The incredible Great Wall of China 8,850k of which more than 6,000k are actual wall. Large parts were built 8-500 b.c. but the wall as such was not joined together and finished until around 200 b.c. by chinas first emperor Qui Shi Huang - built to protect China from hostile northern tribes. Go see it at Simatai where it's authentic contrary to Badaling where it's totally restored and touristic like Disneyland.