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.

Meanwhile, 67 million people are living on land that is subsiding faster than 10 millimeters (0.4 inches) each year.