The most overstressed aquifer is under Saudi Arabia and Yemen

The studies found that eight of the world's 37 biggest aquifers are "overstressed," meaning not enough water is replenished to offset the usage. Topping the list of overstressed aquifers is the Arabian Aquifer System, located beneath Yemen and Saudi Arabia, from which 60 million people draw their water.
"What happens when a highly stressed aquifer is located in a region with socioeconomic or political tensions that can't supplement declining water supplies fast enough? We're trying to raise red flags now to pinpoint where active management today could protect future lives and livelihoods," Alexandra Richey, a graduate student at the University of California, Irvine, and lead author of both studies, said in a statement. [Earth in the Balance: 7 Crucial Tipping Points]
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permanently lose its ability to store groundwater,