2 Best Sights in Sequoia and Kings Canyon National Parks, California

One of more than 200 caves in Sequoia and Kings Canyon, Crystal Cave is composed largely of marble, the result of limestone being hardened under heat and pressure. It contains several eye-popping formations. There used to be more, but some were damaged or obliterated by early-20th-century dynamite blasting. You can see the cave only on a tour. The Daily Tour ($17), a great overview, takes about 50 minutes. To immerse yourself in the cave experience—at times you'll be crawling on your belly—book the exhilarating Wild Cave Tour ($140). Availability is limited—reserve tickets at least 48 hours in advance at or stop by either the Foothills or Lodgepole visitor center first thing in the morning to try to nab a same-day ticket; they're not sold at the cave itself. 

The Kings River has carved out hundreds of caverns, including Boyden, which brims with stalagmite, stalactite, drapery, flowstone, and other formations. In summer, the Bat Grotto shelters a slew of bats. If you can't make it to Crystal Cave in Sequoia, Boyden is a reasonable substitute. Regular tours take about 45 minutes and start with a steep walk uphill.