The Sonoran Desert covers 120,000 square miles in Arizona, California and Mexico. It's the hottest of North America's deserts, but as one of the largest places in the U.S. that hasn't been bombarded with malls, banks and fast-food joints, it's pure, unadulterated nature, renowned for its annual spring flowering and home to a wide range of plants and animals including wild horses, prairie dogs and the famous saguaro cactus. It's the only non-building on our list, just as a reminder to occasionally stop and smell the cactus flowers.