There's no better way to see what life was like underground for prospectors than to descend into the belly of the land. This fascinating tour let's you wander down a chasm into the wonder of an ancient limestone cave. Take the 235-step spiral staircase built in 1922 into this vast cavern. More intrepid explorers can rappel into the chamber—ropes and instruction are provided. Otherwise, the only way inside is via the 45-minute tour, during which you'll see giant (and still growing) stalactites and stalagmites and an archaeological site that holds some of the oldest human remains yet found in America. Outside there are three zip lines, starting at $44 per person.
5350 Moaning Cave Rd., off Parrots Ferry Rd., about 2 miles south of Vallecito, Vallecito, California, 95251, United States