The Veritasium channel visited the drinking water tank in Los Angeles to see why it has been covered by 96 million black plastic balls. The balls are made of high density polyethylene (HDPE), which is less dense than water, so as to float on the surface of the tank even if dissolved. They have a diameter of 10 cm and containing about 210ml water. The primary reasons the tank is to prevent sunlight from entering the water and cause a chemical reaction that converts the bromine in carcinogenic bromate.