The Great Wall is one of the existing megastructures and the world's longest human-made structure, stretching over approximately 6,400 km (4,000 miles) from Shanhai Pass in the east to Lop Nur in the west, along an arc that roughly delineates the southern edge of Inner Mongolia. It is also the largest human-made structure ever built in terms of surface area and mass.

http://en.wikipedia.org/wiki/The_Great_Wall_of_China