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

"It was considered the largest man-made structure lengthwise, second only to the Great Wall of China, and was hailed as the largest earthwork in the world. With more recent work by Patrick Darling, it has been established as the largest man-made structure in the world, larger than Sungbo's Eredo. It enclosed 6,500 km² of community lands. Its length was over 16,000 km of earth boundaries. It was estimated that earliest construction began in 800 AD and continued into the mid 1400's."

Didn't know this. Pretty cool.