China's capital is known for its horrendous¹ smog and occasional sandstorms. Yet one of its major environmental threats lies underground: Beijing is sinking.

 

北京可怕的雾霾和偶尔发生的沙尘暴为世人皆知，而一个重要的环境威胁却被大家忽略了：北京正在下沉。

 

Excessive pumping of groundwater is causing the geology under the city to collapse², according to a new study using satellite imagery that reveals parts of Beijing - particularly its central business district - are subsiding³ each year by as much as 11 centimetres, or more than four inches.

 

The authors of the study warn that continued subsidence poses a safety threat to the city of more than 2o million, with "a strong impact on train operations" one of the predictions.

 

The study on Beijing's subsidence has been published in the peer-reviewed journal Remote Sensing and is based on InSAR, a type of radar⁴ that monitors land elevation⁵ changes. It was written by a team of seven researchers.

 

"We are currently carrying out a detailed⁶ analysis of the impacts of subsidence on critical infrastructure⁷ (eg high-speed railways) in the Beijing plain," they said in an email to the Guardian⁸. "Hopefully a paper summarising our findings will come out later this year."

 

Beijing sits in a dry plain where groundwater has accumulated over millennia⁹. As wells are drilled and the water table drops, the underlying¹⁰ soil compacts, much like a dried-out sponge.

 

The study finds that the entire city is sinking but the subsidence is most pronounced in Beijing's Chaoyang district, which has boomed since 1990 with skyscrapers¹¹, ringroads and other development. The researchers say the uneven¹² nature of the subsidence in some areas poses risks to buildings and other infrastructure.

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