Coastal cities like Miami and Guangzhou face the prospect of massive flooding as sea-levels rise. Yet some cities are confronting an even more urgent threat of flooding than one brought on by climate change. A new study of 99 cities around the world published in the journal Geophysical Research Letters, reveals some of the world’s major cities are sinking even faster than the sea levels around them are rising.

In a process called subsidence, land settles and compacts based on changes to materials below the earth’s surface. This subsidence has caused land in the vast majority of these cities to sink by several millimeters per year. Much of this is brought about by human activity such as groundwater pumping. As water flows out, the land compacts, and the structures built on top fall closer to sea level.

At least 33 cities are falling by more than one centimeter per year,  five times the rate of sea-level rise, based on recent estimates of global sea-level rise. The fastest-sinking cities, […]