Mysterious radioactive spikes are being found across Europe – and nobody quite knows why.

Iodine-131, a man-made radioactive material, is being found in small amounts across the continent. It was found in northern Norway early in January, according to officials, but has been gradually moving across the rest of Europe ever since.

But despite finding the material in January, authorities didn’t announce that it had been found until recent days. That might be because it isn't at all clear where it has come from or how it got to be spread out.

Further information makes the find even more unusual. Iodine-131 is usually found alongside other radioactive materials, but it wasn't. And it has a short half-life – the time required for one half of the atoms of a radioactive substance to disintegrate – but a significant amount of it was found, meaning that it is likely that it was introduced very recently.

It isn't clear where Iodine-131 would ever have been released from. The US has deployed specialised planes to fly over Europe and isolate it – but have mostly failed.

Conspiracy theorists have pointed to the fact that the spread appeared emanate from northern Norway, where Russia may have run a secret nuclear test. Iodine-131 is perhaps most closely associated with atomic bombs, and was found throughout the world after those were tested, as well as after accidents at the Chernobyl and Fukushima nuclear power plants.

But it is also being used to treat some cancers and other illnesses. The fact that the material was found on its own likely suggests that it had been isolated, and so makes it more likely that the leak came from a pharmaceutical company that hasn't reported it to authorities.

It's not possible to say where exactly those leaks would have come from, either. Changing winds mean that it isn't possible to isolate the source, apart from a suggestion that it is likely it was released somewhere near eastern Europe.