National Geographic magazine, May 2012. Reproduced with permission.

Maps: Maggie Smith, Bob Pratt, Theodore A. Sickley; Map Edit: Alexander Stegmaier; Text: Jane Vessels, Amanda Hobbs, Rhett Register; Photo Research: Jeanne M. Modderman; Design: Bill Marr; Graphics: John Tomanio: Text Research: David W. Wooddell; Copy edit: Mary Beth Oelkers-Keegan.