Jammu, April 19 -- Overmining in China's old coal districts has led to shaft crumbling and authorities resorted to injecting concrete to reinforce land.

Nearly half of China's major cities are suffering "moderate to severe" levels of subsidence, putting millions of people at risk of flooding especially as sea levels rise, according to a study of nationwide satellite data released on Friday.

The authors of the paper, published by the journal Science, found 45% of China's urban land was sinking faster than 3 millimetres per year, with 16% at more than 10 mm per year, driven not only by declining water tables but also the sheer weight of the built environment.

With China's urban population already in excess of 900 million people, "even a ...