As many as 20% of groundwater wells worldwide are at risk of running dry if groundwater reserves continue to decline according to a new study, which evaluated data from nearly 39 million wells from across the globe. The findings reveal critical vulnerabilities to even modest reductions in groundwater levels, suggesting an impending threat to drinking water and agricultural irrigation for billions of people. Groundwater is the primary water source for almost half of the planet's population; yet, increasing demand and a general lack of adequate governance or management has resulted in the continued depletion of many major aquifers worldwide. This drawdown has the potential to impact groundwater wells significantly. However, data on groundwater availability are difficult to collect and, despite their importance in supplying crucial water resources, groundwater wells have never been evaluated at a global scale. To address this need, Scott Jasechko and Debra Perrone compiled construction records for almost 39 million groundwater wells in 40 countries worldwide, including local data on well locations, depths, purposes and construction dates. Jasechko and Perrone found that between 6 to 20% are no more than 5 meters deeper than their current local water table, suggesting that millions of wells are at risk of drying up if groundwater levels decline by only a few meters. What's more, the authors found that newer wells are not being constructed deeper than older wells in some areas undergoing rapid groundwater depletion. "Jasechko and Perrone implicitly deliver a timely warning that universal access to groundwater is fundamentally at risk," write James Famiglietti and Grant Ferguson in a related Perspective. "The time is now for key research and exploration and for science-informed governance and policy that address the demand for groundwater and eliminate its overexploitation."