A ball pool that spans 10 acres sounds like the stuff that children’s dreams are made of. Residents of Los Angeles now have one right in front of their doorsteps, although it is not for bathing or playing in, and rather than the colourful ensemble used to attract kids, this particular one contains just black polyethylene balls, covering the surface of Ivanhoe and Silver Lake reservoirs.

What may look like a prank or an extreme case of littering to the outsider is actually the action of the Los Angeles Department of Water and Power. The roughly 6.5m black balls made of plastic – a material often considered hazardous to the environment – actually protect the reservoir’s drinking water by preventing sunlight from mixing with the bromide and chlorine contained in the water, a reaction that would form the carcinogen bromate.