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. Reservations aren't taken for regular tours, which take about 45 minutes and start with a steep walk uphill.
74101 E. Kings Canyon Rd., (Rte. 180), between Grant Grove and Cedar Grove, California, 93633, United States