Norilsk, originally build by slave labour, is the largest of the mining and smelting cities built above the Arctic Circle. With a population of more than 250,000, the it was a model city in the Soviet plan. But the smelters emit a million tons of sulphur a year, polluting the Arctic and damaging the tundra. The cost of maintaining the northern cities has proven prohibitive and more than a million residents are expected to leave. Photo credit: AMAP (Arctic Monitoring and Assessment Program)