According to the Washington Post, new NASA satellite data show the world’s largest underground aquifers are being depleted at unsustainable rates. Aquifers are underground water reserves, which serve as a source of fresh water for hundreds of millions of people.

In various locations, including India, China, the U.S. and more, 21 of the world’s 37 largest aquifers have passed their sustainability tipping points, meaning more water was removed than replaced during the decade-long study, the Post reports. These aquifers have seen increased strain, as a result of larger demands from agriculture, growing populations, industry and drought.

Underground aquifers supply 35 percent of water used by humans worldwide, the Post says, and they can take thousands of years to fill up. Currently, drought-ridden California is getting 60 percent of its water from aquifers. That number is expected to grow close to 100 percent by the end of the year.

“There’s not an infinite supply of water,” Jay Famiglietti, senior water scientist at NASA’s Jet Propulsion Laboratory in California and principal investigator of the University of California Irvine-led studies, told the Post. “We need to get our heads together on how we manage groundwater, because we’re running out of it.”

Read the full story from the Washington Post.