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An ecologist in Canada has located a huge beaver dam stretching 850m (2,788ft) using pictures filmed from outer space.

Jean Thie said he found the dam - the size of eight football pitches - using Google Earth and NASA satellite technology while researching the rate of melting permafrost in the country's far north in 2007.

Using past images and park aerial photography, he estimated that the beavers started building it in the 1970s.