UPDATE, August 15, 2016 at 8:40 a.m.:
Olympic officials have finally figured out why the diving pool in Rio turned bright green last week: Someone mistakenly added hydrogen peroxide to the pool water on August 5, according to the New York Times. The hydrogen peroxide neutralized the chlorine and allowed organic compounds, such as algae, to grow in the water and turn it green. Athletes complained that the chemicals used to treat the green water were making their eyes hurt, and that it was nearly impossible to see underwater.
The pool located right next to the diving pool also turned green last week, but CNN reports that the 1-million-gallon pool has been drained and refilled — just in time for the synchronized swimming event, which is lucky, considering it's essential for synchronized swimmers to see each other underwater.
Now, officials say, the pool "looks as it should." As for the diving pool, officials will instead try to fix its filters. The officials also emphasized that the green water was not a health risk for any athlete.
UPDATE: August 10, 2016 at 8:14 a.m.
Olympic officials have explained what suddenly turned the diving pool at the 2016 Games a bright shade of green. The green was caused "by a proliferation of algae," spokesperson Mario Andrada told the Associated Press. "This was because of heat and a lack of wind."
According to those officials, the water did not pose any health risk to the athletes, and it had passed all routine chemical tests that day. But it did create some issues for divers, because the green hue was so thick that they couldn't see the bottom of the pool. Rio organizers assured participants and onlookers that the pool would return to its blue shade for the rest of the Games.
ORIGINAL POST: August 9, 2016 at 4:37 p.m.
Tuesday afternoon, Olympians set out to compete in the synchronized diving finals in Rio. But onlookers and competitors noticed that the water looked a bit different than it did on Monday. In fact, the water was a bright shade of green.
Lots of spectators, and even British diver Tom Daley, noted that though the water polo pool still looks blue, the diving pool is inexplicably green:
It's unclear at this point what caused the pool to change color, but it hasn't put a stop to the competition. A BBC commentator said on the air that he asked an organizer about the color change, and he said he didn't know.