Scientists estimate there’s about 500,000 cubic kilometers of freshwater underneath our ocean floors. Getting to it will prove to be a challenge. Creative Commons

The last place you might expect to find drinkable water is underneath the ocean. But that’s exactly where scientists have discovered a number of vast, untapped reservoirs of freshwater.

While exploring the ocean floor for pockets of oil and gas, scientists inadvertently came across evidence of massive aquifers sitting right below the ocean floor. According to the new study, published Dec. 5 in the journal Nature, the freshwater is located in continental shelves near Australia, China, North America and South Africa. Researchers estimate there is 500,000 cubic kilometers, or about 120,000 cubic miles, of low-salinity water holed up in seafloor reservoirs.

“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,” Vincent Post, a groundwater hydro geologist from Flinders University in Australia and the lead author of the study, said in a statement.

So how did the water get there in first place? Scientists believe the freshwater reserves have been there for hundreds of thousands of years, long before the Earth looked the way it does today. Back then, oceans were much shallower, with most of the water holed up in glaciers. Rainwater would’ve soaked directly into the ground, becoming stored in vast aquifers.

Then, when the polar ice caps began melting around 20,000 years ago, the sea levels rose. A layer of sediment and clay protected the freshwater from the encroaching ocean water, trapping it underneath the seafloor.

"Our research shows that fresh and brackish aquifers below the seabed are actually quite a common phenomenon," Post said. "Knowing about these reserves is great news because this volume of water could sustain some regions for decades.“

We live in an increasingly water-scarce world, and one where access to freshwater varies greatly across the continents. According to the United Nations, by 2030, 50 percent of Earth’s population will live in conditions of high water stress.

The discovery of the vast, untapped water reserves gives scientists hope that we can use the reserves to provide freshwater to people in need.

"We should use them carefully: once gone, they won't be replenished until the sea level drops again, which is not likely to happen for a very long time," Post said.

The challenge will be getting to the water. Drilling is expensive and can also pose environmental concerns. Also, drilling could endanger the quality of the water if saltwater is allowed to contaminate the reserves.