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As a spectacular sunrise lights up the Sea of Marmara, it's hard to imagine this region as the stage of one of the fiercest naval battles in history. Beneath these glassy waters, from here to the Mediterranean, lie the carcasses of hundreds of war ships - casualties of the Gallipoli Campaign. But today, relations between our Turkish and Australian crew couldn't be better. Their bearings are set on a common goal - to tell the story of a forgotten Australian submarine, the AE2.

Peter Briggs

It is the silent ANZAC. It's the part of ANZAC that people don't know about. It's the part of the ANZAC story that needs to be told.

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But the job ahead - to image the interior of the AE2 and protect the wreck from corrosion - is a huge engineering and scientific challenge.

Tim Smith

We've got a steel shipwreck in a deep marine environment, in an inland sea of another country. So, the logistics are quite enormous.

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And this high-tech deep-sea adventure will prove something of a battle itself, as the AE2 seems determined to hold onto her secrets.

Dr Roger Neill

I'd say frustration, extreme frustration. Yet again, we're so near and yet so far.

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Since 1915, the AE2 has lain right here - 73 metres below us on a dark, silty sea floor. The submarine wreck is at the end of a long, narrow and treacherous strait called The Dardanelles. It's the gateway to Istanbul, formerly Constantinople. Seizing this city was key to breaking the central power block in WWI, but the Allied attempt to do so by sea was a disaster.

Murat Icel

The greatest armada the Earth has ever seen, with high-tech, state-of-the-art machines - canons, submarines, blah, blah - they thought they could just penetrate through The Dardanelles very shortly.

Anja Taylor

Didn't happen.

Murat Icel

No, unfortunately not.

Anja Taylor

Murat Icel's grandfather was one of the defenders at Gallipoli.

Murat Icel

The whole area was very heavily fortified, and it was completely full with mines.

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They devastated the attacking fleet.

Anja Taylor

And so that's what lead to the Gallipoli landing.

Murat Icel

Exactly.

Peter Briggs

Well, AE2 is really the maritime dimension of Gallipoli. We have the brave efforts to scale the cliffs and take the heights, but whilst that was happening, AE2, they were told to create a diversion and they did.

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Under the cover of darkness, on the 25th of April, 1915, just before Anzacs landed on the beaches, Captain Henry Stoker entered the minefield Dardanelles on what many considered was a suicide mission.

Peter Briggs

I mean, one of the submarine's principle attributes is its stealth. He had to expose himself and draw attention to himself, and that's what he did, very successfully and extremely bravely.

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Instead of shelling troops on the peninsula, the entire Ottoman fleet began hunting the AE2. But, miraculously, the submarine made it through.

Peter Briggs

The message that went back was - 'The Australian submarine has got up through the narrows, torpedoed a ship, I'm sure tomorrow will bring you better luck. Dig, dig, dig until you're safe.' And so, this 700-tonne submarine with 32 people in it had a strategic impact.

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The AE2 was eventually hit, and the crew captured as prisoners of war. Captain Henry Stoker, the last man out, scuttled the submarine.

Anja Taylor

She's intact and accessible, her contents undisturbed for nearly 100 years. She's the perfect time capsule from World War I.

Tim Smith

That history beneath the waves is really untold. All around the world at the moment it's becoming a real focus - trying to capture these sights, learn from them and, I guess, unravel the history books.

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Many of the men here have travelled thousands of kilometres and spent many years preparing for the chance to look inside.

Tim Smith

There's over 50 people on this expedition, with all the boat crews, the diving teams, the scientists, maritime archaeologists like myself, historians, naval officers, all the different disciplines.

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But, due to the depths involved, no-one can go inside the sub, and divers only get 20 minutes bottom time to complete each task. Former submarine commander Peter Briggs will need to utilise all his skills to get the mission accomplished.

Peter Briggs

We're limited to three dives a day, and that's really a fundamental restriction. And if we encounter unexpected difficulties we'll have to make some priority calls on what we can do.

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All the visual surveys must be done remotely - by a series of specially-adapted vehicles and cameras, and the first is sent off to sight the wreck.

Peter Briggs

It was really quite a moment for me when we put the remote vehicle off on that bearing. 20 metres, 30 metres, and there was the submarine.

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The AE2 is in remarkably good shape, besides some trawler damage to the casing. The conning tower hatch, left partially open when the crew escaped, is where all the action will take place. The wreck is covered in marine growth, with not a bare patch of metal to be seen.

Dr Ian MacLeod

You've got some soft corals, some hard corals, a whole range of oysters, molluscs and other things.

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Watching from back in Perth, Ian MacLeod - the expedition's rust doctor - is thrilled.

Dr Ian MacLeod

It's not particularly exciting for most people to look at, but for me it says, 'I've wrapped myself in this tight-fitting cocoon, and I'm sitting here and I'm just corroding gently, going, "Tick, tick, tick, tick."'

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Time is also ticking away on-board, as winds slow the installation of the diver support platform. But finally, the divers are suiting up for their first descent.

Murat Icel

Normally, a deep dive is much more complicated diving than recreational diving. And the deal is they cannot use normal air. Normal air is like 79% nitrogen, and under pressure it creates a narcotic effect, so once you're down there, especially at 68 metres, you'll be, like, drinking eight martinis on top of each other. So, you have no idea what you're doing.

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To avoid feeling drunk on nitrogen, divers will breathe a mixture of helium and oxygen fed through an umbilical cord from the main ship.

Murat Icel

Which gives them water, power for their camera and also power for their lights. But they also have a bail-out tank in case the surface supply gets disrupted.

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And now it's time to see if all the years of preparation will pay off. The divers bell is like an elevator to take divers to the bottom, safely, in just a few minutes. As their air mix is switched to heliox, the Turkish divers begin to speak like ducks.

Anja Taylor

One of the most fascinating things about this region to war buffs, scientists and divers alike, is its oceanography. Over there is the Black Sea, which drains into this basin, but from behind me comes salty water from the Mediterranean.

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And you can see the currents change down below. The green layer, from the Black Sea, sits on the top. It's full of jellyfish, and much less salty than normal sea water. As the bell reaches the opposing current from the Mediterranean, the divers appear to go out of focus in the turbulence. Once through the eddies, everything sharpens. You can see the bluish change in colour, and the water nearly doubles in saltiness.

Dr Ian MacLeod

So, that means the density is much higher, the amount of minerals that it contains is higher, and also, when you're diving in a submarine, if you hit that halocline at the wrong level you'll bounce up like a billiard ball hitting a cushion.

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It's a big clue to the AE2's demise. The vessel lost control in a dive just before it was hit. But today, this salty, dense current is key to the AE2's survival.

Dr Ian MacLeod

The saltier the water gets, the less capacity it has to hold oxygen.

Tim Smith

And that's a really good thing for the preservation of shipwrecks - the less oxygen, the less corrosive environment.

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There are hopes for seeing a well-preserved interior, but not on this dive. It takes the rest of the day just to clean the debris from around the hatch.

Anja Taylor

On the next dive the first camera goes inside the submarine. This is a drop camera, and it's been specially designed to fit through the 100 milllimetre gap in the conning tower hatch.

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The idea is to start the survey with the least disturbance, so this small unit will take baseline water measurements and vision before the hatch is opened. But, as we're soon to discover, after all the trials and months of design work, the camera doesn't fit.

Dr Roger Neill

The opening of the hatch was 12cm. We allowed 2cm - so, we designed everything around a 10cm opening. When we went to fit the camera, which is also 10cm, it just barely didn't make it. So, somewhere we have to gain maybe 5mm, half a centimetre, then we'll be able to get the camera inside.

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The next couple of dives finds a solution. They grind away latches that were keeping the hatch down. These were the latches Captain Stoker secured as he scuttled the AE2, allowing her steady descent to the seabed. The drop camera goes in, and we finally get a glimpse of the interior.

Tim Smith

You can see the ladders quite clearly, the guages on the side of the submarine's walls. You can see the trunking, the piping. Some of them we know from the plans, but others, a whole lot of detail we've never seen before. This first examination is just to get an idea of the water quality and how we're going to drive the vehicles at the next stage. It's tantalising. Now we've got to wait a few more hours till the next dive team can go down.

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Each time divers resurface there's a mad rush to get them into the recompression chamber. But in this instance, everything is not OK.

Anja Taylor

One of the divers has just suffered the bends, and he's been put into the therapeutic recompression chamber. He's been taken down to a pressure of 60ft, and he's breathing pure oxygen.

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This will help get rid of the expanded bubbles of gas in his bloodstream. It's only mild case, and Boris recovers, but it's going to be several days before he's cleared to dive. The robotics team have made some final adjustments to the star submersible, and it's ready to go. It's got specially moulded corners to fit in the confines of the hatch, multiple adjustable LEDs to help reduce backscatter, and sonar to help build a clear picture of the interior in 3D. As the hatch is jacked open down below, the ROV is coupled to a huge insertion pole to guide it through the conning tower.

Tim Smith

We should be able to light the control room up as it was in 1915.

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As the ROV gets to the first hatch it squeezed through with some effort. But once inside the conning tower we see beautiful clear images. There's a timber flag locker not shown on any plans, which houses a mix of ensigns and signal flags.

Tim Smith

Look at the detail there.

Anja Taylor

Wow, that's incredible.

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This wheel is mounted in the steering position of the conning tower. There's beautiful old light fittings... and it's thought this could be the wire that sent the message of the AE2's successful penetration of The Dardanelles. But it's all just a big tease, as the ROV reaches the lower conning tower hatch and gets stuck. As they try to force it through on the next dive it becomes completely jammed. They can't get it in or out of the submarine.

Dr Roger Neill

The vehicle just went in at the wrong orientation and failed on perhaps something that's not shown in the plans. So, it's extremely frustrating.

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To get the ROV unjammed, it's down to a boat hook, some electrical tape and quick innovation.

Peter Briggs

It's hard to say where it's caught.

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Precious dive time is ticking away.

Dr Roger Neill

If we don't get this vehicle out then we're starting to run up against the end of the mission.

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And sometimes there are just too many good ideas.

Peter Briggs

I've heard all the arguments. If someone needs to make a decision, here it is. Take that rig out, use what time we have to work on the cathodic protection. Tomorrow we'll come back. The best you guys can do, get the little ROV in.

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After a day of frustrations, the crew return to base at the tiny fishing village of Sarkoy. And tensions begin to ease. A debrief in the hotel gets priorities sorted for the next day. And the next day turns out to be a winner. When we arrive at the ship, huge one-tonne zinc arrays are being craned into the sea. These will be the submarine's new arsenal against rust.

Dr Ian MacLeod

Zinc is a more reactive metal. Zinc will give off its electrons must faster and more readily than iron.

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Down at the wreck, divers blast away marine growth to reveal a bare patch of the iron hull - where the zinc will be attached.

Dr Ian MacLeod

Insulated copper cables take the current from the anode pod onto the submarine. So, the electrons, instead of being wasted in the open ocean, take the easy path.

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Once electrically connected, zinc begins to corrode in place of the iron. Zinc electrons flow into the iron hull and tunnel through the marine growth to the waiting dissolved oxygen in the sea water. The iron electrons stay where they are, preserving the ship. And there's an added bonus - the water around the submarine gradually changes from acid to alkaline as negatively charged chloride ions from the salt are repelled. Within days, corrosion rates drop to around half of what they were.

Dr Ian MacLeod

The absolutely brilliant thing was that the whole submarine is acting as one continuous piece of metal. So, that means they will begin treating everything that is bolted to it. It will completely stop decay.

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Ian's insight into corrosion also reveals why the ROV team had such troubles getting inside. The hatch and the conning tower are made of bronze, which doesn't corrode as readily as iron.

Dr Ian MacLeod

As the iron corroded it protected the hatch, and that made the hatch more alkaline. So, that makes it much easier for the marine organisms to grow, and that's why the conning tower lid had this massive amount of marine growth on it - it was about five or six times thicker.

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But those dramas are now behind us, as the ROV has been unjammed and retrieved. Hopes are now pinned on a much smaller ROV - minus the sonar, with a standard definition camera. And I'm happy to say my high definition camera is being strapped to the package to become part of WWI history. And off it goes. The tension in the control room is palpable. The little ROV passes through the first hatch, the second... and drops its weight. And we're in.

Dr Roger Neill

After a lot of very painful adventures we've finally got the heavily-modified, remotely-operated vehicle into the submarine. We're seeing unbelievable footage. We've got dials that you can read the numbers on, the auxiliary power panel, which the plans just show totally unlabelled. We can see things like the ship's speed, all the light switches.

Peter Briggs

Absolutely spectacular moment. So, you're able to look around and see the control room 99 years after Stoker left it.

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Dr Neill has spent years developing a 3D model of the submarine, which is now being used to identify each object, and help visualise how it fits in the space, and whether it differs from the plans.

Dr Roger Neill

We've already learnt a huge amount. It's just incredible.

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The ROV makes it right to the front of the boat, but a snowstorm of decaying materials and drops of oil from a weeping fuel tank make it hard to see anything at all. But there are a few lovely personal touches in the ward room - what looks like boot polish on the EXO's bunk where some bedding still remains.

Tim Smith

A really fantastic space. This is the stuff we really wanted to see. In 1915, all that drama unfolded, this is like a stage we're watching here. You can almost see the men here working. It's just remarkable. Remarkable to see this.

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As if to salute the team's successful mission, a submarine glides by the expedition ship. With the AE2 successfully protected and a crate-load of data to analyse and add to the museum collections, it's a happy crew that celebrates their final dinner on the Sarkoy pier. And out in the Sea of Marmara, Australia's Gallipoli submarine lies alone again... but no longer forgotten.