This is the most massive construction project undertaken in ancient times. The Great Wall rambles across 4,500 miles of hills, desert, and mountains from Shan-hai Pass to Phai-yü Pass keeping ancient enemies at bay. No single person or government is responsible for building the Great Wall. It is made up of a number of smaller walls erected by the warlords of the various provinces that would be united under Shih Huang-ti around 200bc. He took many of those existing walls and ordered them to be connected into a single defense system. That's why the main wall is only 4,100 miles long -- much of the rest are branches and offshoots left over from before China's unification.

Because of geography, its fragmented past and construction over hundreds of years, different sections of the wall are made from different materials. Some of the earliest fortifications were nothing more than minimal defenses placed on top of existing dikes. Others were huge formal walls, resembling the stonework of European castles, balancing on the crests of windswept mountain ridges. The walls, themselves, vary in form but are generally about 21 feet wide at the bottom, and 19 feet at the top. They stand between 23 and 26 feet tall, depending on the terrain. For the most part the inside of the wall is made of compacted dirt.

The outside is faced with brick, stone, or even wood in some sections. The walls are paralleled by drainage ditches to help reduce water damage. Today, large portions of the wall are gone -- destroyed by both man and nature. After the death of Shih Huang-ti many sections of the wall were abandoned until Emperor Wu-ti rose to power in 141bc. He ordered that beacon fires be lit at regular intervals, and guard towers and castles erected along its most vulnerable section.

This secure zone became known to merchants and traders in the area who used it as a way to move their goods without fear from bandits. It became known as the Silk Route. Since then a number of expansions and restorations have been necessary. Some of them using millions of workers and new advances in technology to create the barrier. Because of the dozens of restoration projects over the centuries, large portions of the wall are still standing as strong as they did 2,500 years ago giving visitors a glimpse into the might that China's ancient dynasties commanded. Children used to be taught in school that the Great Wall of China was the only man-made object visible from space. This is now believed to be an urban legend. The Great Wall became a UNESCO World Heritage site in 1987.