The Great Wall of China is crumbling.

The Jiankou section of the Great Wall of China. | (REUTERS/Damir Sagolj)

The iconic barrier that snakes for 5,500 miles across the Chinese landscape was built in various waves and sections beginning around 220 B.C. and throughout the Ming Dynasty into the 1600s. It was originally meant to protect the newly unified country from barbarian invasions. Later, it helped regulate trade and immigration.

Toward the latter half of the second millennium, as physical barriers like this were rendered unnecessary in the new global order, the wall seemed to act as a psychological barrier between China and the rest of the world, and a reminder of the country's complicated history.

Though it remains one of the world's most impressive and recognizable manmade structures, and a huge tourism draw, time has not been kind to the Great Wall. Many areas, particularly those stretches crossing China's most rugged terrain, are in a state of crumbled disrepair. Stairs have been worn down to suggestive inclines, and entire ledges have been blown off.

In 2005, China began the arduous undertaking of restoring the UNESCO World Heritage site — brick by brick.

(REUTERS/Damir Sagolj)

In Jiankou — a particularly scenic but treacherous 12-mile stretch two hours north of Beijing — the wall winds along a series of sharp cliffs and slopes. This makes it impossible to hoist up or hold modern machinery, so repair efforts have relied on old reconstruction methods: Traditional bricks are laid by hand using extremely basic tools and hauled by pack of very tough mules.

Each day, starting at 6 a.m., mules are saddled with up to 440 pounds of bricks each, which they lug up a steep forest passageway to waiting workers. These beasts of burden traverse the jagged ridges for 10 hours each day.

(REUTERS/Damir Sagolj)

Laborers and their supplies are also held to strict standards by local governments, which insist on preserving the wall's natural beauty and original design. That means bricks are hoisted up to the highest locations in buckets by traditional pulley systems. They are put into place using only very simple hammers, chisels, and shovels. Even the bricks themselves must be actual ancient pieces that have fallen from the wall previously — or exact replicas.

After workers in the northeastern Liaoning province paved their section with sand and cement, the public was outraged over the "ugly" pedestrian-pavement look. So now, authorities are keeping a closer watch on the progress to ensure the wall maintains its authentic look.

"We have to stick to the original format, the original material, and the original craftsmanship, so that we can better preserve the historical and cultural values," Cheng Yongmao, the lead engineer for Jiankou's reconstruction, told Reuters. Cheng, who comes from a long ancestry of traditional brick makers, has helped rebuild 11 miles of the wall over the course of more than a decade.

Have a look at the delicate and sometimes painstaking process of restoring the Great Wall to its original glory:

Workers carry their tools and belongings as they climb down the Great Wall. | (REUTERS/Damir Sagolj)

Vegetation grows over part of Jiankou. | (REUTERS/Damir Sagolj)

A worker waits for bricks and other construction materials to be delivered by mules. | (REUTERS/Damir Sagolj)

(REUTERS/Damir Sagolj)

(REUTERS/Damir Sagolj)

(REUTERS/Damir Sagolj)

(REUTERS/Damir Sagolj)

An engineer points to the line between the old wall (left) and the newly built portion (right). | (REUTERS/Damir Sagolj)

Workers take a break from restoration work on Jiankou. | (REUTERS/Damir Sagolj)

(REUTERS/Damir Sagolj)