TORONTO – They can be walking, talking or doing everyday tasks, when suddenly, they’ll drop to the ground in a deep sleep.

Residents in two villages in northern Kazakhstan have been plagued by a bizarre sleeping sickness that scientists have been struggling to explain since March 2013. People have been falling asleep for days at a time, sometimes as long as six days.

Now, according to media reports, the Kazakhstan government says that scientists have uncovered the cause: carbon monoxide.

The carbon monoxide seems to be emanating from a defunct uranium mine. This isn’t the first time the uranium mine was suspected to be the cause: it had been previously investigated for radon gas and radiation, however, there wasn’t enough found to be of concern.

“The uranium mines were closed at some point, and at times a concentration of carbon monoxide occurs there,” said Kazakhstan’s deputy PM Berdibek Saparbaev, according to The Guardian.

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That build-up released carbon monoxide, reducing the oxygen and causing adults, children and even pets to suddenly fall asleep or act wildly. Some of the 140 people — in a town of 810 — have even reported wild hallucinations, with one child claiming to have seen a winged horse.

Though the symptoms fit, there are some who disagree with the findings, saying that other gases such as carbon dioxide or methane could also be responsible.

No matter what the cause, the government has begun relocating the families.

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