The research, published in the journal Proceedings of the National Academy of Sciences, said this water source is much deeper than traditional aquifers and that tapping it would likely require a lot of money and engineering expertise.
According to a Stanford news release, the water is 1,000 to 3,000 feet underground. Until recently it was considered difficult to retrieve water from that depth. The quality of the water is questionable, researchers said, noting that desalination might be required.
Overpumping groundwater in the Central Valley has caused the ground to sink in some areas, and researchers said this subsidence would need to be considered when drilling.
Still, the researchers said the findings offer potential good news as California enters a fifth year of drought.
"It's not often that you find a 'water windfall,' but we just did," study co-author Robert Jackson told Stanford News Service.