Olympic officials are all at sea over why the official diving pool for the competition suddenly turned from a crystal clear blue to a vivid green overnight.
The green was highlighted further by the fact that the adjacent water polo pool remained blue. Tandem divers complained that the water was so cloudy they had a difficult time seeing their partners.
Athletes took to social media to express bemusement. "Wait, is someone playing a joke or are we celebrating St Paddy's Day early here in Rio?" tweeted silver-medal-winner US diver David Boudia.
"Ermm ... what happened?" asked bronze winner GB diver Tom Daley.
Divers complained that the pool seemed to be growing greener by the minute, but officials dismissed any concerns about toxins or bacteria contaminating the water and gave the green light for the women's synchronised 10m platform diving competition to go ahead.
"Water tests at Maria Lenk's Aquatic Park were conducted and there was found to be no risk to the athletes' health," an official said. "We're investigating what the cause of the situation was, but we are happy to report that the competition was successfully completed."
The change in colour was likely caused by an algae bloom in the water, according to Gizmodo. Leached oxidised copper can also turn water green, but in such cases the water usually remains clear. Warmth also helps to fuel algae blooms and the diving pool is typically kept slightly warmer than the other competition pools.
It is also the first time since the Athens Olympics in 2004 that diving events are being held at an outdoor pool – also a large factor in the cause of algae blooms.
Such a bloom can be sparked by changes in a pool's chlorine level, and the usual way to avoid the problem is to use boosted chlorine overnight to kill any algae in the pool. The city has also been getting heat for raw sewage off Rio's beachfront.