Thousands of feet beneath the surface of the state's Central Valley, one of the world's biggest agricultural hubs, there may be up to 2,700 cubic kilometers of usable groundwater -- nearly three times more than the amount previously thought.
"It's not often that you find a 'water windfall,' but we just did," said study co-author Robert Jackson, a professor at Stanford. "There's far more fresh water and usable water than we expected."
Another one of the study's authors said the findings would be relevant in other places where there are water shortages -- including Texas, China and Australia.
California is in the fifth year of a major drought, according to state officials.
State and local authorities have imposed strict regulations in an effort to conserve water, while also levying fines on violators to enforce conservation efforts.
The drought has affected much of the country,
as California grows more than a third of America's vegetables and two-thirds of its fruit and nuts.
Farmers have already started looking to underground springs to feed their crops, so the windfall may be welcome news.
The discovery is not a panacea, however.
Drilling for water so deep is expensive and can be hazardous.
It may add to the gradual sinking of the land already taking place in the Central Valley, according to a statement from Stanford.
"Groundwater pumping from shallow aquifers has already caused some regions to drop by tens of feet," the researchers said.
Those deep groundwater resources are also vulnerable to contamination from oil and gas -- oil and gas drilling occurs in up to 30% of the sites where deep groundwater is located -- as well as from other human activities, like hydraulic fracturing, according to the study.
"We might need to use this water in a decade, so it's definitely worth protecting," said Mary Kang, another one of the study's co-authors.
Still, Kang and Jackson believe that just because fracking has occurred, it doesn't mean the water has been ruined.
"What we are saying is that no one is monitoring deep aquifers," Kang said. "No one's following them through time to see how and if the water quality is changing."