Over the past few months, China's state-run media has been reporting the existence of a 600-year-old book that is said to prove the country's claim to sovereignty over almost all of the South China Sea.

With an international tribunal ruling on the contested waters expected in the next few weeks, the book is seen as vital evidence for China's insistence that history is on its side.

Media reports describe it as a "cherished" possession, lovingly kept "wrapped in layers of paper".

Our Beijing correspondent John Sudworth went to track it down.