ABOARD LIFTBOAT ROBERT, North Atlantic (AP) -- It turns out the Continental Shelf has been holding a secret from the ages. Beneath the sea floor off the U.S. northeast lies an enormous reserve of fresh water whose existence was long unknown, then was suspected and now is confirmed.

The first global expedition to drill systematically for undersea fresh water has come away with thousands of samples of it. And this comes as rising sea levels and other harms from a warming climate threaten freshwater supplies on land.

Expedition 501, a $25 million collaboration of more than a dozen countries including the U.S., drilled 20 to 30 miles (30 to 50 kilometers) off the coast into what is now believed to be a freshwater reserve stretching from New Jersey to Maine. It's just one of many prospective depositories of fresh water hiding under shallow salt waters around the world that might some day be tapped to slake the planet's intensifying thirst.

Associated Press journalists visited the drilling platform last month, some seven hours out to sea by supply boat from Fall River, Massachusetts, to view the operation. Here are some takeaways:

The potential of the undersea freshwater is enormous, but so are the hurdles of getting it out and figuring out who gets to control it. And yet the need is irresistible.

In just five years, the U.N. says, the global demand for fresh water will exceed supplies by 40%. One growing part of that is the data centers that power AI and cloud computing.

Cape Town, South Africa, came close to running out of fresh water for its nearly 5 million people in 2018 during an epic three-year drought. South Africa is thought to have a coastal undersea freshwater bonanza, too, and there is at least anecdotal evidence that every continent may have the same.

Canada's Prince Edward Island, Hawaii and Indonesia are among places where stressed freshwater supplies coexist with prospective aquifers under the ocean.

Nearly 50 years ago, a U.S. government ship searching for minerals and hydrocarbons off the East Coast drilled into the seafloor over a vast area to see what it could find. It found fresh or freshened water in bore hole after bore hole.

That raised interest but it would take years to connect the dots. In 2015, a revelatory research project mapped contours of an aquifer remotely, using electromagnetic technology, and roughly estimated salinity of the water underneath.

That mission, by the Woods Hole Oceanographic Institution and Lamont-Doherty Earth Observatory, reported evidence of a "massive offshore aquifer system." And it set the state for Expedition 501 a decade later.

Backed principally by the U.S. National Science Foundation and the European Consortium for Ocean Research Drilling, it was quite literally groundbreaking, penetrating Earth below the sea by as many as 1,289 feet or nearly 400 meters.

The work unfolded around the clock over three months from Liftboat Robert, a vessel that, once on site, lowers three enormous pillars to the seafloor and squats above the waves. Normally it services offshore petroleum sites and wind farms.

Scientists went into the project believing the undersea aquifer they were sampling might be enough for a metropolis like New York City for 800 years. They now think the reserve is even larger than that.

Soon after Robert arrived at the first of three drilling sites, samples from below the seabed registered salinity of just 4 parts per thousand. That's far below the oceans' average salt content of 35 parts per thousand but still too briny to meet the U.S. freshwater standard of under 1 part per thousand.

But as Robert moved from site to site, it eventually found water at 1 part per thousand salt content. Some were even lower.

That's water safe enough to drink, in theory.

But months of analysis lie ahead for scientists to test what's in the water and whether it's safe to drink and use.

They also need to figure out where it came from and how old it is. Those answers will determine whether the water is a renewable resource and shape opinion on whether it can be used responsibly.

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Woodward reported from Seekonk, Massachusetts.

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