In which country is the Great Wall?

China, and it is not one wall. It is a set of fortifications built, abandoned and rebuilt by different dynasties over roughly two thousand years, along different lines, out of whatever was to hand - rammed earth and reeds in the desert west, the familiar stone and brick near Beijing, which is Ming work from the 1500s and the most recent part.

The claim that it is visible from space is false and was in circulation long before anyone had been there. It is at most a few metres wide, the same colour as the ground it is built from, and no astronaut has reported seeing it unaided from orbit - Yang Liwei, China's first astronaut, said plainly that he could not. The story dates to at least 1932, when Ripley's Believe It or Not printed it decades before spaceflight.

It also did not really work. Walls of this kind are less a barrier than a tripwire and a logistics system - they slow raiders carrying loot, carry signal beacons and let troops move along the frontier faster than an enemy can move across it. When the Ming dynasty fell in 1644 it was because a general opened the gates at Shanhai Pass and let the Manchus through. The wall was intact, and irrelevant.