The exact amount of water beneath the Earth’s surface has, for the most part, been a mystery. For the first time, the amount has been mapped and approximated, according to a study from British journal Nature Geoscience.

USA Today reports that the study found that there is approximately six quadrillion gallons of water beneath the Earth’s surface. This is enough water to cover the entire planet’s surface in 590 feet of water.

Although some of the water can potentially be used for agriculture or other industries, the water beneath the surface has been in the same place for thousands, or even millions of years. It is likely too stagnant for human consumption.

Only 6 percent of the groundwater is close enough to the surface to be considered part of the water cycle. The water that is close to the surface is the water that is used for human consumption, is the most susceptible to being affected by factors like pollution and climate change.

Groundwater is especially vulnerable in places like California, Texas, and Kansas where groundwater supplies are overused, making sustainability difficult for the necessary water supply.

The study was led by Tom Gleeson of University of Victoria. He and his team combined a few different data sets in an effort to see that groundwater is a finite resource, rather than a continuously renewable one. According to Gleeson, the study will allow researchers to determine how much of the usable groundwater is being depleted. With that information, they will be able to estimate how long it will take for the supply to run out.