The Olympic green swimming pool mystery has been solved. See media coverage from The Independent and Buzzfeed For days one of the pools was changing into an ever darker color of green in sharp contrast to the pool next to it. Swimmers had good reason to fear for their health. The green was of a distinct algea color but the pool was supposed to have chlorine in it? Turns out the people in charge of the pool had been dumping hydrogen peroxide into it, depleting the chlorine. According to Wikipedia's swimming pool sanitation page, basic disinfectants in pools are elemental chlorine gas (professional use) and sodium hypochlorite (NaOCl, home use). Chlorine reacts directly with hydrogen peroxide to form oxygen and HCl. With NaOCl the reaction products are NaCl, oxygen and water.
Question remaining unanswered: why was the hydrogen peroxide dumped in the first place. It is used in general to dechlorinate waste water before it can be safely discarded into the environment. So the presence of hydrogen peroxide at the site is reasonable. Must have been a valve switched in the wrong the direction? Perhaps the amount of chlorine in the pool was miscalculated and too high? And how to quickly fix that error with H2O2? Or some genius decided to disinfect the pool with chlorine AND hydrogen peroxide? Twice the result?
More chemistry at the Chronicle Flask