By China correspondent Stephen McDonell
A study has found the Great Wall of China has deteriorated considerably over the past 100 years, and the battlements themselves are now in need of defending.
The series of walls and battlements dates back to 550 BC and stretches from the Bohai Sea to the Gobi Desert.
A British geographer living in Beijing, William Lindesay, is the pre-eminent non-Chinese expert on the Great Wall, and says it is his passion.
"In terms of wonders of the world you can't find anything to compete with the Ming Dynasty Great Wall," he said.
"It took more time to build, more people to build, consumed more building material than anything else in human history and it will never be surpassed in terms of scale."
But his enthusiasm is tempered by the findings of his own research.
Mr Lindesay has taken hundreds of old photographs of the Great Wall - some 50 years old and some 100 years old - and laboriously re-photographed exactly the same locations.
The results of his re-photography study will be released in Beijing later today.
They show huge sections of the Great Wall, and the towers along it, completely destroyed over the last century.
Mr Lindesay says the wall is crumbling.
"Suddenly people realise that the Great Wall of China needs defending itself," he said.
And he believes he knows why it is deteriorating.
"In the last 50, 60 years, it's people going up to the wall and using the material as a quarry - the bricks especially," he said.
"Most serious in the last few years in the Beijing area has been unmanaged and unauthorised tourism development."
Some parts of the Great Wall have become crowded with tourists and souvenir sellers, as poor local farmers try to convert their piece of wall into the next tourist hot-spot.
The car fumes, the shops and the sheer visitor numbers have all taken their toll on the wall, and in some places the wall has been blasted open to make way for roads.
But Chinese government authorities have shown Mr Lindesay the blueprint for Badaling - the wall's most popular tourism location.
"All of the shlock is going to be moved back three kilometres," he said.
"The car parks, the t-shirt market, the coffee shops and the restaurants - everything is going to be moved back."
Using the research of Mr Lindsay and his Chinese counterparts, the Great Wall is being restored.
As of January 1, there are strong fines for having social functions on the ancient wall or stealing bricks from it.
Despite the huge damage that was also caused by fighting during World War II, thousands of kilometres of Great Wall remain.
It is an enormous and sometimes isolated structure and its preservation will be a difficult task.