The adage “out of sight, out of mind” applies to a whole lot of things in this world, but we usually don’t think of water as being one of them. After all, some 70 percent of the Earth’s surface is covered in the wet stuff and it falls from the sky on a somewhat regular basis. However, scientists have known for decades that there is an enormous amount of water hidden under the planet’s crust as well. It’s been nearly 40 years since experts attempted to determine how much water that is, and a new study purports to do just that, with many more details and measurements than were available in the 1970s when the previous figures were calculated.

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Tom Gleeson, a hydrogeologist at the University of Victoria in Canada, led the study, which was recently published in Nature Geoscience. Gleeson and his team concluded there are some six quintillion gallons of groundwater located in the upper 1.2 miles of the Earth’s crust. If you’re having trouble imagining what six quintillion gallons would look like, you’re not alone. Imagine the entire planet under 600 feet of water and you’re getting close.

Related: The whole world is running out of water

This new figure is still just an estimate, but researchers are feeling confident that this is even more accurate than previous projections. Another interesting product of the study, though, is estimates on the age of the water underground. By looking at clues contained in the water itself, the research team attempted to evaluate how much of the water is “young” or “modern”—or less than 50 years old—and therefore considered to be a more renewable resource than more ancient waters. The team found just 5.6 percent of the total groundwater fell into this category.

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So, before you get too excited about the idea that we can just pump water from underground and solve the drought crises all around the world, you might want to slow down. Although much of the world already draws its drinking water from underground aquifers, of which half are now depleted according to a recent NASA study, nobody knows where the breaking point is.

In California, we’ve seen a perfect example of what happens when groundwater supplies are overtaxed. The underground aquifers collapse and the surface level of the ground essentially sinks. In general, it’s a gradual process with relatively minimal consequences, but that’s when we’re talking about slowly pumping out the reserves over a long period of time. If groundwater is ‘harvested’ rapidly, the above-ground effects could manifest as collapsed buildings, buckling roadways, fallen bridges, broken gas lines, and even gaping sinkholes that swallow up hundreds of feet of land within seconds.

+ Study: The global volume and distribution of modern groundwater

Via The Los Angeles Times

Images via Tom Gleeson et al, and Shutterstock (1, 2)