Scientists have uncovered another hidden threat buried in the icy frozen north—massive natural reserves of mercury, a toxic heavy metal that in some forms can build up in fish and other animals and cause serious health problems in humans. A study published Monday in the journal Geophysical Research Letters reports that the amount of natural mercury bound up in Arctic permafrost may be 10 times greater than all the mercury humans have pumped into the atmosphere from coal-burning and other pollution sources over the last 30 years. As climate change warms the land, this thawing permafrost could release significant quantities...