Smithsonian.com: You just may want to join a ranger-led hike in Southwest Colorado's Mesa Verde National Park. You'll see a spot where the scrubby pinyon-juniper forest drops off. It falls into a sandstone chasm. It reveals a maze of 800-year-old stone dwellings. They are wedged beneath an overhang in the canyon wall. They're well preserved. It's easy to imagine you've stepped back in time. And that nothing has changed in this high desert landscape since the Ancestral Puebloans built these chambers. They were created...