Thirty-two million gallons of mercury are trapped under the immense ice sheets in the Arctic Circle. As the Earth warms and the permafrost thaws, that toxic chemical could be released into the ocean or escape into the atmosphere. Other devastating threats also loom large. Bacteria, viruses and other pathogens that have been immobile for centuries will emerge from their deep freeze, threatening infections against which we have no immunity. Dead plants and animals buried largely intact in the ice will decompose, releasing gases that further hasten warming trends. Just how serious are the risks of thawing permafrost and what can be done?