A millennia ago, Native Americans entered the dark recesses of a cave in eastern Missouri and painted an astonishing array of human, animal, and supernatural creatures on its walls. Known as Picture Cave, it was a hallowed site for sacred rituals and rites of passage, for explaining the multi-layered cosmos, for vision quests, for communing with spirits in the "other world," and for burying the dead. The number, variety, and complexity of images make Picture Cave one of the most significant prehistoric sites in North America, similar in importance to Cahokia and Chaco Canyon. Indeed,...