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 or snow once again.
But the water that's above ground is just a fraction of our planet's total water story.
Hidden in the Earth's crust are vast stores of "groundwater," water that fell from the sky and then trickled into the cracks and crevices between the sand, gravel and rocks beneath our feet. This resource makes up 95% of the freshwater that is not tied up in glaciers or ice caps.
We can't see this groundwater, but more than 2 billion people across the globe drink it every day. In arid areas, it is pumped out of the ground to grow crops. 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 is buried beneath the ground. That calculation had not been updated for 40 years — until this month.
In a study in Nature Geoscience, researchers used tens of thousands of additional data points to determine how much water is stored in the planet's crust. They also examined how long it had been underground.
The results show where on the planet groundwater is quickly being renewed and where it is has been there for a very long time, said Tom Gleeson, a hydrogeologist at the University of Victoria in Canada who led the study.
"This is important because it helps us see the areas that are most sensitive to human contamination and changes due to climate change," he said.
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. That's 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 is stored in various types of rocks across the planet.
The researchers were also interested in the age distribution of that underground water. Previous studies have shown that groundwater could have fallen from the sky as little as a day ago, or as long ago as millions — even billions — of years.
Gleeson wanted to know in particular how much of the Earth's groundwater was "modern," meaning it had entered the ground system within the last 50 years.
Quantifying the amount of young groundwater on Earth is crucial for a variety reasons, the study authors argue: It is a more renewable resource than older "fossil" groundwater, and it's more vulnerable to industrial or agricultural contamination.
To see how much groundwater is modern, they looked at the amount of tritium that had been found in groundwater across the globe. Tritium is a radioactive isotope of hydrogen that spiked in rainwater approximately 50 years ago as a result of above-ground thermonuclear bomb testing.
The team reviewed the scientific literature and eventually found 3,700 tritium measurements of groundwater from 55 countries. From this data set, they determined that just 5.6% of the world's groundwater is less than 50 years old. That's about enough water to reach the top of a stop sign across the continents, if it were pumped out of the ground.
Graham Fogg, a hydrogeologist at UC Davis who was not involved in the study, commended the researchers for trying to figure out how much groundwater was modern, particularly because it represents the water most likely to have been polluted as a result of human activity.
"The groundwater that we pump that is older than 50 or 60 years is generally too old to be contaminated by us," he said.
However, he took issue with their contention that this volume of young water represents the fraction of groundwater that is renewable.
"There are all kinds of water management problems that have to do with taking out more water from the system then can be replaced," he said. But whether that water is 10 years old, 100 years old or 1,000 years old has nothing to do with it.
Fogg explained that the water beneath our feet is connected over vast areas, kind of like an enormous plumbing system. Even if you pump 1,000-year-old groundwater in one area, it could be replenished by much younger water from the surface or by water in groundwater basins miles away. This could happen over the span of a few months or even a few days, he said.
But it will take much longer to replace much of the groundwater that has been used up during California's devastating drought, Gleeson said.
"In the Central Valley of California, people are pumping out water that is tens of thousands years old, deposited during the last glaciation," he said. The water "will be renewed, but not over a human timescale."
The new report "hints at the sluggishness and the vastness of the world's older groundwater stores, which may record the climate and tectonic history over centuries, millennia or even millions of years," wrote Fan Reinfelder, who did not work on the study.
Gleeson said the next step for his team is to take the new estimates of young groundwater and combine them with local estimates of groundwater use.
"We want to find out how long before we run out of this critical resource," he said.