The Ivanhoe Reservoir in LA holds millions of gallons of drinking water. It’s also contaminated with cancer-causing bromate — caused by a reaction between sunlight, chlorine, and naturally occurring bromide.

So what to do? They came up with a really clever solution: Keep sunlight away from the water by covering the water with millions of black balls (called bird balls). Watch a video to see how it went down.

More photos at Curbed.