Researchers may have identified a key to combatting the impending global water crisis: massive reserves of freshwater beneath the ocean seabed.

According to a press release, the recent study estimated a half-million cubic kilometers of low-salinity water hidden around the world beneath the seabed on continental shelves. The water reserves have been identified to exist off the coasts of North America, China, Australia and South Africa.

"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," study lead author Dr. Vincent Post, of the National Centre for Groundwater Research and Training (NCGRT) and the School of the Environment at Flinders University, said in the release.

The researchers published their work Thursday, Dec. 5 in the journal Nature.

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

Some of the world's growing cities along the coasts could theoretically use the reserves of freshwater for their supply. Such reserves were previously known to exist, Post said, but they were believed to be small and rare. Hundreds of thousands of years ago, when the sea levels were much lower and the coastline further out, these reserves began to form under the seafloor.

"It happened all around the world, and when the sea level rose when ice caps started melting some 20,000 years ago, these areas were covered by the ocean," Post said. "Many aquifers were - and are still - protected from seawater by layers of clay and sediment that sit on top of them."

The researchers warned that this resource will not be renewable and it is unclear if more of these exist. Post said, although expensive, offshore drilling for this resource might be worthwhile. Still, because of it rarity, scientists need to be careful in their use of the freshwater reserves.

"Freshwater under the seabed is much less salty than seawater," said Post. "This means it can be converted to drinking water with less energy than seawater desalination, and it would also leave us with a lot less hyper-saline water.

"Freshwater on our planet is increasingly under stress and strain so the discovery of significant new stores off the coast is very exciting. It means that more options can be considered to help reduce the impact of droughts and continental water shortages."