In 2013, the residents of Kalachi, a small village in Kazakhstan, began to take to their beds with a mysterious illness. They couldn’t stop falling asleep. When they woke up, often days later, they remembered nothing. Newspapers (and mental_floss) called it a “sleeping sickness” (unrelated to the African sleeping sickness transmitted by flies) and multiple investigations into the causes—patients’ ages and schedules, the town’s air quality, the food and water people were consuming—turned up nothing. But after years of study, the Kazakh government thinks it has figured out the underlying cause of the outbreak. Maybe. 

The first reported case of the sleeping sickness came in 2010, in a neighboring village. In 2013, the mystery took on new urgency as eight different people from Kalachi (a town of just 640) fell asleep over one weekend, unable to stay awake for any longer than it took to go to the bathroom or eat a little food.

When these otherwise healthy adults finally awoke from their trances, they didn’t remember anything that had happened, even the times when they had seemed awake enough to eat or talk or have a cigarette, as BuzzFeed reported during a week-long investigative trip to the region. One man even woke up in the Kazakh capital of Astana, unable to remember being on the plane that brought him there. People felt nauseous and dizzy; they hallucinated, ranting about images only they could see and at times becoming borderline violent. More than 100 people fell ill at some point. Even a cat was affected. 

This happened again and again over the course of several months, with waves of residents falling prey to the sickness in the beginning of 2014, then later that spring, then again that summer. The nearby uranium mines were a likely culprit, and scientists tested the earth, water, and local food for radon, a gas known to cause cancer. The air was tested for carbon monoxide. People’s hair and fingernails were tested for radiation. Doctors could find nothing wrong with the patients, and no factors to tie them all together. 

Were people being poisoned? Or was it just a case of mass psychogenic illness (essentially mass hysteria), like the “dancing plague” or the numerous population-wide panics throughout history over shrinking penises?

Finally, in the summer of 2015, authorities announced that they had discovered the culprit: high concentrations of carbon monoxide and hydrocarbons coming from the mines caused a lack of oxygen in the air in the area. By the time the announcement came in the summer of 2015, 150 people had already moved away, while another 240 were on a list of people seeking resettlement. Still, a radiologist who had been studying the outbreak told BuzzFeed that the verdict was “only the working theory,” and that researchers were still studying the medical anomaly. In late December of that year, scientists from the National Nuclear Center of Kazakhstan confirmed this explanation.