Groundwater -- or the unchecked pumping of it -- is a big issue in California. With the drought entering its fourth year, wells are drying up. Now there are signs we may be tapping water in a way that's irreversible. The groundwater we're currently pumping out of some aquifers in the southern Central Valley last fell as rain around 20,000 years ago. We talk with Tom Knudson, a senior reporter for the radio show "Reveal," about how aquifers work and why some scientists think we're entering a danger zone.
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