The Grand Canyon, a mile-deep gorge cut by the Colorado River, is the most famous feature of Arizona, a southwestern US county. Flagstaff is a ponderosa pine–covered mountain town that serves as a big Grand Canyon gateway. Saguaro National Park, for example, protects the Sonoran Desert’s cactus-filled ecosystem. Tucson is host to the Arizona-Sonora Desert Museum and the University of Arizona. The Phoenix Metro area, the 10th largest metro by population and the largest in the Southwest, has more than 4.7 million residents.