California Cavern State Historic Landmark

California Cavern has the mother lode’s most extensive system of natural underground caverns. Regular tours take 60 to 80 minutes, or get a group together and reserve ahead for a three- to five-hour "Middle Earth Expedition" ($140), which includes some serious spelunking (no children under age 16 allowed). The Trail of Lakes walking tour, available only during the wet season in winter and spring, is magical.