Around 30% of China’s Ming-era Great Wall has disappeared over time as adverse natural conditions and reckless human activities – including stealing bricks to build houses – erode the Unesco world heritage site, state media has reported.

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

It is so dilapidated in places that estimates of its total length vary from 9,000 to 21,000 kilometres, depending on whether missing sections are included. Despite its length, it is not, as is sometimes claimed, visible from space.

Construction first began in the 3rd century BC, but nearly 6,300km were built in the Ming dynasty of 1368-1644, including the much-visited sections north of Beijing.

Of that, 1,962km has melted away over the centuries, the Beijing Times reported. Some of the construction has weathered away, while plants growing in the walls have accelerated the decay, said the report, 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 (£3) each by locals , it said.

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

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

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, the Times report added.