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Most of us think of the water cycle as something that occurs above ground — water falls from the sky, evaporates back into the atmosphere and then condenses into rain once again.

But above-ground water is just a fraction of our planet’s water story.

Hidden in the Earth’s crust are vast stores of what is known as “groundwater” — water that fell from the sky and then trickled into the cracks and crevices between the sand, gravel and rocks beneath our feet.

We can’t see this groundwater, but more than 2 billion people across the globe rely on it for drinking water every day. In arid areas it is pumped out of the ground to grow crops, and it also plays an important environmental role, keeping streams and rivers running in times of drought.

Back in the 1970s a team of scientists estimated how much of the planet’s water lies buried beneath the ground, but that calculation had not been updated for 40 years — until now.

In a new study in Nature Geoscience, researchers took another stab at estimating how much water is stored in our planet’s crust, this time with tens of thousands more data points. They also looked at the age of that water, or how long it had been underground, to understand how quickly it can be replenished as humans keep pulling it out.

“Our maps and estimates show where the groundwater is quickly being renewed and where it is old and stagnant and nonrenewable,” said Tom Gleeson, a hydrogeologist at the University of Victoria in Canada who led the study.

Gleeson and his team report that there are 6 quintillion gallons of groundwater in the upper 1.2 miles of the Earth’s crust. If you could magically pump it all out of the ground and spread it across the continents, it would form a layer of water 600 feet high, or twice the height of the Statue of Liberty.

To derive that number, the scientists used computer models that take into account 40,000 distinct measurements of how much water can be stored in various types of rocks across the planet.

The researchers were also interested in the age distribution of that underground water.

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