As the drought in California continues to put a strain on the water supply, another long-standing issue comes into play: the ground is sinking.

According to a report by NASA’s Jet Propulsion Laboratory for the State Department of Water Resources (DWR), an occurrence known as subsidence has been causing .

Subsidence is the gradual caving in or sinking of an area of land. In California, the ground is collapsing because . When this water is pumped out of the aquifer underground, the clay in between the pockets of water collapses and causes the ground to deflate.

Between May 2014 and January 2015, parts of the Central Valley sank by as much as 13 inches. (NASA/JPL-Caltech)

This problem is causing the . Despite the fact that the water in these wells - which are now 2,500 feet deep - is thousands of years old, the water is desperately needed to nourish the fruits, vegetables and nuts grown in California, which supply more than half of the country, according to CBS.

"You tell the farmer he can't drill anymore wells ... he can't farm as many acres. You're gonna go into the store buy a gallon of a milk for $10, loaf of bread for $5. Then the public is gonna say, 'Hey, what happened?"' said Steve Arthur, vice president of Arthur & Orum Well Drilling.

These lowered levels aren’t relegated solely to San Joaquin Valley. Michelle Sneed, a hydrologist at the U.S. Geological Survey, says that with some areas being 10 feet lower than they used to be.

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The NASA report also states that the land near Corcoran, in the Tulare basin, , and researchers found that a stretch near the California Aqueduct - one of the main arteries of the State Water Project - sank eight inches a four-month stretch last year. In the last half of 2014, a .

“Because of increased pumping, groundwater levels are reaching record lows – up to 100 feet lower than previous records,” said DWR Director Mark Cowin in a prepared statement. “As extensive groundwater pumping continues, the land is sinking more rapidly and this puts nearby infrastructure at greater risk of costly damage.”

It can also have a variety of effects on the land. “Roads can be broken by fissures, pipelines have been exhumed, and the slope of the land can be altered, changing drainage patterns,” wrote NASA researchers in the report.

Last fall, that requires local governments to regulate pumping, however, in most cases, they won’t take effect until 2020.

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“Groundwater acts as a savings account to provide supplies during drought, but the NASA report shows the consequences of excessive withdrawals as we head into the fifth year of historic drought,” said Cowin. “We will work together with counties, local water districts and affected communities to identify ways to slow the rate of subsidence and protect vital infrastructure such as canals, pumping stations, bridges and wells.”

Jay Famiglietti, senior water scientist for NASA, uses satellite data to show the depletion of California's water resources. CBS also reported that the state has , which is enough to fill Lake Mead - the country’s largest reservoir - twice.

This shocking discovery begs the question, what happens if all the water is depleted?

"That's a question that all of us first of all have a difficult time really fathoming. ... If we still want to have agriculture, we'll have to come up with the water from some other place," said Famiglietti.

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