black plastic ball, Los Angeles Reservoir. 黑色塑膠球, 洛杉磯水庫

The Los Angeles Reservoir is covered with over 90 million black plastic balls in the Sylmar area of Los Angeles, Aug. 12, 2015. The city has completed a program of covering its open-air reservoirs with floating, 4-inch diameter "shade balls" to protect water quality by blocking sunlight from penetrating the 175-acre surface of the reservoir, preventing chemical reactions that can cause algae blooms and other problems.
August 13, 2015
http://www.voanews.com/media/photogallery/august-13-2015-day-in-photos/2916758.html