In the current Olympic games, one of the Olympic pools turned green. It was in the women’s synchronised 10m platform final that this happened. The divers and the spectators was concerned that the water that was azure blue to start with started to turn more and more into a swampy green color. The officials did test the water and it appeared to be safe for the divers.

Rio 2016: ‘I’ve never dived in anything like it’ – Olympic pool turns green – BBC Sport