If the ultimate terror attack were to happen--a nuclear explosion in a city--the resulting death and destruction would be almost unimaginable. But we can imagine the geopolitical consequences. Can we do anything now that would help matters? Michael May, Jay Davis and Raymond Jeanloz argue that we can, by establishing a databank of known nuclear explosive materials. Using this resource it would be possible to establish where the materials came from and who was responsible for the terrorist act. Not just for purposes of retribution, but in order to assess the chances of further nuclear detonations.