Gravity is different on different places on Earth. And gravity can change when masses on or in Earth shift—like in an earthquake or volcanic eruption. Or when a polar ice sheet melts or calves into the sea. Or even when groundwater levels change. It’s this last one we’ll take a closer look at—from space.

Fifteen years ago, NASA launched the Gravity Recovery and Climate Experiment, or GRACE. It’s a pair of satellites with a codependency issue. Rather than measuring Earth’s surface, they keep track of each other. In particular, their exact distance from one another, down to the micron.

When the first satellite passes over a place on Earth with greater gravity, it speeds up very, very slightly, and the distance between the satellites increases—by less than the width of a human hair. When it passes over an area with lower gravity, the distance decreases.

By tracking the expanding and contracting distances between each other, over 2.5 billon miles of Earth orbits, the pair has given us a more accurate picture of our varying gravity than ever before, which has revealed something very important.

Humans have never been able to measure groundwater well. But by sensing the shifting gravity as the water is depleted, the satellites have found that, under many cities in deserts, groundwater reserves have declined by two-thirds since they started measuring, putting these communities at serious risk.

With this remarkable data from space, we can better manage our water resources here on Earth.