What Caused the Olympic Diving Pool to Turn Green?

On August 9, viewers tuning in to the women’s 10-meter synchronized platform final were met with an unexpected sight: the pool water had taken on a startling emerald hue. Olympic officials attributed the unusual color to "an algae bloom triggered by the heat and still conditions," but as New Scientist reveals, the series of events that led to this dramatic shift is actually more intricate.

Anyone who has owned or had to maintain a pool knows that chlorine is key to keeping unwanted organisms at bay—algae included. In the case of the Olympic diving pool, chlorine seemingly did not perform its task, and sunlight might be a contributing factor. This marks one of the first Olympics in recent years where swimming events are held outdoors. The intense sunlight can cause chlorine-based disinfectants to degrade and evaporate, leaving the water vulnerable to external organisms.

But this alone doesn’t account for the algae issue. As viewers noticed on Tuesday, the swimming pool was sparkling clear in stark contrast to the diving pool just a few feet away. One major difference was temperature. The diving pool is maintained at a warmer temperature, creating a more favorable environment for algae and other life forms. The water in the diving pool is also much calmer than the swimming pool, which aids divers with their jumps but also results in chlorine circulating at a slower pace throughout the water.

Soon after the incident, Rio spokesperson Mario Andrada told AP that the pool would return to its proper color by the following day. However, when the next day’s events began, the pool’s unusual green tint remained. Meanwhile, the nearby water polo pool has been gradually turning green to match. Fortunately, tests confirmed that the water is safe for swimming.