In a major breakthrough, scientists have for the first time determined and mapped the total amount of water that's stored under the Earth's surface.
According to a new study led by Tom Gleeson at the University of Victoria and his team of researchers from the University of Texas, University of Calgary and University of Göttingen, the planet holds 23 million cubic kilometers of groundwater worldwide or roughly about six quintillion (6,000,000,000,000,000,000) gallons, reported BBC.
"This has never been known before," study lead author Tom Gleeson said in the release.
"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."
For the purpose of the study, the researchers build a groundwater map based on the data from 40,000 groundwater models, comprising data from nearly a million watersheds.
Using geochemical, hydrological, geospacial and geological data sets for the study, the researchers found that about 96 percent of the ground water is "old" while only 6 percent of the water trapped in the upper layer of Earth's landmass belongs to the "modern" category, according to USAToday.
"Old" groundwater is typically found deeper in the Earth and is more likely to be salty and nonrenewable, while "modern" groundwater refers to the category of water that is less than 50 years old and is more vulnerable to climate change and other environmental stressors.
The study found that most of the world's modern groundwater is located in tropical and mountain regions. Some of the largest deposits are in the Amazon Basin, the Congo, Indonesia, and in North and Central America along the Rockies and western cordillera, while the least amount of groundwater is in arid regions such as the Sahara.
The next step for the researchers will be to understand how quickly humans are depleting both old and modern groundwater.
"Since we now know how much groundwater is being depleted and how much there is, we will be able to estimate how long until we run out," said Gleeson.
The study is published in the journal Nature Geoscience.
