Apart from Mark Twain's celebrated jumping frog, the feature Calaveras County in central California may be best known for is its array of fantastic limestone caverns. While some local spots offer guided tours, the trail to Natural Bridges cave, near Vallecito about 90 miles southeast of Sacramento, takes you to an otherworldly landscape you can explore on your own.
Coyote Creek runs through the cave, which is covered with calcium deposits, creating spiky rock formations that seem both alien and prehistoric. Condensation drips from the vaulted ceiling. You can swim, kayak, or float into the darkness, or scamper among the rocks and walk in the water to gape at the walls, provided you're careful not to do any damage.
To find the cave, start at the trailhead at Parrots Ferry and Airola Roads, then follow a moderate downhill path less than a mile to the hollow at the end. If the weather is warm, head out in the morning and bring drinking water, because the area can get hot in the afternoon. Inside the cave, all is cool and quiet.