The water in the Olympic diving pool was bright green on Tuesday, baffling at least one competitor who said she could not see her partner underwater, although organisers said that it did not pose any danger to health.
The mysterious shade of the water, which contrasted sharply against the blue hue of water polo pool beside it, generated jokes about algae and dye on social media and overshadowed the women's 10 metre synchronised platform final.
British diver Tonia Couch said the pool's water was so green she could not see her partner Lois Toulson underwater, The Times said on Twitter.
Rio 2016 organisers said tests at the Maria Lenk Aquatic Centre's diving pool found that the water was no risk to health, although they were still investigating.
How green?It has been deemed safe though.... #Rio2016 https://t.co/2lbwFnU2Vz pic.twitter.com/UICQRPdAMI— BBC Sport (@BBCSport) August 9, 2016
The executive director of swimming's world governing body FINA told Reuters that the problem may have come from a faulty filter or problematic water quality.
"No danger for divers, just not a good image for Olympic Broadcasting Services," said FINA's Cornel Marculescu, adding that he was not sure how quickly the pool water could be brought back to its normal colour.
The problem comes on top of worries over dangerous levels of pollution in Rio's Guanabara Bay and concerns that floating garbage could damage or slow sailors.
Most divers, however, said the colour was no issue, even though the pool's water was perfectly blue during the men's 10 metre platform event on Monday.
"When we were practising to get used to this venue (the water) was always sky blue...But we're always mentally prepared for unexpected situations," China's Liu Huixia, who won the gold medal with partner Chen Ruolin, told Reuters.
Bronze medallist Meaghan Benfeito of Canada told reporters that the colour had made her and her partner Roseline Filion want to laugh, but the green actually helped. "It's not the same colour as the sky so that was really on our side today."