Oil sand is a porous rock layer often a mixture of sand, clay, water and bitumen. Oil sands most often refer to Canada where over 170 million barrels of bitumen are estimated to be in place. It is a sedimentary rocks containing heavy oil that cannot be extracted by conventional petroleum recovery methods. The extraction of oil from tar sands is the world’s most capital-intensive method for extracting oil and is also one of the most environmentally destructive. Transforming the tar/bitumen, which is mixed with sand into petroleum is energy intensive and creates significant carbon emissions. Steam created by burning natural gas separates the semisolid bitumen. Then more natural gas is needed to turn the bitumen into synthetic crude, which can be processed by refineries. Spent water used in oil sands projects is placed in lake-sized tailings ponds, one of which killed hundreds of migrating birds in Canada in 2008. Seepage from the ponds is polluting rivers. Strip mining of the oil sands, the most common method of extraction, has destroyed large swaths of forest.