Switzerland’s Linthal hydropower plant produces as much power as a nuclear plant, using nothing but the energy of gravity. It’s the world’s biggest pumped-storage facility, which means that it works as a kind of giant battery, using water to store electricity in the form of potential energy.

A pumped storage plant is just what it sounds like. It uses two reservoirs, each at different heights. When power is needed, it runs from the higher reservoir to the lower, passing through turbines in the way, which spin and generate electricity. At night, or at other times of low demand, excess power from the grid is used to reverse the turbines. They become pumps, which pump the water back up into the upper reservoir, effectively storing that surplus electricity for future use.

The Linthal plant is remarkable mostly because of scale. It uses two mountain lakes 2,000 vertical feet apart, connected by a man-made tunnel. Water runs between Lake Mutt to Lake Limmern, a dammed valley below. Limmern holds 23 billion gallons of water and uses new variable-speed motor generators from General Electric, which improve on fixed-speed designs because their speed can be tailored to suit the amount of power available. Thanks to these motors, the efficiency of the plant is around 80%—that is, just 20% of the generated power is used in pumping the water back up to the top lake.

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The plant itself is mostly underground, with not much but the dam and a cable-car used by workers as evidence of a station that, once fully operational, will generate 1,450 megawatts, enough to power 1 million homes. The cable car takes workers up the first 3,300 feet, and then they switch to a Mercedes truck, which takes them through a two-mile tunnel to the power plant. There is also a subterranean funicular railway.

As an engineering project, it is impressive enough, but as a power source it is also pretty amazing. 1,450 megawatts is about the same generating capacity the nuclear plants used by Germany. But the possibility of storing energy so (relatively) efficiently is something that nuclear can’t do. Also, that excess energy can come from other renewable sources like wind and solar, or even other hydroelectric plants. Plus, a pair of lakes connected by a tunnel will never go into meltdown or produce waste.

Right now, GE is involved in building similar plants in Nant de Drance in Switzerland and Tehri in India, but it seems like an ideal solution as long as you have mountains, lakes, and the engineering ability to dig holes in them.