Forget about peak oil—we should be worrying about peak water: Groundwater basins that supply 2 billion people are being rapidly depleted, according to a new study. Worse: No one knows how long those reserves will last.

A research team led by scientists at the University of California, Irvine, examined the world’s 37 largest aquifers between 2003 and 2013 and found that one-third of them were “stressed,” meaning more water was being removed than replenished, according to one of two studies published Tuesday in the journal Water Resources Research.

The eight worst-off aquifers, labeled “overstressed,” had virtually no natural replenishment to offset human consumption.

The scientists determined the Northwest Sahara Aquifer System, which supplies water to 60 million people, to be the most overstressed. The Indus Basin aquifer of northwestern India and Pakistan is the second-most overstressed, followed by the Murzuk-Djado Basin in northern Africa. California’s Central Valley is “suffering rapid depletion” and was classified as highly stressed, according to the study.

The findings are alarming, especially as humans increasingly pump groundwater during times of drought.

So, Why You Should Care? As lakes and rivers diminish owing to climate change, aquifers become “an increasingly important water supply source globally,” the study said. “Understanding the amount of groundwater used versus the volume available is crucial to evaluate future water availability.”

The authors analyzed data from two NASA satellites that detect dips and bumps in the earth’s gravity, which is affected by the weight of groundwater.

Even that state-of-the-art technology cannot determine how much water remains beneath the surface.

There is a severe shortage of data on global groundwater availability, making it almost impossible to estimate how long an aquifer will last given its current rate of depletion and replenishment, scientists concluded in the second study.

The researchers found wildly ranging projections for “time to depletion.”

“Available physical and chemical measurements are simply insufficient,” study author Jay Famiglietti, a UCI professor who is also the senior water scientist at NASA’s Jet Propulsion Laboratory, said in a statement. “Given how quickly we are consuming the world’s groundwater reserves, we need a coordinated global effort to determine how much is left.”

If underground water is so important to human survival, why the knowledge gap?

“It is expensive and time consuming to study groundwater,” Alexandra Richey, a study coauthor and a UCI doctoral student in civil engineering, said in an email.

That’s because researchers must drill deep beneath the surface to test aquifer levels, which can vary from place to place, depending on how much pore space there is in the soil, among other factors.

“As a result, many measurements need to be taken across the whole aquifer area instead of just being able to look at the level of a lake or reservoir,” Richey said.

Even so, she added, such painstaking research will be critical for the world to manage whatever groundwater resources it has left.

“[It] is not going to happen overnight,” Richey said. “It will be a long, coordinated project that ideally will connect researchers and decision makers to build the science into management plans.”

The city of Irvine, California, for example, has extensively studied its aquifer. “They know how low they can let the aquifer go before needing to either stop pumping or to supplement supply,” Richey said. “There’s a paper that says basically you can’t manage what you don’t know and right now we aren’t really managing groundwater well, if at all.”

Lance Larsen, a science center fellow at the Natural Resources Defense Council, said that it’s critical to monitor whatever groundwater is left.

“These studies clearly demonstrate that there are significant issues with groundwater management in the U.S. and we think more can be done to protect this limited resource,” Larsen said in an email. “Protecting groundwater supplies is a key piece to the puzzle of ensuring that future generations have access to fresh water.”

“When these aquifers dry up,” he said, “they are gone forever.”

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