“It’s somewhere in South Russia,” agency director Jean-Christophe Gariel told NPR last week, after the French traced the cloud’s most likely origins to a region between the Volga River and the Ural Mountains that mark Europe’s eastern edge.

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Several nuclear facilities exist in the region, NPR noted, including a problematic plant that blew up in 1957. The agency ruled out a meltdown, concluding this week such a catastrophe would have spewed out far more radioactive material than the niche element that covered Europe in the first weeks of October.

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Ruthenium-106 is used in medical research, nuclear fuel reprocessing and sometimes to power satellites. No suspect satellites were known to have fallen to earth in late September, the French agency wrote. Russian officials have denied any knowledge of an accident, according to the Associated Press.

In any case, Austria detected Ruthenium in its atmosphere on Oct. 3. Germany the next day. Over the next two weeks the levels peaked, faded and finally disappeared.

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At no point was the cloud a danger, Germany’s radiation agency wrote. You could inhale from that country’s Ruthenium cloud for a straight week and still have breathed in no more radiation than you naturally do in an hour.

By mid-October, the French agency wrote, the Ruthenium was gone altogether.

“It is currently no longer detected in Europe,” it wrote, concurring with Germany that there had been no health hazard.

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Which isn’t to say the cloud’s mysterious origin is not a concern. Or that the French wouldn’t have handled things differently, had it started within its borders instead of . . . wherever it was that it happened.

“The consequences of an accident of this magnitude in France would have required to implement locally measures of protection,” the agency wrote — evacuation or sheltering for at least a couple miles around the incident.

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Contaminated food would be expected in an even wider radius — though the French government sounded confident it wasn’t importing any bad mushrooms from Russia.

Still, the cloud remains a mystery. This isn’t the first time radiation of unknown source made its way across Europe. Scientists were also vexed when Iodine-131 spread over the continent in January.