2. Great Wall of China

It is by far mankind's most extensive construction endeavor. In its heyday in the 16th and 17th centuries, it snaked 6400 kilometers or 4000 miles across northern China.

It zigzags up and down steep ridges and is punctuated with imposing watchtowers, making the Great Wall of China one of the most photogenic man-made structures on earth.

Today, it is the travel icon of China - and one of the leading vacation travel destinations in the world.

Why the Great Wall of China was built. Among the many reasons:

• To serve as lookout posts - The ancient Chinese had rational fears about being invaded by nomadic armies from the north.

• To provide the army with a swift early warning system - Fire signals (nighttime) and smoke signals (daytime) were relayed from one watchtower to another. Messages could be rapidly sent over great distances.

• To create an elevated military roadway through the rugged terrain - This helped speed the deployment of soldiers from one area to another along the Great Wall of China.

• To impede invaders - This worked against feeble armies. However, the wall would serve more as a psychological than a physical barrier against a determined, well-manned military force. A formidable invader could easily breach a lightly guarded part of the wall. Or, it could muscle its way through one of the gaps between the individual wall sections.

No one ever said, "Let's build the Great Wall of China". There was never a master plan. Initially, powerful regional kingdoms built sections solely for their own defensive military needs.

The earliest known sections were constructed in the 7th century BC. Building activity by the regional kingdoms flourished in the 5th and 3rd centuries BC. It was only after the Qin Dynasty (221-206 BC) unified China did the wall become a collective entity. The Qin dynasty repaired, renovated and linked old sections and built new ones. Note: The modern word "China" derives from the Qin (pronounced "Chin") Dynasty. Over the next 1700 years, the Great Wall of China was enlarged and enhanced. Most of what tourists see today is the fruit of the work done in the 16th and 17th centuries.

Although much of it is in disrepair or deteriorated, some wall sections are in fairly good condition, thanks to repairs, renovations and protection programs by the Chinese government. There are currently slightly over 100 known sections ranging in states from deteriorated ruins to skilled restorations. One section was discovered just a few years ago. Archaeologists believe that some sections still lie buried and are waiting to be discovered.

It is popularly written that the wall is so long that you could see it with the unaided eye from an orbiting spacecraft (and some say from the moon). That would be impossible. Despite the Great Wall's extraordinary length, it's too narrow to be seen from an orbiting spacecraft without a telescope or super binoculars. Spotting the Great Wall from that height is analogous to being able to see a stretched-out mile-long thread on the ground while standing on top of a ten-story building.

Over the centuries more than a million people (peasants, soldiers and prisoners) helped build the wall. Thousands died in the process.

In ancient times the Great Wall of China was called the Ten Thousand Li Wall. This name referred to the wall's length (a “li” is a unit of measurement equaling roughly a half kilometer or one-third mile).

While the sections in eastern China were mainly made with bricks and chiseled stones, those in western China were made with less durable materials (often with clay or pounded earth reinforced with tree branches).

The path stretches from a seaport on China's east coast to Xinjiang in China's north west. In between, it passes through a variety of terrains, including mountains, plateaus and deserts.

The width and height of some sections are impressive. The average dimensions are roughly 6 meters (18 feet) wide and 8 meters (25 feet) high. The watchtowers normally add about 4 meters (13 feet) to the height.

The Great Wall of China watchtowers are fairly close to each other along many stretches of the Great Wall. Some are a stone's throw apart.

Most watchtowers were not garrisoned at any given time. Troops were regularly redeployed between one and another watchtower. The military goal was to keep the invading enemy guessing how many defending soldiers might be occupying a given watchtower.

Weathering through the centuries caused the lion's share of the damage to the wall. Other culprits were local residents seeking free building materials, souvenir-hungry travelers, and uncaring hikers.