The Britannic was commandeered as a hospital ship
Forgotten by many and unheard of by most. Yet the sister-ship of the Titanic is starting to escape from the shadow of the iconic shipwreck.
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.
Thirty people lost their lives when the Britannic went down
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.
"It's one way to understand the technical changes made to large ships after Titanic. It's also the only easily accessed example of the giant Edwardian leviathans that were being built at the time.
"We were able to film the three huge propellers on the ship and understand the sheer scale of these things. On Titanic they are all but inaccessible and are in a very dangerous part of the wreck. But on Britannic they are very visible."
The Britannic, like the Titanic, was built in Belfast
The Britannic lies on its starboard side, the side which took the impact of the explosion. So it's impossible to discover if it was a mine or torpedo from this evidence. At the time eyewitnesses claimed they saw two torpedo tracks heading towards the ship.
But after the war, the log of the German U boat U 73 was examined and revealed that it had laid 12 mines in the Kea Channel, just two miles from where the Britannic lies. Mike's documentary shows the seabed at the location where the mines were supposed to be and reveals what they found there.
Unlike its famous sister, Britannic made few headlines. Sadly, the 30 deaths were negligible compared to the huge death toll of the First World War. It has just been forgotten. But modern technology and the renewed interest in the Titanic and Belfast shipbuilding has also renewed interest in the Britannic.
Built with a double skin, taller internal bulkheads to prevent flooding and lots more lifeboats than the Titanic, the Britannic should have remained afloat and been a lot safer. But in fact it sank three times quicker than the Titanic. The probable reason is that special watertight doors were left open in the panic to abandon ship.
Finding out more about the ship and what led to its downfall has become the passion of its owner, Simon Mills. He bought the wreck in 1996 and has set about protecting it while still allowing divers access to the ship.
The Britannic lies off the coast of Greece
"My interest was more the historical conservation," explained Simon. "Diving has become a part of it. I have to work with divers to get what I require as much as they have to work with me to get what they require.
"I bought the ship for marine conservation. It's also the conserving of the artefacts. We want to create some sort of unique attraction around the world. It will combine science, history archaeology.
"I want to see Britannic conserved for future generations and if I can start the process rolling, that's brilliant."
Plans are already well advanced to start regular submarine visits to the ship for tourists. And for many, it is as close as they will ever get to one of the Titanic family. It takes just 10 minutes to reach the Britannic by mini-sub.
"That's much quicker than the two-and-a-half hours required to dive to Titanic," said Mike, who has made both dives. "After being lost for more than half a century, the Britannic could soon be the biggest must-see attraction in the Mediterranean. It's a very spectacular wreck and a very special one."
Already Greek biologists are fascinated by the wreck. It has become a "living" artificial reef in what is a very empty part of the Aegean Sea. Taken over by a wide variety of marine life, it is turning into a natural laboratory for scientists who want to learn more about how such reefs might benefit the marine biodiversity of the sea-bed.
"In the Shadow of Titanic" can be seen on BBC One Northern Ireland, Sunday 23 November at 1625 GMT.