City officials in the prefecture of Saitama in central Japan have promised to get to the bottom of a mysterious soot-coloured rain that coated the city of Hasuda and surrounds on Monday.

The strange phenomenon has prompted wild conspiracies theories on social media.

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There are claims the sludge is the fallout from a mass incineration of coronavirus bodies in China, the aftermath of a nuclear test in North Korea or the result of fuel dumping from planes flying overhead.

Authorities in the area just north of Tokyo have tested the radiation levels in the black puddles that pooled on the city’s streets and concluded that there was “nothing out of the ordinary”.

The population’s anxiety over the event is understandable, however, given that radioactive black rain did fall over Japan after the US dropped atomic bombs on Hiroshima and Nagasaki in August 1945, claiming an estimated 120,000 lives.

“That’s about as bad an omen as you can get these days,” one person posted on Twitter, using the #blackrain handle.

“Are they secretly burning the bodies of coronavirus victims?” another tweeted.

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“Didn’t North Korea fire missiles on that day?” asked another.

The most plausible explanation appears to be a recent fire at a plastics factory in Noda, in the neighbouring prefecture of Chiba, but this has not been confirmed either.