yes that's quintillions
More than two billion people around the world rely on groundwater for drinking water and to grow crops, and scientists decided it was time to figure out how long it takes for the water to be replenished after it's pulled out.
Groundwater is the water that slips through the rocks and sand and is stored beneath the Earth's surface, and the last time scientists estimated how much is down below, it was the 1970s. A team at Nature Geoscience, using computer models that take into consideration 40,000 different measurements of how water can be stored in various rocks, determined that there are six quintillion gallons of groundwater in the top 1.2 miles of the Earth's crust. As the Los Angeles Times points out, that's enough water that, spread across every continent, would form a layer 600 feet high (that's twice the height of the Statue of Liberty).
The team also wanted to know how much water entered the ground system less than 50 years ago, and figured it out by testing tritium, a radioactive isotope of hydrogen that appeared in rain water about five decades ago because of above-ground thermonuclear testing, the Times reports. The scientists found that only 5.6 percent of groundwater is less than 50 years old, which Tom Gleeson, a hydrogeologist at the University of Victoria in Canada who led the study, said was the biggest shock of all. The next step is for the researchers to take their new estimates and combine them with local estimates of groundwater use in order to "find out how long before we run out of this critical resource," Gleeson said.