In the spring of 1956, a doctor in the Japanese village of Minamata reported an outbreak of a troubling new disease. It was seen mainly among children, and it affected the central nervous system. The disease quickly spread, with hundreds of cases reported, then thousands. It took years for scientists to work out the cause: poisoning from industrial pollution in Minamata Bay—the first known case of a disease caused by polluted seawater.

A chemical factory was pumping huge amounts of wastewater into the bay. The water was laced with mercury. Some of it was methylmercury—an especially nasty form.

Microscopic organisms gobbled the stuff up, then were eaten by larger organisms. The amount of mercury built up to higher and higher levels with each link in the food chain. So the fish and shellfish eaten by people were filled with it. That triggered Minamata disease. Symptoms included numbness, problems with vision and hearing, trouble walking, and tremors. The disease killed hundreds, and may have afflicted millions. And its effects are still being felt.

The company dredged the bay to remove contaminated sediments. And the nations of the world crafted a treaty to reduce the amount of methylmercury in the environment. It calls for less mercury in products and manufacturing, fewer emissions of it from coal-fired power plants, and better storage and disposal.

Even so, mercury and other chemicals still cause problems as they work their way up the marine food chain.