A paper published in the journal Science Advances this week has some bad news for San Francisco and the Bay Area in California: Sea-level rise projections for the area had not previously factored in a geological phenomenon called subsidence.

That’s a problem.

Subsidence is the settling—or sinking—of land. It most often ends up in the news during a drought, when so much groundwater is pumped out of underground aquifers that the land on top literally sinks.

In San Francisco’s case, subsidence is happening in areas developed atop artificial landfill, and in places where neighborhoods were built on top of mud deposits. And when that is factored into sea-level rise models, the area around the bay in jeopardy of being underwater by 2100 increases from 48 sq miles (124 sq kilometers) to 166 (430 km2).

Among the fastest-sinking locales is Treasure Island, an artificial island in the bay between San Francisco and Oakland. It was built in the 1930s on top of landfill, and now it is sinking—or “subsiding”—by a third of an inch per year. That’s much more than the overall subsidence of the Bay Area, which is less than a tenth of an inch (2 millimeters) per year. That makes it more vulnerable to sea-level rise over time.

San Francisco International Airport is also subsiding faster than the average. When added to sea-level rise projections, that means half of its runways are likely to be underwater by 2100.