Beginning in the 7th century BC, a series of massive defensive fortifications were constructed along China's northern border. Built to protect China from northern attacks, the walls stretched out for thousands of kilometers, many joining together to become the Great Wall of China. Over several centuries, the wall and thousands of supporting structures were built across mountains, deserts, and rivers, eventually stretching more than 20,000 kilometers in length. Sections of the wall near large cities are well-maintained, but many remote areas are slowly being reclaimed by nature. Gathered here are images of the Great Wall over the years, from its westernmost pass at Jiayuguan to where it meets the sea in Qinhuangdao.
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Part of the Great Wall of China at Jinshan, framed by an arch, on the outskirts of Beijing, on August 12, 2003.
Visitors walk outside the Jiayuguan Pass Town in Jiayuguan, near the westernmost end of the Great Wall, in northwest China's Gansu province, on April 28, 2007.
A man walks past the remains of the western most tower of the Great Wall of China, left, next to a crumbling section of the wall at right, near Jiayuguan, in China's northwest Gansu province, on October 11, 2005.
(AP Photo/Greg Baker)
A train moves through a break in the Great Wall of China, at Jiayuguan, built in the Ming Dynasty (1372 AD), once part of the Old Silk Road, photographed on October 14, 2003.
(Goh Chai Hin/AFP/Getty Images)
Chinese tourists walk on a rebuilt section of the Great Wall of China, near Jiayuguan, in Gansu province, on October 11, 2005. The section, known as the Shiguan Gorge Overhanging Great Wall, is believed to have been built in the 16th century and had crumbled to almost nothing before being rebuilt in 1987.
(AP Photo/Greg Baker)
A woman is helped to the top of a mound at the Dajingmen Great Wall, once used as a watchtower overlooking the historic garrison town of Zhangjiakou, some 180 kilometers north of Beijing, in China's northern Hebei province, on May 24, 2006. Unlike other Great Wall sites nearer to Beijing which have been restored for tourism, the wall around Dajingmen Gateway, which marked China's northern border dating back to the early Ming Dynasty (around 1368) and rebuilt again in 1546 during the Qing Dynasty, has been left largely untouched.
(Frederic J. Brown/AFP/Getty Images)
A 124-mile (200-kilometer) stone section of the Great Wall along the middle of the Yinshan Mountains in central Inner Mongolia is shown in this April 1, 1998 photo. Discovered by archaeologists from the Inner Mongolia Autonomous Region, the section was part of the original wall built during the Qin Dynasty (221-207 BC).
(AP Photo/Wang Yebiao, Xinhua)
Chinese hikers make their way up a section of the Great Wall that is decaying and overgrown with vegetation located near Xiang Shui Hu village, located 80 km (50 miles) northwest of Beijing, on September 30, 2012. Voted one of the New Seven Wonders of the World in 2007, the 6,400 km (4,000 miles) wall draws millions of tourists every year, mostly to restored sections near the capital, Beijing. But away from the tourist trail, some parts of the wall are being allowed to crumble away.
A woman climbs part of the Jinshanling section of the Great Wall of China, in Hebei province, on July 17, 2012.
(Ed Jones/AFP/Getty Images)
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On February 24, 1972, U.S. President Richard M. Nixon stands at the Great Wall of China, near Beijing. Nixon and his party toured the wall before resuming talks with Communist Chinese leaders during his historic trip to the People's Republic of China, a step toward formally normalizing relations between the United States and China.
Tourists visit Laolongtou, or Old Dragon's Head, section of the Great Wall in Qinhuangdao, Hebei Province, on July 9, 2009. Old Dragon's Head is the eastern end of the Ming Dynasty (1368-1644) Great Wall. It extends about 20 meters (66 feet) into the Bohai Sea like a dragon drinking water, hence its name.
(Andrew Wong/Getty Images)