It was the most serious release of radioactive material since Fukushima 2011, but the public took little notice of it: In September 2017, a slightly radioactive cloud moved across Europe. Now, a study has been published, analyzing more than 1300 measurements from all over Europe and other regions of the world to find out the cause of this incident. The result: it was not a reactor accident, but an accident in a nuclear reprocessing plant. The exact origin of the radioactivity is difficult to determine, but the data suggests a release site in the southern Urals. This is where the Russian nuclear facility Majak is located. The incident never caused any kind of health risks for the European population.