In a surprising new study, Stanford researchers have found that drought-ravaged California is sitting on top of a vast and previously unrecognized water resource, in the form of deep groundwater, residing at depths between 1,000 and nearly 10,000 feet below the surface of the state’s always thirsty Central Valley.

The resource amounts to 2,700 billion tons of freshwater, mostly less than about 3,250 feet deep, according to the paper published Monday in the influential Proceedings of the National Academy of Sciences.