Originally broadcast by the BBC, this documentary investigates what caused ill-fated Russian submarine the Kursk to sink in the Barents Sea in August 2000. One hundred and eighteen lives were lost. It was a tragedy which shocked the world. But to many the tragedy remains incomprehensible, for the Kursk had been built to be unsinkable. How could this submarine have foundered? Scientists have pieced together evidence to disprove theories that the disaster occurred after a deep-sea collision with an American vessel, and believe the answer to the riddle can be found in the tragedy that befell a British submarine 50 years earlier.