Longevity Hill in Beijing. (Photo: shizhao/CC BY-SA 2.0)

Beijing needs water—a lot of it—and the city for years has relied on its groundwater to slake the thirst of a metropolis that has nearly doubled in size since 1990, to over 21 million people.

But taking such massive amounts of groundwater has had at least one bad side effect, according to the Los Angeles Times: parts of the city are sinking.

That’s because of a phenomenon known as subsidence, in which land sinks after parts of it, in this case water, are displaced.

And in Beijing, that sinking has been happening for a long time, even if the city recently has been making efforts to do something about it. Most worrisome is that the worst sinking is happening in the city’s Central Business District, which researchers said was dropping over four inches a year, according to the LA Times.

Researchers uncovered the sinking after looking at eight years of images from satellites and GPS data. In the worst case, the sinking could threaten rail lines and buildings, researchers said, since it is occurring unevenly.

For now, officials will have to do what they can. Last year, they announced they would stop using 367 water wells, though Beijing may later wonder if that was too little, too late.