Sled dogs have been used in the Arctic for at least 9,000 years and were important for 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 and northern Canada. Sled dogs today are still used by some rural communities, especially in areas of Alaska and Canada and throughout Greenland. They are used for recreational purposes and racing events, such as the Iditarod Trail and the Yukon Quest.