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.