Police in north China have arrested four people allegedly involved in an illegal mining operation that destroyed a section of the Great Wall.

The alleged ringleader of the gang, a 40-year-old man with the surname Wang, allegedly claimed that they destroyed part of the Great Wall with mining machines over a weeklong period in middle October.

An investigation by the local cultural relics bureau found a section of the Wall 10 meter high and 23 meters long had been destroyed at Luliang Mountain, Qingshuihe County, Inner Mongolia Autonomous Region.

The section had totally collapsed and a 1,000-square-meter protection area around the Wall has also been damaged, the bureau officials said.

The part of Great Wall, originally built in the Han Dynasty (206 B.C.- 220 A.D.) and rebuilt in the Ming Dynasty (1368-1644), is on the main trunk of the Wall, which runs from the Shanhaiguan Pass in north China's Hebei Province westward to Gansu's Jiayuguan Pass.

"This section of Great Wall was made of mud rather than brick and stone and is more prone to erosion or damage from human activities," said Wang Dashan, a regional cultural protection expert.

The Great Wall, which was listed as the United Nations World Heritage Site in 1987, was first built in the Warring States Period (475-221 B.C.) to defend China against invasion by northern nomadic tribes. It was rebuilt and extended many times through history and earlier records indicated it stretched about 6,000 kilometers.

Like any other architectural site in the world, the Great Wall is at risk of damage caused by natural and human activities. In some sections, its bricks and dirt have even been used as construction materials.

"Only a small portion of the Great Wall is under protection, and about 90 percent of it, mostly in remote areas, lacks proper protection," said Dong yaohui, deputy chairman of the China Great Wall Association.

In 2004, the Great Wall was listed as an endangered site by the World Monuments Fund, a New York-based nonprofit organization on preservation of cultural and architecture sites.

The State Council, or China's Cabinet, issued a regulation in September last year banning vandalism and driving on the Great Wall, taking soil or bricks, and building on it.

Meanwhile, China's State Administration of Cultural Heritage (SACH) and State Bureau of Surveying and Mapping (SBSM) are carrying out a geographical survey of the Great Wall. Statistics including its exact length and layout will be released in 2008.

(Xinhua News Agency November 8, 2007)