The Great Wall
Seen from space?
Contrary to popular belief, it can't be seen from the Moon. It was not built as one long wall - different Chinese dynasties built different walls, and they connect in places. But the Great Wall of China is truly great, and it is one of the places in the world you have to visit.
For many years, people said the Great Wall was around 5,000km long. But in April, a survey found it was actually longer than 8,800km. That's about the same distance as from Los Angeles to Tokyo - 12 hours by plane.
A huge endeavour
The early Chinese states probably built primitive walls seven centuries before the birth of Christ. But they got serious about building after Qin Shi Huang conquered all of China in 221BC. He destroyed all the old walls and built a new one in the north.
Qin Shi Huang's wall, like all the walls that came after it, aimed to keep out the 'barbarian' people who lived on the plains. The Chinese called them the Xiongnu. But they were actually a mix of many different tribes - mostly Turkic and Mongol - who sometimes banded together to try to take over China.
Waves of walls
Qin Shi Huang's dynasty, the Qin, lasted only 13 years. The next great dynasty, the Tang (618-907), was so powerful it didn't need a wall. But, after many costly battles, the Ming dynasty (1368-1644) spent around 100 years building a wall to protect itself from the Manchurians and the Mongols.
Most of what you can visit today is the Ming wall. It was particularly strong near Beijing in order to protect the capital. This is why Beijing is the best place to see the Great Wall. In many other parts of China the great wall is very eroded. Some sections have almost disappeared completely.
The same survey that found the wall is longer than we thought also found many sections of it are threatened. In some sections, it is simple erosion. In other sections, construction of roads and buildings have affected it. Villagers even take stones from the wall to build homes.
'The Great Wall is under great threat, climate change and the country's massive infrastructure building being the biggest two,' State Administration of Cultural Heritage director Shan Jixiang told the China Daily early this year.
See it while you can.