Great Wall of China

A stone dragon 21,000 km long

The Great Wall is the longest structure ever built by humans: all its sections together stretch 21,000 km — halfway around the Earth! It was built over 2,000 years to protect against nomads. The wall crawls over the mountains like a dragon. Every few hundred metres there's a watchtower — when danger came, they signalled with smoke.

The Great Wall of China is a system of fortifications built by various dynasties from the 7th century BC to the 17th century AD; a 2012 survey measured all sections at 21,196 km. Qin Shi Huang (221 BC) was first to join the walls; the brick walls seen today are mostly from the Ming dynasty (1368–1644), 8,850 km long. It is 6–8 m high and 5–6 m wide, with 25,000 watchtowers; smoke and fire signals travelled 800 km in a day. Millions of soldiers and peasants built it, and many died. It cannot be seen from space with the naked eye — that's a myth. 30% of the Ming wall has been lost. A UNESCO site (1987), it draws 10 million visitors a year (Badaling, Mutianyu).

Fun facts

• The wall can't be seen from space — one of the most famous myths.
• Sticky rice was added to the mortar — that's why it has stood for 600 years.
• A smoke signal travelled 800 km along the wall in a day — the 'internet' of its time.