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.