21. Portions of the Great Wall of China

With great walls come great responsibility. And for China, the 5,000 mile Great Wall comes with the great responsibility of upkeeping such an impressive structure. But try as they might, almost two-thirds of the wall has already been closed off to the public.

Of course, tourists still flock to the Great Wall of China to get to walk on a bit into history. The Chinese government has allowed some of the Wall to be torn down as China builds up infrastructure, but other portions are crumbling due to vandalism and natural causes. And if the erosion continues, there could be a day when the Great Wall was just a great memory.