Titanic's Tragic Twin: The Britannic Disaster

With the outbreak of WW1 Titanic's twin sister, the Britannic, was transformed from the world's largest ocean liner to the greatest hospital ship in the world - and yet she is virtually unknown today. Despite being bigger and better built she sank and has remained a mystery for 100 years. Presenters Kate Humble and Andy Torbet go in search of what caused the demise of the Britannic.