Large stretches of California's Central Valley are sinking fast, and it's no mystery what the culprit is: water pumping.

Groundwater accounts for about 60 percent of the state's water supply in dry years, with most of that used by agriculture, according to the state Department of Water Resources.

Joseph Poland of the U.S. Geological Survey used a utility pole to document where a farmer would have been standing in 1925, 1955 and where Poland was then standing in 1977 after land in the San Joaquin Valley had sunk nearly 30 feet. — Joseph Poland of the U.S. Geological Survey in 1977 in the San Joaquin Valley, standing by a utility pole measuring the roughly 30-feet of land subsidence since 1925. The land here continues to sink today as groundwater pumping increases. (Credit: U.S. Geological Survey)

As California's historic drought drags on and surface water supplies (from rain and snowpack) continue to dry up, farmers are drilling ever-deeper wells to pump up the dwindling supplies of subterranean water. But when too much water is extracted, the land quickly begins to subside.

A recent investigation by CIR's Reveal project calculated that the tens of thousands of groundwater pumps, mostly in the Central Valley, running around the clock, use up roughly 5 percent of the state's total electricity, or "enough to power every home in San Francisco for three years."

Since settlers first started pumping water here more than a century ago, groundwater use in California has been largely unregulated and unmonitored, leaving landowners free to pump as much as they've wanted. And the state's first ever groundwater regulations, enacted last year, won't go into effect for decades.

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The vast Central Valley aquifer, the state's largest reservoir, extends for about 400 miles under the Sacramento and San Joaquin valleys. Contrary to popular belief, aquifers are not huge underground lakes, but rather a porous layer of sand, rock and gravel that water has seeped into and accumulated in large volumes over thousands of years. And the one that millions of people in California rely on is quickly disappearing.

For more on aquifers and why you should care about them, scroll through this visualization by Newsbound.

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What's an Aquifer? An Interactive Guide to Understanding California's Growing Groundwater Crisis