HMHS Britannic was completed at Belfast's Harland and Wolff shipyard two years after Titanic was lost in 1912. But she in turn went to the bottom, the victim of enemy action in the First World War.

BBC News correspondent Mike McKimm joined a Greek scientific expedition and dived to the Britannic to bring back dramatic footage of one of the world's biggest wrecks.

And the expedition also set out to try to discover what sank the vessel. Was it a torpedo or a mine?

The Britannic was bigger, better and safer than its ill-fated sister. But before it could carry a single fare-paying passenger across the Atlantic it was commandeered as a hospital ship.

And so, in its white livery with huge red crosses, His Majesty's Hospital Ship Britannic plied to and forth from Southampton to the eastern Mediterranean, bringing home thousands of wounded service men.

Just after breakfast, on 21 November 1916, off Greece, there was an explosion near the bow and within 55 minutes the ship had sunk.

Thirty lives were lost when two lifeboats were smashed by a still-rotating propeller. It was wartime and the loss of the ship was soon forgotten - until Frenchman Jacques Cousteau found it in 1975. It lay just over two miles off the island of Kea in just over 100 metres of water.

Using a mini-sub belonging to the Hellenic Centre for Marine Research, Mike McKimm was able to dive and film much of the ship for a BBC documentary. The pictures will also form part of a special website which already offers considerable underwater footage of the Titanic, also filmed by Mike.

"The Britannic is an important ship for historians," said Mike.