Chisso's toxic effluents with methyl mercury in them was discharged into the sea, untreated, for 36 years (from 1932 to 1968). (They included other poisonous heavy metals and chemicals, such as selenium, thallium and manganese.) The total amount of methyl mercury released is estimated at 400--600 tons. It is widely believed that all the mercurial sludge at the sea bottom cannot be recovered.