In the 1970s, engineers drilled a series of wells off the U.S. Atlantic Coast, looking for oil. Instead they found water. Fresh water.

Puzzled, they wondered if they’d found isolated pockets or a continuous aquifer.

Thirty years later, a new technology arrived to help answer this question. Electromagnetic imaging could read the electrical conductivities of subsurface fluids to identify them.

Saltwater is very conductive. Oil and fresh water are not.

They surveyed the same area, and what they found astonished them: a freshwater aquifer 600 ft thick, stretching from New Jersey to Massachusetts, extending more than 50 miles out from the shore.

Geologists set to work to determine where it had come from.

At the peak of the last Ice Age, much of Earth’s water was locked up in continental ice sheets, causing sea level to drop 400 ft—exposing most of the Atlantic continental shelf.

As the climate naturally warmed and the ice receded, meltwater flowed for thousands of years into sedimentary rocks of the continental shelf, charging them with water.

When the rate of glacial melt slowed over the last few thousand years, sediments deposited on the seafloor sealed the aquifer beneath them.

This huge reserve of Ice Age glacial melt could one day supply fresh water to the Eastern U.S.

And it has led scientists to probe other continental shelves, to see if they, too, hold large stores of fresh water.