Why did the Olympic diving pool turn green?
Consternation over the colour of water in the pool threatened to overshadow the women's 10m synchronised diving final on Tuesday at the Olympics.
Far from the glimmering blue seen in Monday's men's event, the pool was an increasingly murky green as the women progressed through their dives.
Commentators could only speculate as to what had caused the "green abyss", as the BBC team termed it, while spectators channelled their alarm on social media.
Tom Daley, who won bronze for Team GB in the diving pool, tweeted a pictures of the water, saying: "Ermmm, what happened?!"
One early and scarcely believable rumour was that the pool had been deliberately turned green to emulate the dominant colour of the Brazilian flag.
More serious suggestions led on the idea that event organisers may not have 'shocked' the pool overnight with the usual treatment of chlorine.
@SamJDean it depends on current cholrine level and type of algae. Extra cholrine should have been added this morning to combat problem.— Theresa Stewart (@theresamstewart) August 9, 2016
BBC Sport quoted an American photographer in the Rio press room as saying: "If you don't shock the pool water it turns green. It doesn't look nice, but it isn't dangerous."
A green colour can be born of algae, which breaks out in the absence of chlorine in the water.
A spokesman for the Rio 2016 organisers said: "We're investigating what the cause of the situation was.
"It's very important to the Rio 2016 organising committee to ensure a very high quality field of play.
"Water tests at Maria Lenk Aquatics Centre diving pool have been conducted and there was found to be no risk whatsover to athletes."
Canada team leader Mitch Geller suspected the cause was algae that multiplied quickly in the day's warm and sunny conditions.
"Everybody was scratching their heads going, 'What's going on?'" he said. "I think that the filter is busted, but I'm not sure. It's not really dangerous. It's not like it's toxic or dirty or any of that. It seemed to get worse over the course of the competition."
The kale-colored water wasn't just a cosmetic nuisance; it was so dark that divers couldn't see the bottom of the pool.
"They're used to seeing the water," Geller said. "The visuals are really, really important in diving."
Chen Ruolin, who teamed to win gold with Liu Huixia, said it didn't affect them.
Paola Espinosa of Mexico, competing in her fourth Olympics, noticed the pool getting increasingly darker throughout the six-round competition. But she said the water didn't smell or affect her skin.
"I haven't seen anything like it before," Espinosa said. "But it's Brazil and everything is green down here, so maybe it was a decoration to make it look pretty."
American Jessica Parratto wears contacts and said the water didn't burn her eyes.
Bronze medalists Meaghan Benfeito and Roseline Filion of Canada tried not to laugh as they gazed at the water from atop the 33-foot tower. They liked that the dark green color offered a helpful contrast with the blue sky.
"The only thing we said is don't open your mouth in the water, just in case," Benfeito said.
In the competition itself, the British pair of Tonia Couch and Lois Toulson could not emulate the bronze-medal-winning exploits of their male compatriots Tom Daley and Dan Goodfellow the night before.
An imprecise final dive left the women in fifth, still tucked up behind the North Koreans, while Canada leapfrogged both to claim the final medal behind China and Malaysia.