The Great Wall of China.

The Great Wall of China actually consists of numerous walls and fortifications, many running parallel to each other. Originally conceived by Emperor Qin Shi Huang (c. 259-210 B.C.) in the third century B.C. as a means of preventing incursions from barbarian nomads into the Chinese Empire, the wall is one of the most extensive construction projects ever completed.

The best-known and best-preserved section of the Great Wall was built in the 14th through 17th centuries A.D., during the Ming dynasty (1368-1644). Though the Great Wall never effectively prevented invaders from entering China, it came to function more as a psychological barrier between Chinese civilization and the world, and remains a powerful symbol of the country's enduring strength.
When Emperor Qin Shi Huang ordered construction of the Great Wall around 221 B.C., the labor force that built the wall was made up largely of soldiers and convicts. It is said that as many as 400,000 people died during the wall's construction; many of these workers were buried within the wall itself.
This wall is 4000 miles long.