Rio’s Olympic diving pool turned green overnight thanks to algae

There are two pools in Rio for the Olympics. Yesterday, both of them were clear and blue. Today, one of them is green.

OK, the pool’s green. That’s fine. Surely there’s a good reason for it ... right? Riiiiight?

Seriously, this pool is VERY green and we aren’t sure why.

Like, REALLY FREAKING GREEN.

I’m told by swimming pool connoisseur and SB Nation coworker Seth Rosenthal that green pools usually happen because there’s too much algae and not enough chlorine. The pool is covered but not completely indoors, so it is possible for that algae to sneak in and break out there overnight.

From the website pooluniversity.org, which certainly seems like it might be a good source. It also says that if it is algae, it’s not harmful to swimmers, so that’s good.

Algae spores may get into the pool through rain, wind, or contaminated equipment and swimsuits. When the swimming pool conditions are not well balanced, an algae bloom can occur overnight. These pool conditions include an improper imbalance of water chemicals, presence of nitrates and or carbon dioxide, warm temperatures and sunlight. Of course, poor circulation, filtration and sanitation are always the main cause of the algae. Algae blocks filter pores causing a decrease in the filter effectiveness which may require you to do more backwashing. It consumes chlorine in the water that should be used on other contaminants creating a higher chlorine demand in the water. Algae itself does not harm swimmers but if a pool has algae, it may be a haven for pathogens like E-coli bacteria. Although algae do not harm swimmers, no one wants to go swimming in a pool that is full of algae.

Update: CNN confirmed that, just as we suspected, algae was indeed the reason the diving pool turned green, thanks to “the heat and lack of wind in the venue.” The pool should be back to a normal blue for Wednesday’s events.