Doug Allan relives his remarkable career and Gordon Buchanan confronts a huge polar bear. Narrated by David Attenborough - the first episode of this stunning new series reveals the hardships and hazards involved when filming wildlife in the world's most extreme environments - the Arctic and Antarctic. Legendary polar cameraman and diver Doug Allan relives some of his most hair-raising encounters and returns to the place where his underwater career began - the freezing depths of a Highland river