This wasn’t part of Rio’s plan to host the greenest Olympics ever.

When the diving pool turned a frightening shade of St. Patty’s Day sometime Monday night, it caught us all off guard — including Tom Daley, a British diver.

Help Grist raise $25,000 by September 30 to further advance our climate reporting

Less than 24 hours later, it happened again.

The leading theory is that the green color was caused by algae. The “heat and a lack of wind” sapped the chlorine in the pool, a Rio spokesperson said. Apparently, it’s safe to swim in — at least, compared to the actual bodies of water that surround Rio, which are teeming with sewage and superbacteria.

Algae are a familiar menace in many waterways (not just of the swimming-pool variety). The harmful blue-green variety is made worse by phosphorous- and nitrogen-rich fertilizers, but climate change hasn’t helped matters either. Toxic algae blooms thrive best in warm, tepid waters, and the consequences are much bigger in freshwater than a change in color.

Grist thanks its sponsors. Become one.

To support our nonprofit environmental journalism, please consider disabling your ad-blocker to allow ads on Grist. Here's How