After concerns over the water quality near Rio de Janeiro beaches before the Rio 2016 Olympic Games, now a supposedly clean Olympic pool has inexplicably turned green. At the Maria Lenk Aquatics Centre, startling pictures showed a blue pool and a green pool next to one another. Officials allowed Olympians to continue diving in the green pool as they seek the cause of the weird color change.

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The change occurred overnight. One day the pool was blue and the next day it was green. Olympic officials tested the water quality and said there were “no risks for the athletes” in a tweet. They also said they’d be “investigating the cause.”

Related: Experts to Rio Olympic athletes: Don’t put your head underwater

While some speculated urine produced the color, Jim’s Pool Care national manager Brett Blair told The Guardian the Olympic pool was too large to have turned green from urine. Blair speculated poor filtration could be a cause. He told The Guardian, “…the main reason a pool normally goes green is lack of sanitation…The scary part is how at a world event, a pool could go green. It’s unbelievable.”

Another leading theory is that algae caused the change, because the water is so cloudy. Some Olympic divers said they couldn’t see their partners when they dove into the green water. Algae blooms can also happen when chlorine levels in a pool change, and if that’s the case at the Rio pool, it might mean some worker wasn’t doing their job.

Chlorine can probably solve the pool problem; Blair said the issue could be resolved in 24 to 48 hours. While the AP reported Rio spokesperson Mario Andrada saying that algae proliferated “because of heat and a lack of wind,” we’re still waiting for the official word from the Olympic committee on why the issue happened.

Via The Guardian and Gizmodo

Images via Tom Daley on Twitter and screenshot