Guides lead tourist in the Machu Picchu citadel, on July 5, 2011 during preparations for the centennial commemoration of its discovery by American adventurer and archaeologist Hiram Bingham in 1911. The compound, which sits at 2,350 meters above sea level in the heart of the Urubamba valley in southern Peru, 510 kilometers south of Lima, remained unknown to Spanish conquerors and is now visited by up to 250,000 a year. AFP PHOTO/Cris BOURONCLE