Exploring the Britannic wreck, Titanic's sister ship

Sunk by a German mine in November 1916, the Britannic was the largest of the three Olympic class luxury liners built by the White Star Line at Belfast's Harland & Wolff shipyards. She was commissioned as a transatlantic passenger liner and underwent crucial design changes after the disastrous sinking of the Titanic in 1912 and the subsequent accident inquiries. These safety alterations included the raising of some watertight bulkheads to B deck, introducing a double hull at the engine room and boiler room levels and changing the design of the lifeboat launch system. The number of lifeboats was also increased.

Launched in 1914 just before the outbreak of the First World War, the vast four-funnelled ship was repainted in hospital colours - white with a green stripe and prominent red crosses on her side - when she was refitted. Britannic entered service in December 1915 under the command of Captain Charles Bartlett. She had cost over £1.9m and was the largest ship in the world in active service. Her early deployments involved the evacuation of wounded men during the ill-fated Gallipoli campaign in the Eastern Mediterranean, and her work continued as casualties on the Macedonian front mounted rapidly.

On her sixth and final voyage on 21 November 1916, with 1,065 people on board, the ship was transiting a narrow strait south of the Greek port of Piraeus, near the island of Kea. At 8.12am a large explosion was heard and initial reports suggested the cause was either a mine or a torpedo. In October, a German U-boat U-73 had laid mines in the area, but the German Navy claimed the sinking as a torpedo hit. It took until the 1990s to confirm definitively that the ship was sunk by a mine. Although damage was extensive, only six of the watertight compartments flooded and the ship remained afloat, but as it listed, water began to enter open portholes on the starboard side.

In an attempt to beach the ship off Kea, Capt. Charles Bartlett ordered full speed ahead. The movement, however, caused more water to enter, and Bartlett quickly ordered the engines stopped. At approximately 9.07am the Britannic sank. Breached in the bow section just forward of the bridge, she went down far faster than the Titanic, in just 55 minutes, but with much less loss of life. Thirty people died when the Britannic sank. Those critical design changes helped, as she was equipped with lifeboats for 3,500 people, her maximum load on return from the front with casualties. Much warmer waters would have helped the survival rate too. The grand liner now sits on the seabed, 120m below the surface of the Aegean Sea.

The underwater explorer Jacques Cousteau rediscovered Britannic in 1975. The French team was joined by MIT's Dr Harold Edgerton, whose recently-developed side-scan sonar helped to locate the wreck. Cousteau later explored Britannic using a submersible named Denise, recovering the ship’s engraved sextant. The ensuing documentary included a visit to her former workplace by 86-year-old Mrs Sheila Macbeth, who had been 26 and a serving nurse on the ship at the time of her sinking.

Visiting the Britannic

British filmmaker and maritime historian Simon Mills has owned the UK government's legal title to the wreck since 1996. Mills has been visiting the Britannic since 1995, when he accompanied the marine explorer Bob Ballard to her final resting place. That first visit took place in a US Navy nuclear submarine. "It's a far better preserved example of the Olympic class liner than the Titanic," says the author of several books about the sister ships."She lies in 120m of water and is relatively intact apart from structural damage at the bow section".

"The sheer length of the 50,000-tonne ship meant that her bow hit the bottom while her stern was still above the surface and the huge pressure further cracked the bow like an eggshell in the area of the mine strike. The impact buried part of the bow section under the seabed." Mills added that "judging from the imagery shot over that time, we've all aged more than the Britannic", explaining that the curse of the Titanic - iron-eating bacteria - is much less prevalent on the Britannic, probably because she is in much warmer, more oxygenated, shallower water and is covered by a more diverse ecosystem of organisms that compete with the destructive rust-feeders.

The 25m support vessel U-Boat Navigator that the team operates in the Kea channel above the wreck is equipped with two Triton manned submersibles: one three-man vessel and a one-man sub. Dmitri Tomashov is one of the sub pilots. He has been visiting and filming the wreck for a documentary series since 2013 and has logged 65 hours on the Britannic since then. "Our main goal is filming and surveying the whole ship, so both subs are equipped with 6K Red Dragon cameras and powerful LED lighting, the secret to high quality underwater filming at depth," he explains.

The addition of the second Triton this year is an added element of safety and each Triton can film the other working, or they can shoot the same subject simultaneously from two different angles. Another advantage of these Tritons, which are depth-rated to 1,000m, is the viewing sphere made of optical glass allowing filming at desirable angles without distorting the image.

"There is nothing else out there right now that can outperform the Tritons," says Tomashov. "I can be in the water for seven or eight hours at a time, though there is a limit because concentration has to be absolute and over time you do get pretty exhausted, even though comfort for both the pilot and the passengers is at a high level in these machines." The U-Boat Navigator is also equipped with an Ageotec Perseo remotely operated vehicle (ROV) and the one-man Triton can deploy a tiny ROV of its own for close inspection of particular areas that the subs cannot approach.

Her depth at 120m places the wreck just inside the depth limits for exploration by human divers. "Britannic lies in that sweet spot where we can use technical divers, ROVs and manned submersibles to explore the exterior of the ship. The 2016 expedition has pretty much completed what we need in terms of exterior surveys. We've done as much imaging as we reasonably can," says Mills. The next phase will be penetration of the wreck to identify, retrieve and conserve selected artefacts. However, that is now in the hands of the diplomats from the UK Foreign Office and the Greek Ministry of Culture, as no historic artefacts may be removed from Greek territory without the permission of the Minister of Culture.

Deep-diving dangers

Marine exploration technology has massively improved since the mid-1990s, especially on the technical diving front. The advances have been in the development of closed-circuit rebreather (CCR) technology where the diver’s exhaled air is first chemically scrubbed of carbon dioxide, before being topped up with a small amount of additional oxygen.

A CCR operates in much the same way as an astronaut’s backpack and uses the same technology as the life-support systems of the submarines. As the name suggests, they emit virtually no excess gas. This is important for the next stage of exploration in the wreck. Bubbles or pools of exhaled gases would disrupt the delicate marine ecology of the interior, speeding up the degradation of fragile wood panelling, for example.

The depth is near the limit for even experienced technical divers and their time at the bottom is limited to 40 minutes or so. Even that short dive time on a gas mix of helium, oxygen and nitrogen requires a slow decompression of over five hours to reach the surface safely. Oxygen levels are reduced in the mix breathed at depth to prevent the very real dangers of oxygen toxicity - the gas can trigger convulsions and deaths at increased pressures at depth.

Nitrogen levels are also reduced relative to normal air and replaced with helium to avoid the dangers of nitrogen narcosis, a "rapture of the deep" that impairs judgement and incurs penalties in the divers' bodies as nitrogen bubbles form in the tissues. These dangers are reduced by a long, slow ascent to the surface, stopping at precise depths and changing gas mixes until the divers are breathing air in the shallows.

Evan Kovacs is a technical diver and director of underwater photography at the Advanced Imaging Lab, Woods Hole Oceanographic Institution in Massachusetts. He has dived on the Britannic many times over four expeditions in 2006, 2009, 2015 and 2016. Kovacs explored the Titanic in 2005 with the History Channel and opted to visit the Britannic in 2006 to see if they could learn something about the Titanic by looking at the differences in construction and the changes made to her sister ship.

"Britannic is a magnificent ship, beautiful to look at and massive in all ways. Unlike the Titanic she is almost intact; she is one of my favourite long-term projects," he says. "This year we were using the U-Boat Navigator as a support vessel. It's the best in the world in my view, just brilliantly designed and perfectly equipped for this kind of work."

One of the big advantages for the divers and image makers in 2016 was the presence of a wet bell. This is a platform with a breathable air bubble supplied with hot water and communications from the surface. "We can pump down unlimited gases to a diver in distress, which is a huge safety bonus. Plus it's easy to keep hydrated and to be able to eat and listen to our iPods during the long hours of decompression. The only thing missing is a cup of hot tea," he jokes.

To get around this huge wreck in the limited time - only 45 minutes - that the divers have at depth, they use underwater scooters. Even with rigorous safety measures in place, the sea is still unpredictable, especially at these depths. Britannic claimed the life of world-renowned technical diver Carl Spencer in 2013 and Kovacs says that it's not uncommon to hear the explosions of illegal fishermen nearby.

Kovacs' holy grail for this wreck is a rivet-accurate blended acoustic and optical model of the entire exterior yielding a hugely accurate picture of the bulkheads. What this means in practice is a 3D volumetric model. The acoustic images have been taken with multibeam and side-scan sonar from the Perseo ROV and these will be overlaid with the optical results from the divers.

On land, this is not such a difficult proposition, but underwater there is a classic mosaicking problem. It is impossible to go super-wide to gain the entire perspective - underwater visibility prevents that on a structure of this scale. The team must first build an acoustic model to ground-truth the optics. Currents, camera lenses and pitch and yaw of the diver’s attitude all introduce errors where it is important to run a straight line. "If we can do this and in real time, then this has huge implications not just for marine archaeology, but also the oil and gas industries, where the results from visual pipeline inspection robots could be overlaid on an acoustic examination for faults and leaks," says Kovacs.

While owner Simon Mills has been offered the opportunity to dive to the wreck, he has declined "I am a recreational diver," he says. "What these technical guys do is a different ballgame. The next phase is to enter the wreck with the permission of the Greek government and retrieve and conserve artefacts by deploying small ROVs and manned dives. I prefer to remain on the surface and communicate with the divers, who have the dexterity and ability to weigh up the hazards that no robot can ever have, while ROVs can continue to work safely after the divers have exceeded their safe bottom times. I hope that we will be back in 2017."