Land is sinking underneath millions of peoples’ feet in China’s major cities due to human activities, putting the country’s coastal areas more at risk of flooding and rising sea levels, new research shows.

Nearly half of China’s urban areas comprising 29% of the country’s population are sinking faster than 3 millimeters (about 0.12 inches) per year, according to the study published Thursday in the journal Science. That’s 270 million people living on sinking land.

China’s rampant groundwater extraction is one of the primary factors for subsidence, researchers said. Cities have been pumping water from underground aquifers faster than it can be replenished, a situation exacerbated by climate change-fueled drought. Excessive pumping lowers the water table and causes the overlying land to sink.

The land is also sinking due to the growing weight of cities themselves. Soil can compact, naturally from the weight of sediments accumulating over time and from heavy buildings pressing down on the ground, causing the land to steadily sink.

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📸: AFP/Getty Images