On the left is a photo of the pool on Tuesday morning, and on the right is the pool on Tuesday afternoon.
Athletes who competed in the green pool say it didn't affect their performance. But the big question is...why did it turn green in the first place?!
Rio spokesperson Marlo Andrada said the green colour was due to a "proliferation of algae" that happened "because of heat and a lack of wind".
The algae explanation fits with the fact that the pool seems to have turned not just green, but cloudy too.
Algae are naturally found in small numbers in swimming pools, but they're so tiny you wouldn't normally be able to see them.
In the right conditions, algae can bloom very rapidly.
Without looking at them under a microscope it's impossible to tell what kind of algae turned the pool green.
"There's hundred of species of these green algae which could grow quite happily in a swimming pool, so it's hard to say without seeing them," said Henson.
"It might look a bit unsightly, but algae are natural, and they've said these ones are safe, so there should be no problem for the swimmers."
Another explanation floating around was copper. But though copper can turn water green, it doesn't tend to make it cloudy.
Either way, officials said that the water had been tested and did not pose any risk for athletes.
UPDATE: A spokesperson has now said the colour change was due to a "sudden decrease in the alkalinity in the diving pool".
Henson said that "it's possible" that this change could cause an algae bloom.
"Different species of algae are adapted to different ranges in temperature, nutrients and pH," she said. "So if something caused the pH to increase (i.e. water becomes more alkaline) that might have triggered a bloom of a particular type of algae."
UPDATE: A spokesperson confirmed to BuzzFeed News that the "decrease in alkalinity" was the reason for the green colour, and said that this was due to the "increased use of the pool in the last few weeks."
Another pool that is used for water polo and synchronised swimming pool is also affected, they said. The chlorine levels and pH of both the pools are "within the required standards" and are expected to return to normal in the next few days.