While 70 to 75 percent of the Earth’s surface is covered by water, more fresh water is stored underground in aquifers. However, a 2015 study shows that 21 of the world’s 37 known aquifers are struggling, with 13 of them dangerously over-stressed. Swelling populations, industries like mining and the world’s growing agriculture demands are all cited as reasons for the problems.
The Arabian Aquifer, used by 60 million people, is the most stressed aquifer, while the Central Valley Aquifer in California is the most troubled in the United States. You can blame California’s droughts, as that aquifer is being drained and more water wells drilled to help irrigate farms.