The amount is enormous, they say, around six quadrillion gallons — that a six followed by 15 zeroes — which, if it was on the surface, would cover our planet's entire land area with a layer almost 600 feet deep.
However, while the amount is impressive, most of it is too far below the surface to be accessible, and would be unfit to drink even if we could access it, researchers report in a study appearing in Nature Geoscience.
That water is known as "ancient" groundwater because it has existed deep in the Earth for thousands of millions of years.
Only a small portion of our planet's groundwater — about six percent — is considered "modern," renewable within a human lifetime and available for human use because it exists only in the first mile or so of the Earth's land area surface, the researchers note.
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