Many—if not most—of the Earth’s aquifers are in trouble.

That’s the finding of a group of NASA scientists, who published their study of global groundwater this week in the journal Water Resources Research. Water levels in 21 of the world’s 37 largest known aquifers, they report, are trending negative.

The study is the first major accounting of groundwater change over time on the planetary scale. It was accomplished, not with wells or surveys, but with satellites.

Groundwater reserves are one of the environmental phenomena that’s hardest to conceptualize. Droughts are systemic and complex, sure, but a curious person can always go stand in a reservoir and see where the water is supposed to be. Dry lawns and fallow fields present another view on to what a drought is. But aquifers remain hidden, hard to measure, hard even to imagine: What does it mean that, beneath much of the United States, there are invisible seas full of drinkable freshwater? How can we think usefully about both their vastness and their finitude?

Already, 2 billion people worldwide rely on groundwater for daily use. That will have slosh-over effects: A 2012 study reported that water from aquifers, moved to the surface by human activity like farming and mining, would constitute 25 percent of sea level rise before 2050, and possibly even more after that. Relocated groundwater, by that paper’s estimate, would be the third-most significant cause of sea level rise this century, after the melting ice sheets of Antarctica and Greenland.

In the study released on Tuesday, researchers found that eight aquifers—particularly those in arid climates—were dangerously overstressed. Eleven major aquifers were “negatively recharging,” meaning people were pumping water out of them much faster than they were putting it in. “The water table is dropping all over the world,” Jay Famiglietti, a water researcher at NASA and one of the authors of the study, told The Washington Post. “There’s not an infinite supply of water.”

This study notably only allowed scientists to measure how aquifers were changing, not how big they are. But its methods seem to offer significant improvements on previous techniques.

Earlier groundwater research occurred by a sort of census. Government or independent researchers collect data about how people are accessing an aquifier—how deep they’re drilling, how much water they’re pumping, and how quickly they’re extracting it. They then combine this figure with other calculations about the size and depth of the aquifier to arrive at an estimate of its health.

The satellites in the Gravity Recovery and Climate Experiment—nicknamed GRACE—works by a different method: It observes the mass of water beneath the ground. The two satellites in GRACE chase each other across orbit, measuring their distance from each other. When they pass over something with more gravity—like a continent—the front satellite accelerates away from its partner. By measuring these accelerations, scientists can measure and estimate the planet’s gravity and heaviest regions. They can then estimate the presence of large, massy agglomerations—such as the underwater seas that are aquifers—that can’t otherwise be observed.

The two GRACE satellites launched in 2002, and their own methods have improved over time. Last year, NASA researchers found they could better measure groundwater by comparing GRACE’s findings to irrigation records, rather than by estimating the amount of water in the soil from climate records.

GRACE’s findings sometimes differed wildly from those on the ground. The study’s authors say that the aquifer beneath California’s Central Valley is in better shape than statistics would indicate. The aquifer beneath Democratic Republic of Congo, meanwhile, appears to be in considerably worse shape, losing water at least three times as quickly as statistics would estimate, the study says.

The sum of all these findings is that we’ll soon have to start treating and monitoring global groundwater with the same precision we track aboveground reservoirs. And last week, California took its first serious steps in that regard, issuing major regulations limiting how farmers could use water from the Central Valley aquifer. The mandate remains to treat groundwater—hard as it is to imagine—as the finite resource that it is.