave is the longest), Jewel Cave isn't renowned for its size. Rather, 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.