Scientists say they've completed the first measurements and mapping of the total amount of groundwater under the Earth's surface.

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.

"This has never been known before," says study leader Tom Gleeson of the University of Victoria in British Columbia, Canada. "We already know that water levels in lots of aquifers are dropping. We're using our groundwater resources too fast — faster than they're being renewed."

An accurate estimate of available groundwater has been considered vital as the world increasingly depends on groundwater from springs and wells for both drinking and particularly agriculture, says Gleeson, a hydrogeologist.

"It is important to know how much groundwater there is because more than a third of the population in the USA and the world drink groundwater every day and it is crucial to agriculture and the environment," he says.

While the amount of groundwater is dwarfed by the amount in the world's oceans, it accounts for most of the available fresh water on Earth, researchers point out.

The "modern" available groundwater is the main component of Earth's active water cycle, but because it's closer to the surface, it is more vulnerable, they say.

It is one of the world's most precious resources but also among the most exploited, perhaps unsustainable, they warn.

"The main implication of our study is to show that our youngest groundwater resources — those that are most renewable, yet also most sensitive to human contamination and climate change — are a finite resource," Gleeson says.