On June 4th — a date that has become nervously known as “Day Zero” — it’s expected that Cape Town, South Africa, a city of 4 million, will run dry in the wake of what could arguably be the most alarming and severe water shortage a modern city has ever experienced. But research suggests that a solution to Cape Town’s looming crisis — quadrillions of liters of fresh water — may be sitting practically beneath the city’s feet, and it’s going entirely untapped.

The current historic drought began after Cape Town experienced an unseasonably dry winter in 2015. The lack of rainfall that year caused water levels in the city’s dams to plummet by 20 percent, only to be followed by two more dry winters. A changing climate has made drought conditions worse, and poor water management exacerbated the situation, bringing the city’s water supply to the critically low level it sits at today.

In response, the Cape Town government is calling on inhabitants to curb water use: the city rolled out social media campaigns around messages such as “We Can Beat Day Zero” to curb water use to 50 liters or less per person per day versus the more typical 80 to 100 liters. Advice includes one load of laundry per week, using hand sanitizer instead of soap and water, and not washing your hair as often as you might like. For residents like Joe Appel, who lives in a Cape Town suburb called Ottery, the crisis has prompted him to carry out a simulated “dry run” in preparation for Day Zero.

“I decided to limit myself to 25 liters per day, which is what the ration will be after Day Zero,” he said. “It was very funny, as I had to use a very small basin to wash myself.”

It’s a dire state of affairs, and one that may have been avoidable: more than a mile below the seafloor off the coast of South Africa lies a vast sea of fresh water that, if tapped, could serve as a backup water supply for the water-starved city.

This hidden subsea water supply sits at the very southern tip of the continent in an 18,000-square-mile basin that shares its name with the South Africa town of Bredasdorp. The Bredasdorp basin, along with similar offshore aquifers found along portions of every other continent, was documented in a 2013 paper in Nature by scientists from Flinders University and the National Centre for Groundwater Research and Training.

The study put the phenomenon in a global context, but it wasn’t the first time subsea fresh water was reported. In 1976, scientists with the US Geological Survey found subsea freshwater reserves extending roughly 60 miles off the New Jersey coast during a scientific drilling expedition in the Atlantic.

While geoscientists have been able to estimate how far offshore aquifers extend in different continents through well sampling, they haven’t been able to determine the absolute sizes of offshore aquifers in different regions. This is mainly due to the fact that the technology required to map them in 3D — Controlled Source Electromagnetic (CSEM) surveying — has only been recently applied to the study of offshore aquifers. The technique, which the oil industry has traditionally used to detect the presence of offshore oil and gas, works by beaming electromagnetic signals into the ocean, typically by a transmitter draped off a ship. The signals diffuse down through the seafloor and into the subsurface, where pockets of fresh water fill porous sandstone, sandwiched in between layers of marine clay. As the electromagnetic signals penetrate the subsurface, their intensity changes depending on how easy or hard it is for the fluids to transmit an electrical current. Since fresh water is a poor conductor of current, the technology is able to distinguish it from salt water (which is an effective conductor) and thereby determine its presence.

Even though the fresh water is largely kept isolated from seawater by the layers of clay, some seawater salts can permeate the sediment over time through diffusion. This can make some of the fresh water slightly salty, or brackish.

According to the 2013 study, there’s an estimated 120,000 cubic miles of subsea fresh water globally — roughly 1,000 to 1,200 times the amount of water used in the US annually.

That would be more than enough to provide backup water supplies to other cities facing water shortages beyond Cape Town, like São Paulo, Brazil and Mexico City. To date, however, none of it has been pumped up for public use.

But why?

“It’s complicated,” says Brandon Dugan, a geophysicist and associate professor with the Colorado School of Mines, who has been studying offshore freshwater aquifers since 2002. “We don’t exactly understand the plumbing of the system or the precise volume of fresh water that’s down there. So that makes it difficult to devise a pumping strategy to maximize use of the resource.”

In order to extract the water, Dugan explains, geoscientists must understand how the reserves were generated in the first place. If the water was originally deposited by a melted freshwater glacier during the ice ages — when the sea level was hundreds of feet lower than it is today — then it’s an exhaustible supply that would run out once depleted. But if the fresh water seeped its way down there from land-based water supplies, the reservoirs could prove to be massive renewable resources.

Then, there’s the question of legal rights. According to Renee Martin-Nagle, an aquifer law expert, if subsea freshwater reserves lie within a country’s 200-mile exclusive economic zone, as defined under the UN Convention on the Law of the Sea, it belongs to that country. Because the Cape Town-adjacent underwater reserves sit just 50 to 75 miles offshore, accessing and developing it would be within the city’s legal right. “However, if the aquifer straddles another country’s exclusive economic zone, the law is silent and the parties would have to work out an understanding between themselves,” she said.

Accessing the reserve also poses a problem. Mark Willett, an engineer and director of the Wannacomet Water Company in Nantucket, Massachusetts, says the cost of accessing the reserve could be staggering. “Offshore fresh water would be a great option for regions that don’t have a good water supply, but there are several challenges in getting it to shore,” he said. “You’d need an offshore rig to drill the well, and divers would have to go down and weld or fuse the pipe to the well. If the pipe was laid on a rocky bottom, it would have to be engineered to withstand the shifting ocean currents. It could be $4 to $7 million in well construction and approximately $100,000 per mile of pipe plus the cost of any water treatment that is needed.”

Chris Hartnady, research and technical director of the Cape Town-based environmental consultancy Umvoto Africa, notes that there had been local interest in offshore freshwater aquifers a decade ago after drillers struck fresh water several hundred miles east of Cape Town during offshore oil exploration. But he says it was never pursued, presumably due to costs. “It is considerably less expensive to develop onshore wellfields,” he said.

While the costs of tapping offshore aquifers would likely dwarf the expense of traditional water well drilling, they would arguably be a small price to pay compared to the financial blow the region could face on Day Zero.

No one knows how the crisis will play out, but as devastating as the situation is for Cape Town, Dugan sees an upside. “When a developed city that has a high respect level around the world all of a sudden runs out of water, it’s going to drive innovation and creativity that will help prevent this from happening in the future. It makes the idea of tapping offshore reserves seem more viable than ever.”

Correction: This article initially identified “Zero Day” as May 11th. That date has recently been pushed back to June 4th.