Everyone knows the story of Titanic, the ‘unsinkable’ ship that sank in 1912 with massive loss of life.

Less well known is the story of her sister ship, Britannic, just as big and even more ‘indestructible’ than Titanic, which went down in the Mediterranean four years later, sinking in just 55 minutes.

Britannic was being used as a World War One hospital ship at the time, and was en route to pick up soldiers wounded in battle.

Kate Humble talks to relatives of the survivors and pieces together Britannic’s terrible story, while Andy Torbet dives down to the wreck of the ship itself, a ragged giant on the sea floor.