The water has remained shielded from seawater thanks to the accumulation of a protective layer of sediment and clay. And it’s not a local phenomenon. Such reserves are to be found under continental shelves off Australia, China, North America and South Africa.

The discovery was made by researchers at the National Centre for Groundwater Research and Training (NCGRT) and the School of the Environment at Flinders University. The scientists estimate there is around half a million cubic kilometers of what they describe as “low salinity” water, which means it could be processed into fresh, potable water economically.

The reserves formed when ocean levels were lower and rainwater made its way into the ground in land areas that were not covered until the ice caps melted 20,000 years ago, causing sea levels to rise.

"The volume of this water resource is a hundred times greater than the amount we’ve extracted from the Earth’s sub-surface in the past century since 1900," says study lead author Dr. Vincent Post. "Our research shows that fresh and brackish aquifers below the seabed are actually quite a common phenomenon."