Why is the Olympic diving pool's water suddenly green?

On Monday, Rio's diving pool and swimming pool were the same clear shade of blue. But by Tuesday, the diving pool looked like it had been dyed green for a St. Patrick's Day celebration:

The green water was "so dark that divers couldn't see the bottom of the pool," The Associated Press reported. "I've never dived in anything like it," said Britain's Tonia Couch after competing in the emerald waters.

Though a test reportedly confirmed the green water poses "no risk whatsoever" to athletes, officials still aren't sure why exactly the water would turn this hue. The owner of AAA Pool Service in Santa Rosa, California, told The New York Times there's a good chance the color change was caused by a "sudden algae bloom" or a "chemical reaction between chlorine and a metal in the water, most likely manganese." Another pool and spa adviser told The Independent that "it's almost certainly a lack of chlorine in the water." Yet another expert told The Guardian that pools usually turn green because of a "lack of sanitation." And, per a Rio spokesman, the water-color change was caused by a "proliferation of algae" because of "heat and a lack of wind."

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Officials promise the pool will be back to its normal color by Wednesday. In the meantime, divers are avoiding opening their mouths in the water — just in case.