Sand storms in northwest China are reducing sections of Great Wall to mounds of dirt and may cause them to disappear in about 20 years, state media said on Wednesday.
The Great Wall, which was chosen last month as top of the new seven wonders of the world, snakes its way across more than 6,400 km (3,980 miles) and receives an estimated 10 million visitors a year.
More than 60 km of the wall in Minqin county in Gansu province, built in the Han Dynasty which lasted from 206 BC to 220 AD, had been "rapidly disappearing", Xinhua said, citing the head of the local museum, Zhou Shengrui.
"This section of Great Wall was made of mud rather than brick and stone, so is more prone to erosion," it quoted Zhou as saying, adding the wall had become brittle and the mud sanded down and blown away over time.
"Similar erosion happened to the Great Wall in other places, but the situation is much worse here," he was quoted as saying.
Extensive farming since the 1950s had sapped underground water in Minqin and destroyed the local ecology, which made the county a major source of sand storms in northwest China, the agency said.
More than 40 km of the wall had disappeared in the past 20 years and only about 10 km remained, it said.
The height of the wall had been reduced from five metres to less than two metres in places and the square lookout towers had disappeared completely, it added.
The Great Wall, which the United Nations listed as a World Heritage Site in 1987, has been rebuilt many times through the centuries, and many sections of it have suffered serious damage from weather erosion and human destruction.
Visitors climb wilder, crumbling sections that are not officially open to the public and stretches have become popular sites for summer raves.