Around 30 percent of China's Ming-era Great Wall has disappeared over time as adverse natural conditions and reckless human activities -- including stealing the bricks to build houses -- erode the UNESCO World Heritage site, state media reported.

The Great Wall is not a single unbroken structure but stretches for thousands of kilometres in sections, from Shanhaiguan on the east coast to Jiayuguan in the windswept sands on the edge of the Gobi desert.

In places it is so dilapidated that estimates of its total length vary from 9,000 to 21,000 kilometres (5,600 to 13,000 miles), depending on whether missing sections are included. Despite its length it is not, as is sometimes claimed, visible from space.

Construction first begun in the third century BC, but nearly 6,300 kilometres were built in the Ming Dynasty of 1368-1644, including the much-visited sectors north of the capital Beijing.

Of that, 1,962 kilometres has melted away over the centuries, the Beijing Times reported.

Some of the construction weathered away, while plants growing in the walls have accelerated the decay, said the report Sunday, citing a survey last year by the Great Wall of China Society.

"Even though some of the walls are built of bricks and stones, they cannot withstand the perennial exposure to wind and rain," the paper quoted Dong Yaohui, a vice president of the society, as saying.

"Many towers are becoming increasingly shaky and may collapse in a single rain storm in summer."

Tourism and local residents' activities are also damaging the longest human construction in the world, the paper added.

Poor villagers in Lulong county in the northern province of Hebei used to knock thick grey bricks from a section of wall in their village to build homes, and slabs engraved with Chinese characters were sold for 30 yuan ($4.80) each by local residents, it said.

Under Chinese regulations people who take bricks from the Great Wall can be fined up to 5,000 yuan, the state-run Global Times said Monday.

"But there is no specific organisation to enforce the rules. Damage could only be reported to higher authorities and it is hard to solve when it happened on the border of two provinces," said Jia Hailin, a cultural relics protection official in Hebei, according to the report.

It added that explorations of undeveloped parts of the Great Wall -- an increasingly popular leisure activity in recent years -- had brought those sections more tourists than they could bear, damaging them severely.

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Great Wall