It’s part of a $34.5 million project to protect the city’s water supply from the sun.

No, the folks at Las Virgenes’ municipal water district in Los Angeles are not making the world’s biggest ball pit in this mesmerizing video from June.

What’s being dumped from the back of the truck are “shade balls”—hollow, plastic balls measuring 4 inches in diameter that, as the name suggests, block sunlight. They cost 36 cents each, but to the city of L.A., they’re worth much more.

Local news outlets in L.A. reported Monday that 20,000 balls were released into a reservoir at the Van Norman Complex in the Sylmar area of the city. With that move, city officials completed a $34.5 million project that dumped 96 million balls into four reservoirs to protect what’s left of the city’s drinking-water supply.

For a city facing one of the worst droughts in U.S. history, the sun is a tough adversary. Not only does it evaporate millions of gallons of precious water, but it can also cause chemical reactions that make the water unsafe to drink. That’s where the shade balls come in.

The balls float at the surface of the reservoirs to form a cover that prevents sunlight from triggering a reaction between bromide (found naturally in ground water) and chlorine (added to kill bacteria)—which forms carcinogen bromate. They’re expected to last for 10 years and save 300 million gallons of water from evaporating each year. That’s enough drinking water for 8,100 people.

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