The first map showing the world's hidden groundwater was published on Monday, bringing us closer to estimating how much there is, and when it will run out if we over-use the resource.
Using data and computer models, an international team of researchers estimated that less than six percent and perhaps as little as one percent of water found close to the Earth's surface is renewable in a human lifetime.
"This has never been known before," Tom Gleeson of Canada's University of Victoria and the lead author of the study, said in a statement.
"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."
The study, published in the journal Nature Geoscience, estimated a total volume of underground water to be almost 23 million cubic kilometres, of which 0.35 million cubic kilometres is younger than 50 years old.
Underground water is found beneath the Earth's surface and is recharged by rain, snow or water that leaks from the bottom of lakes and rivers.
Its age can be a few months to millions of years. It can be found as deep as 30,000 feet (around 9 km), according to the United States Geological Survey (USGS).
"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," Gleeson said.
Although water found closer to the surface is being renewed quicker than the water found deeper in the Earth, it is more sensitive to contamination and climate change, but it can also serve to temper climate extremes, Gleeson said.
Water found deeper in the Earth is often used for agriculture and industry. It can contain arsenic or uranium and is often more salty than seawater, he added.
"Groundwater can and should be thought of as a very useful buffer to climate extremes," Gleeson told the Thomson Reuters Foundation in a phone interview from Canada.
"If properly managed it flows to rivers during times of drought so it's a valuable and strategic resource for mitigating the extreme impacts of climate on water availability."
According to the study, most groundwater is found in tropical and mountainous regions, with some of the largest deposits in the Amazon Basin, the Congo, Indonesia and along the western borders of North and South America.
Not surprisingly, the least amount is in arid regions such as the Sahara.