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.

Here’s how the pool complex looked as the Games were being set up. Business Insider notes that on Monday, the diving pool was the same shade of blue as the water polo pool:

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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:

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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.