BEIJING - Meandering on mountain ridges across north China, the Great Wall, one of the world's great wonders, has for thousands of years protected residents from invasion.

Gone are the beacons and soldiers on the mountaintops. But another group of rangers has appeared, protecting the ancient landmark from a new kind of invasion.

In Yanqing District, on the northern outskirts of Beijing, home to a 179- km-long section of the Great Wall, a team of more than 60 government-employed rangers and some 200 volunteers have ensured that there is no further man-made destruction of the centuries-old wall.

Mei Jingtian, 73, is the oldest of them.

He lives in Shixia Village near Badaling, where the oldest sections of the Great Wall have been dated back to the Northern Qi Dynasty (550 - 557). "I grew up at the foot of the Great Wall and it was an important part of my childhood," he said.

Mei remembers the imposing wall of his youth, before it went through massive destruction during the Cultural Revolution in 1960s and 1970s, when it was considered a remnant of feudal history to be pulled down. During that time, some locals even took bricks from the ancient wall to build their houses.

The Great Wall, a symbol of China, is actually not just one wall, but many interconnected walls built between the third century B.C. and the Ming Dynasty (1368-1644). It was listed as a World Heritage Site by UNESCO in 1987, but its preservation faces long-term challenges.

According to the Yanqing cultural relics authority, only 10 percent of the Great Wall in the district has been protected, 15 percent has completely disappeared and the rest is in ruins.

Mei became a voluntary ranger in 1983. Carrying a bottle of water and a sickle, he leaves home at dawn, humming an old song while picking up the garbage and weeding around the foot of the Great Wall.

He walks around 20 kilometers each day. Over the decades, he had worn through more than 200 pairs of shoes. "I feel reassured standing on the mountain," he said. "The Great Wall is unique. If we don't do anything, I think one day it might disappear completely."