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A sled dog is a dog trained and used to pull a land vehicle in harness, most commonly a sled over snow. Sled dogs have been used in the Arctic for at least 8,000 years and, along with watercraft, were the only transportation in Arctic areas until the introduction of semi-trailer trucks, snowmobiles and airplanes in the 20th century, hauling supplies in areas that were inaccessible by other methods. They were used with varying success in the explorations of both poles, as well as during the Alaskan gold rush. Sled dog teams delivered mail to rural communities in Alaska, Yukon, Northwest Territories and Nunavut. Sled dogs today are still used by some rural communities, especially in areas of Russia, Canada, and Alaska as well as much of Greenland. They are used for recreational purposes and racing events, such as the Iditarod Trail and the Yukon Quest.