An ancient Chinese city wall with a history of more than 14 centuries is being eaten away by a kind of mite in Xi'an, capital of northwestern Shaanxi Province, archaeologists have said.

Experts were surprised to find it had been the mites undermining the wall at the Hanguang Entrance Remains Museum during their one-year research for further protection.

"The discovery is the world's first of this kind. The mites had damaged the wall seriously," said Li Yuhu, archaeologist of Shaanxi Normal University.

The Hanguang Entrance is one of the 18 entrances in the city wall in Xi'an, which served as the nation's capital for 13 dynasties in history. The eight-meter-high entrance with three gates, one 5.5-meter-wide gate in the middle and two 5.3-meter-wide gates in the east and west, is the best preserved one, and was built at the beginning of the seventh century.

The wall had created a suitable environment for bacteria which feeds mites, Li said.

The mites had lived and built their dens in the wall for a thousand years, and had eaten it away from the inside, Li said.

Li and his colleagues have begun killing the mites with insecticide, by transfusing and spraying the pollution-free chemical into the wall once every 20 days. "The insecticide is widely used across the world and has proven harmless to human being, vegetation and the environment," said Li.

Because of its involatile nature, its effect could last seven to 14 years, he said.

Meanwhile, experts have also developed a kind of material to reinforce the wall against a number of other mite-related problems, including bacteria, loosening and cracks, Li said.

The material, invented by Li in 1990, had proven to be effective in the protection of the terracotta warriors and Banpo Ruins in the province during the last decade, Li said.

A protection team set up last year has carried out a one-year study on Li's protection method and tried to figure out a way to improve the method exclusively for the wall, said Wang Su, head of the protection bureau of the Hanguang Entrance Remains Museum.

The experts tested the method on the earth floor from the Tang Dynasty (618-907) before using it on the real wall, which proved effective, said Wang Danhu, an expert with the technological section of the State Administration of Cultural Heritage.

The new material made the earth stronger and more permeable, Wang said.

If successful, the method will have a broad future as most of China's historic buildings were built using earth, which had been a difficult problem for the world's archaeologists, An Jiayao, archaeologist of Institute of Archaeology, Chinese Academy of Social Sciences.

Relics in Japan and the Republic of Korea also face the same problem, An said.

"It is just the beginning. We know very little about the mites and we want to further investigate them," Li Yuhu said.