About 4% of Canadians are directly employed in primary resource fields, and they account for 6.2% of GDP. They are still paramount in many parts of the country. Many, if not most, towns in northern Canada, where agriculture is difficult, exist because of a nearby mine or source of timber. Canada is a world leader in the production of many natural resources such as gold, nickel, uranium, diamonds, lead, and in recent years, crude petroleum, which, with the world's second-largest oil reserves, is taking an increasingly prominent position in natural resources extraction. Several of Canada's largest companies are based in natural resource industries, such as Encana, Cameco, Goldcorp, and Barrick Gold. The vast majority of these products are exported, mainly to the United States. There are also many secondary and service industries that are directly linked to primary ones. For instance one of Canada's largest manufacturing industries is the pulp and paper sector, which is directly linked to the logging business.