Nearly 50 years ago, a US government ship, which was originally looking for minerals and hydrocarbons, unexpectedly stumbled upon freshwater while drilling into the sea floor off the country’s northeastern coast. Now, a group of international scientists have followed up on the surprise discovery and confirmed the existence of a large aquifer — a body of porous rock or sediment saturated with freshwater — under the Atlantic Ocean, stretching from New Jersey to Maine.The scientists drilled for freshwater under the sea by as many as 1,289 feet and collected nearly 50,000 litres of it for analysis.Brandon Dugan, a geophysicist and hydrologist at the Colorado School of Mines, told the Associated Press, “It’s one of the last places you would probably look for freshwater on Earth.”The offshore aquifer is just one of many depositories of freshwater known to exist in shallow salt waters around the globe.Here is a look at what offshore aquifer systems are, how they get freshwater, and why they are significant.What are offshore aquifers?Much like aquifers on land, offshore aquifers are bodies of rock or sediment which contain freshwater. The difference is that they are located beneath the sea floor. Scientists believe that offshore aquifers can extend up to 90 kilometres off the coast and contain more freshwater than all the water that has ever been removed from aquifers on land.A 2021 study, ‘Offshore fresh groundwater in coastal unconsolidated sediment systems as a potential fresh water source in the 21st century’, published in the journal Environmental Research Letters, estimated that there was a million cubic kilometres of freshwater beneath the sea floor — about 10% of all terrestrial fresh groundwater.Story continues below this adOver the years, scientists have confirmed the existence of offshore aquifers at several locations, including off the coasts of Australia, China, North America, and South Africa. However, they have not been really explored primarily due to an exclusive focus on aquifers on land and logistical reasons.That’s why the latest exploration of the offshore aquifer, located off the US northeast, is crucial. It is the first global expedition to drill systematically for undersea freshwater. The scientists drilled between 30 and 50 kilometres beneath the sea floor, and have estimated that the aquifer has more than enough water for a metropolis like New York City for 800 years.How do offshore aquifers get freshwater?Aquifers on land get freshwater primarily from rain and snow-melt, which soaks into the soil and trickles down through the rocks. However, this is not how offshore aquifers can get freshwater.There are competing hypotheses about how such water might get there. According to one theory, water might have reached beneath the sea floor during past ice ages, when sea levels were lower and large areas that are currently covered by the ocean were dry land. As a result, rain trickled into the ground, building up large volumes of freshwater. Also, during this time, the size of ice sheets had grown, and their weight may have forced water into porous rocks that extended out to sea.Story continues below this adAnother theory is that offshore aquifers could be connected to onshore aquifers, which regularly supply them with water after rainfall.Note that the freshwater beneath offshore aquifers may not be getting mixed with salty water due to a cap rock layer, built from compacted clay-rich sediment, over the aquifers. “Clay is paradoxical: it can hold a lot of water when loose, but when it is compacted, it becomes almost impervious. This cap prevents the less dense fresh water from rising up to the seafloor,” according to a report by Scientific American.To ascertain the source of freshwater at the offshore aquifer located off the US northeastern coast, scientists have extracted thousands of samples. If the water is not renewable and has been there since the ice age, it will be a finite resource to be used sparingly, if at all.Also in Explained | How climate change is affecting Earth’s water cycleWhy are offshore aquifers significant?Offshore aquifers are significant as they can be untapped freshwater reserves and help deal with water scarcity on land. According to a 2023 United Nations report, the world is currently facing an unprecedented water crisis, with global freshwater demand predicted to exceed supply by 40% by 2030.Story continues below this adThe crisis has been exacerbated by rising global warming, which has led to droughts and changes in rainfall and snowfall patterns. This has severely affected the recharge of freshwater sources. The scarcity has also been driven by pollution and wasteful use of water.“The scientific evidence is that we have a water crisis. We are misusing water, polluting water, and changing the whole global hydrological cycle, through what we are doing to the climate. It’s a triple crisis,” Director of the Potsdam Institute for Climate Impact Research Johan Rockstrom, who is co-chair of the Global Commission on the Economics of Water (GCEW), told The Guardian in 2023.What are the challenges in extracting water from offshore aquifers?The road to turning offshore aquifers into a dependable municipal supply of freshwater is long and dotted with challenges. For instance, offshore drilling is quite expensive. The latest extraction off the US northeastern coast cost around $25 million.There will also be a need to design wells which will operate beneath the sea floor, transport water onshore, and control pumping to ensure that salt water is not mixed with freshwater.Story continues below this adIt is also unclear how extraction from offshore aquifers will impact the ecology and marine life.Most significantly, there are issues of ownership and rights. For example, who will manage the extracted water? How much say will indigenous, fishing, and coastal communities get in the management of water?“If we were to go out and start pumping these waters, there would almost certainly be unforeseen consequences,” Rob Evans, a geophysicist, told AP.