Located between the Volga river and the Ural mountains, the leak coming from the Mayak nuclear development facility released up to 100 times the amount of radiation into the atmosphere as the Fukushima disaster. 

Italian scientists were the first to raise the alarm on 2 October, when they noticed a burst of the radioactive ruthenium-106 in the atmosphere. This was quickly corroborated by other monitoring laboratories across Europe.

Georg Steinhauser at Leibniz University Hannover in Germany says he was "stunned" when he first noticed the event. Routine surveillance detects several radiation leaks each year, mostly of extremely low levels of radionuclides used in medicine. But this event was different.

"The ruthenium-106 was one of a kind. We had never measured anything like this before," says Steinhauser. -NewScientist