The Shiranui Sea.

The sea around Minamata was heavily polluted with mercury during the 1950s and 1960s from the Chisso Corporation's chemical factory. This highly toxic chemical bioaccumulated in shellfish and fish in the Yatsushiro Sea which, when eaten by the local populace, gave rise to Minamata disease. The disease was responsible for the deaths and disabling of thousands of residents, all around the Yatsushiro Sea. The marine ecosystem was also extensively damaged.