Environmental scientists have known that high levels of the toxic element, mercury, have been accumulating in the Arctic Ocean for some time. It was believed to be mostly caused by atmospheric sources stemming from the combustion of coal. However, a new study from the Harvard School of Engineering and Applied Sciences and the Harvard School of Public Health has found that the great majority of Arctic mercury arrives via circumpolar rivers. Some of the largest rivers in the world flow north into the Arctic in Eurasia and North America.