The World's Biggest Battery

California is generating a ton of solar power--some days, more than people can use. It could power the whole state soon, except for one problem: There's no way to hold on to all that energy at night.

Some of the country's biggest energy companies are trying to figure out how to store the power. They're buying giant lithium batteries, and imagining ways to tap the batteries in electric cars. But there's another option.

Today on the show, we visit the world's biggest battery, built out of a mountain and a lake.