Humans are rapidly depleting about one-third of the world’s largest groundwater basins without knowing when they might run dry, two new studies suggest. Groundwater is a key source of water for drinking or for commercial and industrial uses such as agriculture. Arid and drought-ridden regions are especially reliant on groundwater. It’s tough to measure how much water groundwater aquifers contain, lose, or gain, however, given how large and difficult to access they are. Now, researchers have used satellite data for the first time to measure groundwater use from the world’s 37 largest basins. From slight changes that twin satellites in NASA’s Gravity Recovery and Climate Experiment mission picked up in the basins’ gravitational tug, the team could estimate how fast they were gaining or losing water. And a water model the scientists used told them which basins were getting at least some water fed into them naturally. Between 2003 and 2013, 21 of the 37 basins’ water levels were declining, the team reports this week in Water Resources Research. Eight of those 21 weren’t being naturally replenished at all, with another five only being slightly replenished. Basins in the world’s driest areas, such as the Arabian Peninsula, are generally in the worst shape of those 13. Moreover, comparing their findings with old data on how much water the basins can store, the researchers found widely varying estimates on when basins would run 90% dry—from 10 years to tens of thousands of years, in the case of the northwest Sahara. Climate change and population growth may simply hasten the demise of many basins, they warn.