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Melvyn Bragg and guests discuss the KT Boundary. Across the entire planet, where it hasn't been eroded or destroyed in land movements, there is a thin grey line. In Italy it is 1 cm thick, in America it stretches to three centimetres, but it is all the same thin grey line laid into the rock some 65 million years ago and it bears witness to a cataclysmic event experienced only once in Earth's history. It is called the KT Boundary and geologists believe it is the clue to the death of the dinosaurs and the ultimate reason why mammals and humans inherited the Earth.But exactly what did happen 65 million years ago? How was this extraordinary line created across the Earth and does it really hold the key to the death of the dinosaurs?With Simon Kelley, Head of Department in the Department of Earth Sciences, Open University, Jane Francis, Professor of Palaeoclimatology, University of Leeds; Mike Benton, Professor of Vertebrate Palaeontology in the Department of Earth Sciences, University of Bristol.