If you were looking for the perfect place to live in the 1950s, you couldn't do much better than Minimata in southern Japan. It's surrounded by mountains right next to the ocean, and there are hot springs just up the road. It was basically a paradise. Plus, if you were looking for work and fishing wasn't your thing, there was a giant chemical manufacturing plant, the Chisso Corporation. So, you had employment, idyllic scenery, and endless amounts of seafood - Minimata was perfect. Except for the cats. 
One day locals started noticing that cats were acting weird. And it turns out (in the least shocking plot twist ever) that the chemical manufacturing company was responsible.
About 30 years earlier, the Chisso Corporation had started making a chemical called acetaldehyde, a super important precursor to things like acetic acid and some plastics. The most efficient way of making acetaldehyde at the time used a particular catalyst - a thing to kick-start the reactions. And, unfortunately for the unsuspecting residents of Minimata, the Chisso Corporation decided to use mercury and then dump the leftovers from the reaction into the bay.
We've known for a long time that mercury is one of the nastier elements on the periodic table, which is why we don't use it so much anymore, although you can still find it in old thermometers or batteries. Eating plain mercury doesn't actually cause too many problems; like, it's not great, so don't just do it, but your gut doesn't absorb too much of the metal. The Chisso Corporation wasn't dumping elemental mercury, though - it wasn't pure metallic mercury. Heavy metals, which are elements that are mostly in the middle section of the periodic table, can lose electrons to form positively charged ions. When mercury does this, it forms what chemists call inorganic mercury, and it's the ions with a positive charge of 1 or 2 that really cause problems. Hat makers in the 1800s, for example, used a compound called mercuric nitrate to make felt for their hats. Workers would spend hours every day inhaling mercury fumes, and the effects were not super-awesome - most would have severe personality changes, hallucinations and uncontrollable shaking; in fact, that is where the Mad Hatter in Alice in Wonderland came from. And the Chisso Corporation in Minimata was dumping similarly toxic kinds of mercury into the water. And the cats weren't just inhaling fumes - it was worse than that, so much worse than that. All because of a bunch of anaerobic bacteria living in the Minimata bay. 
Instead of using oxygen as their main source of energy, anaerobic bacteria use sulfur, and they're not too picky about the quality of the water they're living in or the ions that might be floating around in it. When the bacteria came into contact with the extra inorganic mercury at Minimata, they transformed it into the most poisonous form of the metal - methylmercury, which is a carbon attached to a mercury with a single positive charge. Inorganic and organic compounds don't tend to want to mix - they're sort of like oil and water. So when the original mercury ions were floating around all by themselves, the plants on the floor of the bay basically ignored them. But the methylmercury produced by the bacteria was bio-available, meaning that the plants were able to absorb it. And methylmercury is especially poisonous for humans and other animals, because, unlike elemental mercury, it is almost entirely absorbed in the gut.
Here's where it gets bad - once methylmercury is in, it doesn't leave, and these mercury-laced plants were being eaten by fish. The more the fish ate, the more methylmercury they accumulated in their cells, so the larger they grew, the more deadly they became. And, of course, the biggest source of food for the cats of Minimata was fish. 
In the early 1950s the locals started seeing what looked like the cats dancing, which sounds kinda funny, but was actually horrible - they would jerk around uncontrollably, making terrifying noises, and then they would die. Around 1956 the human residents of Minimata started experiencing the same symptoms. They started flooding hospitals with numbness in their hands and feet, high fever, uncontrollable flailing and loss of sight and hearing. A lot of them ended up losing consciousness and dying. Doctors and researchers launched an investigation into what they started calling Minimata disease, and by 1959 they had found the cause - methylmercury poisoning from fish. But they couldn't figure out why it was causing severe birth defects; there's a membrane called placental barrier that's supposed to stop really terrible things like mercury from getting through to the fetus. We now know that methylmercury gets through by disguising itself as a super important component of proteins, the essential amino acid methionine. When it gets inside the body, methylmercury binds to a different amino acid called cysteine, and to your cells the cysteine-methylmercury combo looks pretty much exactly like methionine. The thing about methionine is that it's really easily absorbed across the almost impenetrable placental barrier, as well as the similar barrier that's supposed to protect the brain.
By 1962, researchers had proven that the chemical processes used by the Chisso factory could cause the methylmercury poisoning, but it wasn't until 1962, 12 years after the first victims died, that the Chisso Corporation stopped dumping the factory's wastewater into the bay. The government started cleaning up the bay in 1977, and by 1997 the water was considered safe. But it will be a long time before people start thinking of Minimata as a perfect paradise again.
Thanks for watching this episode of SciShow. For more on the worst environmental disasters ever, you can check out our video on seven most toxic sites in the US.