In the 1950s, residents of Minamata, Japan, noticed that something was wrong with the city's cats. As the SciShow's Hank Green recounts in the video below, the cats would convulse, make strange noises and jerking "dancing" motions—and eventually die. Soon, these symptoms spread to the local townspeople, and scientists began searching for the cause of the terrifying sickness.

The culprit was soon revealed to be the Chisso Corporation, a Japanese chemical company with a factory in Minamata. About 30 years earlier, the company had begun making an organic chemical called acetaldehyde, using mercury as a catalyst to trigger the needed reactions. Afterwards, the company dumped the leftover chemicals into Minamata Bay, where the mercury came into contact with bacteria that transformed it into the most noxious form of the metal: methylmercury. This toxic substance was absorbed by plants, which were in turn eaten by fish. Eventually, the methylmercury made its way up the food chain and poisoned both felines and humans. Birth defects became rampant, and more than 900 people died. Thousands of victims have since been identified. The neurological syndrome caused by extreme mercury poisoning is now known as Minamata disease.

You can learn more about the worst mercury poisoning disaster the world has ever seen by watching the video below.