They're hypnotic and incredibly odd. Thousands of black plastic "shade balls" float and bob on the surface of the Los Angeles Reservoir, looking like a kind of dark playground ball pit.

Roughly 96 million plastic balls now reside in the 175-acre reservoir, the culmination of a $34.5 million initiative to protect the water supply.

“In the midst of California’s historic drought, it takes bold ingenuity to maximize my goals for water conservation,” said Los Angeles Mayor Eric Garcetti, who helped release the last batch of balls in August 2015. “This effort by LADWP is emblematic of the kind of the creative thinking we need to meet those challenges."

The balls are intended to prevent sunlight-triggered chemical reactions that encourage algae — creating cleaner water, says Garcetti. The bobbing balls also protect the water from wildlife. But the key benefit is that the floating ball will prevent evaporation. Los Angeles officials estimate the balls will save about 300 million gallons of water each year.

The shade balls are BPA-free and should not release any chemicals. Garcetti said the orbs, which are manufactured by minority, women-owned facilities in Los Angeles County, require no parts, labor or maintenance aside from occasional rotation. They're recyclable and should last 10 years before they need to be replaced.

Plus, they're saving the city a lot of money compared to other alternatives, which included splitting the reservoir with a bisecting dam and installing floating covers that would have cost more than $300 million. According to a Facebook post from Garcetti, “with these shade balls, we ended up spending only $0.36 for each ball coming in at just $34.5 million to get the same result."

Shade balls aren't a new concept; they've been used in open-air reservoirs in Los Angeles since 2008. They're the brainchild of Dr. Brian White, a now-retired biologist with the Los Angeles Department of Water and Power, who said he got the idea when he learned about the application of “bird balls” that were placed in ponds along airfield runways to keep birds from congregating too close to planes.

In addition to the Los Angeles Reservoir, the balls are floating at Upper Stone, Elysian and Ivanhoe reservoirs and other areas.

Watch science educator Derek Muller take a boat through the millions of black plastic balls in the Los Angeles reservoir.

Editor's note: This story has been updated with new information since it was published in August 2015.

Mary Jo DiLonardo writes about everything from health to parenting — and anything that helps explain why her dog does what he does.

Why California dumped 96 million plastic balls into a reservoir
Millions of black 'shade balls' float on the surface of reservoirs, slowing evaporation.