Arctic Circle level, although in mountainous regions there is often no direct view of the true horizon. Only four million people live north of the Arctic Circle due to the severe climate; nonetheless, some areas have been settled for thousands of years by indigenous peoples, who today make up 10% of the region's population. Tens of thousands of years ago, waves of people migrated from eastern Siberia across the Bering Strait into North America to settle. The largest communities north of the Arctic Circle are situated in Russia, Norway and Sweden: Murmansk (population 307,257), Norilsk (175,365), Tromsø (71,295), Vorkuta (59,231) and Kiruna
Petroleum exploration in the Arctic Bay. However, only half the basins – such as the Beaufort Sea and the West Barents Sea – have been explored. A 2008 United States Geological Survey estimates that areas north of the Arctic Circle have 90 billion barrels of undiscovered, technically recoverable oil (and 44 billion barrels of natural gas liquids ) in 25 geologically defined areas thought to have potential for petroleum. This represents 13% of the undiscovered oil in the world. Of the estimated totals, more than half of the undiscovered oil resources are estimated to occur in just three geologic provinces – Arctic Alaska, the Amerasian