The Great Wall of China is actually a series of walls and fortifications built and maintained over 2,000 years since as early as the 7th century BC.
Overall, the wall measures over 8,800 km (5,500 mi) - almost twice the distance across the United States!
Contrary to popular belief, the Great Wall is sadly not visible from the Moon, however, it may (barely) be from low-earth orbit in perfect conditions, so if you're planning a trip to space, don't be set your hopes too high.
Regardless of how you view the wall, it is, without a doubt, a wonder in its own right.
You can visit many different parts of the wall, which vary from fully-constructed to ruins. The most popular portions are Badaling and Juyongguan, and can be reached easily from Beijing by bus and train.