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At an estimated 175 billion barrels, Alberta's oil sands are the second largest oil reserve in the world behind Saudi Arabia, but they were neglected for years, except by local companies, because of high extraction costs. Since 2000, skyrocketing crude oil prices and improved extraction methods have made exploitation more economical, and have lured several multinational oil companies to mine the sands. Pictured, an aerial view of a tailings pond, leftovers from the extraction process, near Fort McMurray on Oct. 23, 2009.