The diving and swimming pools down in Rio de Janeiro looked a sparkling blue colour for the first few days of the 2016 Summer Olympics, but then two of them suddenly turned a dark and cloudy green colour. The diving pool was the first to be affected, but the majority of competitors said it didn’t really affect their performances. However, the neighbouring outdoor swimming pool turned the same shade of green a couple of days later and water polo players started to complain that it was hurting their eyes. The Olympic organizers then decided on August 13th to drain the million-gallon swimming pool and refill it with fresh water since synchronized swimmers need to see each other below the water level.

The green water resulted in several theories with the most sensible one being that it was caused by algae. It’s no secret that algae can grow quickly in relatively warm water if the right chemicals aren’t used to combat it. The green pools certainly didn’t look pleasing on the eye, but tests allegedly proved the water was safe. However, the Olympic organizers were definitely embarrassed by the sight. Algae growth can also happen when the chlorine level in a pool changes drastically to a low level. But finally after several days of speculation, the International Olympic Committee stated that the pools turned colour because somebody added 80 litres of hydrogen peroxide to each of them by mistake.

According to reports, hydrogen peroxide is an effective cleaning chemical for swimming pools, but not when it’s used in combination with chlorine. Hydrogen peroxide supposedly neutralizes the chlorine in the water if it’s added to it and the chlorine can’t do its job properly. This combination of chemicals somehow confuses the electronic systems that are in place to monitor the balance of the water. Gustavo Nascimento, who is in charge of the Olympic venues in Rio, admitted that somebody made a mistake as he didn’t know that hydrogen peroxide had been added to the pools.

When the outdoor Olympic pool is drained it takes approximately 10 hours to empty and then refill it. But the solution may not be just as simple as adding fresh water to the pool. This is because algae often grow on the walls and floor of the pool itself. If this is the case, then the pool will more or less need to be scrubbed clean before new water is added to it. And once the pool is refilled, it’ll be just as crucial to make sure the right balance of chemicals is used to keep the algae at bay.