The Chinese began working on this 4,000-mile long wall in the 770 B.C. It was built to protect China's borders from nomadic tribes and to keep invaders out. The Wall is the longest man-made structure in the world. Its five stone sections include forts and towers, where soldiers used to keep watch. Considered one of the Seven Wonders of the World, the Great Wall is visited by an estimated 13 million people a year. (JAN COBB—GETTY IMAGES)