Story highlights The thirsty city has depleted its groundwater, leading to the sinking

Its central districts are among the most affected

Hong Kong (CNN) Beijing residents have put up with choking smog, trash-filled rivers and toxic running tracks. Now they have another concern -- sinking.

An international study led by Beijing-based researchers has discovered that the city is dropping by as much as 11 centimeters (4 inches) in some districts per year.

The thirsty city has depleted its groundwater, which the study identified as the cause of the sinking.

Using satellite imagery and GPS data, the team analyzed topographical trends from 2003 to 2010 and found that the city, which has a population of over 20 million, was sinking at an alarming rate.

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