According to a recently released report from the journal Geophysical Research Letters, There are about 15 million gallons trapped underneath permafrost in the Northern Hemisphere, mostly under the Alaskan, Canadian, and Russian regions of the Arctic Circle. As global warming increases temperatures across the globe, this permafrost will melt and free huge amounts of mercury into the ocean and waterways of the world. Fifteen million gallons is nearly twice as much as the Exxon-Valdez oil spill.
While the amount might sound alarming, it's worth a reminder that there are about 326 million cubic miles of water. To put that roughly, if all the world's ocean water was a basketball, 150 million gallons would be about the size of a head of a pin.
The real danger, though, is what that amount of mercury could do to the food chain. Salmon would be immediately affected, and would quickly carry toxic (if not immediately lethal) amounts of mercury. Any increase in mercury intake would carry with it sharply increased risks of certain types of cancers.