the oil in alaska, for example, is very important to the united states. the main source is the prudhoe bay oil field on alaska's north coast. it was discovered in 1968. plans were quickly made to build a pipeline to bring the oil to an ice-free port so it could be shipped south. oil companies spent billions of dollars to build the trans-alaska pipeline in the mid-1970s. it had to be built above the ground in the northern section because of the permafrost. the warm oil in the pipes could have melted the permafrost and caused the ground to sink. that would have broken the pipes and caused an oil spill. the pipeline was completed in 1977. it brings oil to the seaport of valdez on alaska's south coast. from there the oil is taken by huge tankers to refineries along the west coast of the united states. if you live in western america, your family car may have alaskan gasoline in its tank.