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 Basin, and the East Greenland Rift Basins. More than 70% of the mean undiscovered oil resources is estimated to occur in five provinces: Arctic Alaska, Amerasia Basin, East Greenland Rift Basins, East Barents Basins, and West Greenland–East Canada. It is further estimated that approximately 84% of the undiscovered oil and gas occurs offshore. The USGS did not consider economic factors such as the effects of permanent sea ice or oceanic water depth in its assessment of undiscovered oil and gas resources. This assessment is lower than a 2000 survey, which had included lands south of the arctic circle. A recent