Ben Chafin sees the future of clean energy in abandoned coal shafts.

The Virginia state senator, whose Appalachian district is pockmarked with empty mines, pushed through legislation in April that encourages companies to transform those tunnels into giant storage devices to hold vast amounts of renewable power.

The idea, which Dominion Energy Inc. has been studying, is to fill mines with water and then use electricity from wind and solar farms to pump it up to a reservoir on the surface. When utilities need power, operators open floodgates, letting water gush back into turbines on its way down.

“Voila—you have electricity,” said Chafin, a Republican. “These deep mines can act just like a giant battery.”

The technology Chafin is pushing is not new—its first use was for a Swiss hydroelectric plant in 1909. But it wasn’t until the 1960s and 1970s that the technology flourished, mostly to store surplus energy from nuclear plants. It has since been largely ignored—until now. As wind and solar boom, the need for grid-scale alternatives to lithium ion batteries has increased.

Unlike coal and gas generators, wind and solar farms don’t provide a steady flow of electricity—the sun doesn’t always shine and winds kick up and die down, defying forecasts. Giant batteries have long been considered the elusive solution to balancing clean energy’s ebbs and flows, allowing it to be more widely used.

The problem is that lithium ion batteries—the technology used, for instance, in Tesla cars—just can’t be built large enough to get the job done. That’s got developers scouting deep holes in the ground, reimagining this old technology that relies on little more than gravity and millions of gallons of water.

It remains to be seen whether pumped storage technology will actually work in old mines. Nonetheless, developers from Germany, to the U.K., to the U.S. are giving the idea a try.

High above the sand and sagebrush of southeastern California, a transmission line runs along Interstate 10, connecting solar farms in the Sonoran Desert to Los Angeles. From daybreak to nightfall, that line crackles with electricity as photovoltaic panels soak up the sun. It goes quiet just as millions of people turn on their lights.

“When the sun sets, that line goes empty,” said Steve Lowe, president of Eagle Crest Energy Co. “We want to do something about it.”

Eagle Crest is partnering with NextEra Energy, Inc., the U.S.’s biggest clean energy developer, to build a 1.3 gigawatt, pumped-hydro facility near a sun-baked crossroads named Desert Center. The $2 billion project, already approved by federal regulators, could stockpile enough electricity to power nearly 1 million homes.

It will utilize two pits left over from a defunct iron-ore mine dug into the side of a mountain. During the day—when solar power floods the grid—pumps will move water to the upper pit, ready for release at night. Lowe estimates it will take six years to build.

For all the talk of lithium ion as the future, pumped hydro is by far the most prevalent form of energy storage in the U.S., accounting for about 97 percent of capacity. There are more than 40 existing facilities nationwide, with a combined capacity of more than 20 gigawatts. That’s enough to power all of New York City for several hours.

“They are the hidden story of the electricity grid,” said Jeff Leahey, deputy executive director of the National Hydropower Association. “But they are not sexy.”

Nor are pumped-hydro facilities easy or quick to build. The technology may be elementary, but it requires just the right spot, with a steep ascent and room for lots of water. Even a small project can cost $1 billion, Leahey said. And since almost no one is building nuclear reactors these days, just a handful of pumped-hydro plants have been constructed in the U.S. over the last 25 years.

Renewables may change that.

Amid the stony ridges of Pennsylvania and Maryland, Adam Rousselle, a former transmission-industry executive, has teamed up with his son to create a new company, Merchant Hydro Developers LLC. The pair are like modern-day wildcatters, using a computing mapping system to pinpoint 21 spots on which to build pumped hydro projects adjacent to wind farms. The Rouselles have applied for licenses to build on those sites and are in talks with utilities and potential investors.

Nearly all of their potential building sites are on abandoned strip mines. The advantage, Rousselle said, is that they are already environmentally despoiled.

“You can’t do anything worse to an abandoned mine than what humankind has already done,” said Rousselle, the company’s president.

Back in Virginia, Chafin envisions pumped hydro as an economic shot in the arm for his district. The region, 300 miles (483 kilometers) west of Richmond, has double-digit unemployment. A $1 billion construction project, Chafin said, could put plenty of miners back to work. His legislation, which takes effect in July, would fast-track the permitting process for pumped hydro in the state’s coal region.

Shoehorning generators into mine shafts won’t be easy, said Rick Miller, who specializes in hydropower for HDR Inc., an engineering and construction company based in Omaha. “Theoretically, you could do it,” Miller said. “But it would be incredibly expensive.”

It’s also unclear whether the mid-Atlantic region needs more electricity. The area is already awash with cheap power, thanks to inexpensive Marcellus shale gas. And the region’s grid manager, PJM Interconnection LLC, recently slashed its 10-year demand outlook.

Chafin, undeterred, believes hydro power could save coal country.

“I’d love to see the coal industry boom again,” he said. “But we need to diversify our economy and get people back to work.”