The Los Angeles reservoir holds some 12.5 billion liters of water or roughly 3.3 billion gallons, the same deposit where most LA residents get their personal drinking water. But why would officials put 96 million black plastic balls in the Los Angeles reservoir? Well, they were originally thought to minimize evaporation. However, minus what has often been reported as the main purpose of these balls, its not actually for preventing evaporation. In a new episode from Veritasium, Derek Muller investigates the story behind this wild idea.

Despite their efficiency on water conservation, the plastic balls or "shade balls" as they are most commonly referred to, were initially used to combat a potentially harmful carcinogenic that forms in the reservoir. The problem began with bromide, a natural property found in salt water. Bromide, on its own, is not necessarily a health risk to humans, but if some of this salty water sneaks into the reservoir and experiences ozone treatment with the rest of LA's drinking water, it can form the compound bromate. And, as we know, bromate is a carcinogen.

The LA Department of Water and Power assumed they were keeping tabs on these bromate levels, but for some reason, the carcinogenic levels kept spiking when water entered the reservoir. Come to find out, when bromide and chlorine intermingle with sunlight, the reaction produces even more bromate than when simply interacting with ozone treatment.

Initially dubbed "bird balls", the solution was both strange and strangely perfect. These balls were generally used around airports to prevent birds from landing in nearby water, and essentially becoming a casualty of airplane engines. Scientists noticed, however, that the black balls also turned out to be highly effective at keeping out sunlight.

"They knocked out the problem immediately," chief of LADWP Marty Adams told Muller in the video.

Adams also explains that their carbon black color sustains each sphere and they can work for up to ten years, with no chance of toxic waste mixing into the water below. Even better, once these balls are placed on the reservoir, the treatment facility can use less chlorine for algae growth, which tends to thrive in sunlight. And while evaporation was not the original reason these balls were used, they do in fact keep the water below much cooler. "So for all of these reasons, shade balls reduce evaporation by 80 to 90 percent," Derek explains. "That's pretty significant for a dry climate like Los Angeles."

Between the savings in chlorination and evaporation alone, Adams says these balls will pay for at least half their original price, approximately three-for-a-dollar, by the time they are through with their job.