Whether or not it can be seen from space, the Great Wall of China is incredibly long. After five years of investigation, Chinese archeologists have determined that the Great Wall is actually more than 13,000 miles long, TWICE as long as it was previously believed to be. This makes it by far the largest structure ever created by man.
The earlier length of 5,500 miles was based on historical records. Also, a lot of it has collapsed: only about 8% of the part of the wall that was built during the Ming Dynasty remains intact, and about 75% is in poor condition. In some sections of the wall, only the foundation remains.
The wall was begun in 500 BC, and took approximately 300 years to complete. It was maintained and extended for more than a thousand years afterwards. These days, it's threatened primarily by tourism.
We're still learning about the past, and (of course) we're worried about the future. What if you had a painting that was really a time machine? That's the wonderful premise of Whitley Strieber's novel "The Omega Point."