Discovered in 1918, this unique cave has marble formations estimated to be 10,000 years old. Tickets for the 50-minute introductory tour are only sold online in advance or, during October and November, in person at the Giant Forest Museum and Foothills Visitor Center, not at the cave. Bring a jacket.

Accidentally discovered by two parks employees who were going fishing, the cave was carved by an underground river. Stalactites hang like daggers from the ceiling, and milky-white marble formations take the shape of ethereal curtains, domes, columns and shields. The cave is also a unique biodiverse habitat for spiders, bats and tiny aquatic insects that are found nowhere else on earth.

Tours fill up quickly, especially on weekends, so buy tickets online at least a month before your trip. Teens and adults can take a more in-depth, 90-minute ‘Discovery Tour’ ($18) or sign up in advance for a full-day spelunking adventure ($135). Wheelchairs, baby backpacks, purses, strollers, tripods and walking sticks are prohibited inside the cave. No drinking water is available. Note the cave may close during rainy weather (refunds available).

From the Lodgepole or Foothills areas, allow at least 90 minutes to get to the cave, which is a half-mile walk from the parking lot at the end of a twisty 6.5-mile-long paved road. Look for the signed turnoff for Crystal Cave Rd about 2 miles south of the Giant Forest Museum.