The story of the biggest man-made explosion before to the atomic bomb, when a munitions ship and, ironically, a refugee relief vessel collided and Halifax, Nova Scotia , was devastated. Two thousand people were killed, many thousands were injured and the world was changed: this initiated the first global response to a disaster and the academic study of catastrophe. Sean Street visits the scene of the disaster and, 90 years on, meets Some Of the survivors. Producer Julian May