The water that lies beneath the earth’s surface — known as groundwater — has been a vital resource for thousands of years. Communities that are far away from lakes and rivers use groundwater to irrigate crops and provide drinking water.

For most of human history, groundwater has existed in a convenient equilibrium. The pockets of water under the surface need years or decades to replenish as rainwater and other moisture seep into the earth. Fortunately, though, people have used groundwater slowly, allowing replenishment to happen.

Now that equilibrium is at risk.

Several of my colleagues — led by Mira Rojanasakul and Christopher Flavelle — have spent months compiling data on groundwater levels across the U.S., based on more than 80,000 monitoring stations. Chris and Mira did so after discovering that no comprehensive database existed. The statistics tended to be local and fragmented, making it difficult to understand national patterns.

The trends in this new database are alarming. Over the past 40 years, groundwater levels at most of the sites have declined. At 11 percent of the sites, levels last year fell to their lowest level on record.

The U.S., in other words, is taking water out of the ground more quickly than nature is replenishing it. “There’s almost no way to convey how important it is,” Don Cline, the associate director for water resources at the United States Geological Survey, told The Times.

Already, there are consequences. In parts of Kansas, the shortage of water has reduced the amount of corn that an average acre can produce.

In Norfolk, Va., officials have resorted to pumping treated wastewater into underground rock layers that store groundwater — known as aquifers — to replenish them. On Long Island, the depletion of aquifers has allowed saltwater to seep in and threatened the groundwater that remains.

“We’ve built whole parts of the country and whole parts of the economy on groundwater, which is fine so long as you have groundwater,” Chris told me. “I don’t think people realize quite how quickly we’re burning through it.”

Giant wells

Unlike many other environmental trends, this story is not primarily about climate change, although the warming planet plays an aggravating role. There are three main reasons for the groundwater declines:

• Pumping technology has improved, allowing communities to draw water out of the earth much more quickly than in the past. Some wells can pump more than 100,000 gallons a day.

• Economic growth and urban sprawl have increased the demand for water. Although the U.S. economy has not been growing rapidly in recent decades, American farms help feed other countries where the economy and population have been growing faster.

• Climate change has reduced the amount of water that comes from alternative sources, like rivers: A warmer planet leads to less rainfall and faster evaporation of the rain that does fall. These declines have led communities to increase groundwater use.

These forces are not unique to the U.S. Other countries are coping with groundwater declines that are sometimes worse. This summer, my colleagues Vivian Yee and Leily Nikounazar reported on the dire shortages in parts of Iran, while Alissa Rubin and Bryan Denton did so in Iraq. The photographs and videos from Iraq are especially jarring.

Protecting the commons

Is there any solution?

Slowing climate change, by reducing carbon emissions, would help in the long term — and the long term is obviously important. More immediately, the answer may need to involve stricter rules on how much water towns, farms and companies can remove from the ground. “In many places,” Chris said, “the rules are weak or nonexistent.”

The federal government neither tracks the situation nor does much to regulate it. Some state and local governments — in parts of Arizona, for instance, and Texas — also have lax rules.

It’s a classic tragedy of the commons. The ecologist Garrett Hardin popularized that term in a 1968 essay based on a 19th-century pamphlet by William Forster Lloyd, an English economist. In the pamphlet, Lloyd explained that any individual farmer had an incentive for his cattle to eat as much grass as possible in any field that the community shared. But if all the farmers did so, the field would be ruined. The solution is for the farmers to agree on a set of rules that benefit all of them in the long run.

You can read The Times’s groundwater investigation here.

Related: Children have a right to sue their countries over climate change, a U.N. panel determined.

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