THE BIGGEST CONSTRUCTION PROJECTS OF WORLD WAR II.

ORDERED BY HITLER TO SECURE WORLD DOMINATION.

NOW THEY SURVIVE AS DARK REMINDERS OF THE FUHRER'S FANATICAL MILITARY AMBITION.

THESE ARE THE SECRETS OF THE NAZI MEGA WEAPONS.

Max!

THE ATLANTIC OCEAN.

Torpedo 25 knots.

1940.

U-BOAT CAPTAIN GUNTHER PRIEN IS ON HIS 7TH PATROL, LEADING A WOLFPACK OF NAZI SUBMARINES INTO BATTLE.

Range 500.

Range 500.

THEIR MISSION?

TO DESTROY ALLIED MERCHANT SHIPS CARRYING VITAL SUPPLIES TO BRITAIN.

Keep it steady!

THE OUTCOME OF THE WAR DEPENDS ON WHETHER THESE CONVOYS GET THROUGH.

IF THE U-BOATS CAN STOP THEM, THEN THE NAZIS COULD WIN.

- Fire.

- Fire.

[Shouts in German] SINKING 730 ALLIED SHIPS IN THE FIRST TWO YEARS OF WAR, PRIEN AND THE WOLFPACKS ARE A DEADLY UNSEEN THREAT BELOW THE WAVES.

AND WHEN THEY SURFACE BACK AT BASE, THEY TURN FROM UNSEEN TO INDESTRUCTIBLE... BECAUSE HOME IS ONE OF THE GREATEST FORTRESSES EVER BUILT.

THE NAZI SUBMARINE BASE AT LORIENT WAS ONE OF THE MOST FORMIDABLE DEFENSIVE STRUCTURES ON THE PLANET.

NAVAL HISTORIAN AND AUTHOR DR. GLYN PRYSOR IS EXPLORING THAT BASE TO UNCOVER ITS INCREDIBLE SECRETS.

This whole site covers more than 50 acres.

At the time it was incredibly ambitious.

It was one of the biggest construction projects ever attempted anywhere in the world, second only perhaps to the Hoover Dam in America, and I think it perfectly illustrates the ambitions of the Nazi regime to dominate both at land and at sea.

THE TRANSFORMATION OF THIS PEACEFUL FRENCH FISHING PORT INTO A HUGE NAZI SUBMARINE BASE WAS THE BRAINCHILD OF ONE MAN.

JUNE 22ND 1940.

HITLER'S FORCES HAVE RAMPAGED THROUGH EUROPE.

AND NOW THE SURRENDER OF FRANCE COMPLETES NAZI DOMINATION OF THE CONTINENT.

BUT 300 MILES AWAY ONE GERMAN COMMANDER IS FOCUSED ON HIS OWN MISSION.

ADMIRAL KARL DOENITZ IS THE COMMANDER OF THE GERMAN NAVY'S U-BOAT FORCE.

CONVINCED THAT U-BOATS CAN WIN THE WAR IN THE WEST, HE IS PERSONALLY SEARCHING THE NEWLY CONQUERED FRENCH COAST FOR SOMEWHERE TO BASE HIS SUBMARINES.

At this stage of the war, Britain was standing alone against Germany.

And of course Britain's weakness, its Achilles heel, was the fact that it needed to import raw materials for industrial production, food to feed its population, so cutting off shipping, blockading the whole of Britain, would bring Britain to its knees.

JOE MAIOLO IS A PROFESSOR OF WAR STUDIES AND A LEADING EXPERT ON NAVAL WARFARE.

After the Germans defeated the French army and occupied France, they now controlled a coastline stretching all the way from Norway to the Bay of Biscay.

Instead of having to sail all the way from bases in Germany, now Doenitz had bases in France.

The bases in France gave the U-Boats ready access, safer access to the Atlantic.

WITH DIRECT ACCESS TO THE OCEAN, DOENITZ BELIEVES HE HAS THE CHANCE TO DESTROY BRITAIN'S NAVAL SUPPLY LINES.

HE QUICKLY SELECTS THE PERFECT SITES TO BUILD FIVE NEW BASES IN FRANCE.

BUT THE GREATEST OF THEM ALL IS LORIENT, THE BRITTANY FISHING PORT WHICH DOENITZ NAMES AS HIS STRATEGIC COMMAND POST.

DOENITZ WOULD OVERSEE THE BASE'S DEVELOPMENT FROM A CHATEAU OVERLOOKING THE PORT.

This is the inside of the Chateau Kernevel.

This is where Doenitz spent his time.

This place hasn't really changed much since the summer of 1940, when Doenitz moved here and set up his headquarters.

This is really the heart of the entire U-Boat operation.

The Chateau is right next to the sea, opposite Lorient base.

Doenitz would've stood here and watched the U-Boats coming back.

He was very close to his crews, and he really cared about them.

At that time, Doenitz only had a handful of U-Boats available to him.

And he knew just how precious they were to Germany's chances of success in the Atlantic.

BUT DOENITZ'S CHATEAU IS HOME TO A SECRET JUST AS VITAL TO NAZI SUCCESS AS THE U-BOATS THEMSELVES.

This is incredible.

This is the bunker that Doenitz ordered to be built beneath the Chateau to house his headquarters and base of operations.

ALONG WITH HIDDEN ACCOMMODATION IN THE GARDEN FOR THE 80 SOLDIERS GUARDING THE HEADQUARTERS, THESE BUNKERS ARE THE START OF LORIENT'S TRANSFORMATION.

The entire thing is around 10,000 square feet, made out of steel and reinforced concrete.

This would have been a hive of activity during the war.

There were 200 people at work down here.

This was really the first bunker that was built at Lorient.

And of course it was from here that Doenitz would have overseen the construction of the rest of the base.

DOENITZ WANTS TO TURN THE PORT INTO AN INDESTRUCTIBLE FORTRESS, SO THAT HE CAN PROTECT HIS PRECIOUS U-BOATS AND KEEP THEM OPERATIONAL.

THE CHALLENGE IS THAT, UNLIKE MODERN SUBMARINES, U-BOATS CONSTANTLY NEED TO RETURN TO BASE.

SOMETHING THAT BOB MEALINGS, CURATOR OF THE ROYAL NAVY SUBMARINE MUSEUM, KNOWS ALL ABOUT.

A modern nuclear submarine can go on patrol for months at a time because of the nature of its nuclear technology.

U-Boats were very different.

Using the combination of diesel engines, electric motors and batteries, they could only really go to sea for a matter of weeks, because, as the fuel ran out, they would have to return to base.

And these weren't very big craft, so there was a limited amount of fresh water and food that they could store on board.

So, that was almost just as important, the fact that they were running out of the kind of stores that would keep the crew fit and healthy and capable of actually fighting.

ONCE THEIR STOCKS OF FOOD AND TORPEDOES RUN OUT, U-BOAT CAPTAINS LIKE GUNTHER PRIEN MUST DOCK FOR FRESH SUPPLIES.

How did it go?

Good.

More torpedoes, and I would have given you some more prizes.

Ah, you'll have your chance.

BY SEPTEMBER 1940, PRIEN IS ALREADY A HERO IN GERMANY AFTER SINKING A BRITISH BATTLESHIP AT THE START OF THE WAR.

NOW, PRIEN AND HIS FELLOW COMMANDERS RECEIVE A FURTHER BOOST.

THE NEW BASE AT LORIENT OPENS FOR U-BOAT REPAIRS.

There's not a drop of fuel left, we were running on fumes!

And we need a full complement of torpedoes, including aft.

AS WELL AS BEING RESUPPLIED, EACH U-BOAT MUST BE CAREFULLY INSPECTED EVERY TIME IT RETURNS FROM PATROL.

Crew were trained to carry out as much maintenance when they were at sea as possible, but inevitably when these boats came back from patrol they'd often suffered quite considerably from operating in quite a hostile part of the world's oceans, and that's only if they weren't attacked.

If they'd been attacked, they could have incurred all sorts of damage.

A U-BOAT IS MADE OF TWO LAYERS, AN INNER PRESSURE HULL THAT KEEPS IT WATERTIGHT, AND AN OUTER HULL, CONTAINING THE BALLAST TANKS THAT ENABLE THE SUB TO DIVE.

ANY WEAKNESS IN THE STRUCTURE COULD PROVE CATASTROPHIC AT SEA, SO IT'S ESSENTIAL TO BRING THE HULL OUT OF THE WATER AND CHECK FOR DAMAGE.

BUT HOW DO YOU GET SOMETHING THAT WEIGHS OVER 1,000 TONS OUT OF THE WATER?

THE FIRST ANSWER FOR THE NAZIS IS TO USE LORIENT'S EXISTING FACILITIES.

This was a fishing harbor.

And just here there was a ramp with a cradle at the bottom and a winch at the top, which was used to pull fishing boats up out of the water and onto rails.

WHEN THE NAZIS TAKE OVER, THEY USE THE SAME SYSTEM FOR THEIR U-BOATS.

Back then, they used a giant turntable, which was here.

U-Boats would be brought up from the harbor and sat onto the cradle, which would then rotate and move the boat out to a siding for repairs.

HOWEVER, THERE'S A PROBLEM.

AT SEA, THE U-BOATS CAN HIDE UNDER WATER, BUT ON LAND, THE SUBS ARE INCREDIBLY VULNERABLE.

Being out here in the open wasn't ideal, because allied bombers began to target this base as early as the summer of 1940.

TO PROTECT DOENITZ'S PRECIOUS U-BOATS, THE BASE NEEDS TO EVOLVE.

SO THE NAZIS ENGINEER A QUICK FIX.

The U-Boats would be brought along these rails and into this building here.

52 FEET WIDE, 82 FEET HIGH, AND 275 FEET LONG, WITH WALLS OVER FIVE FEET THICK, THESE TWO STRUCTURES, BUILT IN JUST FOUR MONTHS, ARE THE FIRST COVERED REPAIR BUNKERS AT LORIENT.

This is actually the original trolley that U-Boats laid on.

It's been modified to take the weight of larger boats by building these extra struts along the side, but it's basically the same equipment.

And you can see just how important it was to be able to get at the hull to make it accessible for all those really crucial repairs.

WHAT MAKES THESE BUILDINGS VITAL IN THE EARLY DAYS, IS THE PROTECTION OFFERED BY THEIR ARCH LIKE CONSTRUCTION.

From up here you can quite clearly see the distinctive shape of the bunker with its curved roof and steep, sloped walls.

It's built this way with the intention that any bombs dropped from the air would simply bounce off the sides.

Without another name for it, the Germans began to call it a "Dombunker".

Literally "cathedral bunker" in German.

And it's easy to see why.

DESPITE THE FACT THAT THEY'RE STILL IN USE TODAY, THE DOMBUNKERS ARE A TEMPORARY FIX.

THEY'RE ONLY BIG ENOUGH TO HOLD SMALLER U-BOATS, AND NOT THE LARGE VESSELS THAT DOENITZ PLANS TO BUILD.

Good.

The plans for the bunker, please.

WHAT DOENITZ DREAMS OF IS A BASE MADE UP OF GIGANTIC SUBMARINE PENS ENCASED IN INDESTRUCTIBLE CONCRETE BUNKERS TO PROTECT AND SUPPLY HIS FLEET.

It looks excellent, right?

AND TO COMPLETELY SEVER THE LINK BETWEEN BRITAIN AND AMERICA HE NEEDS HUNDREDS MORE U-BOATS.

But... how many?

Enough?

Enough batteries?

Enough batteries?

Five?

Six?

Admiral Doenitz needed many more U-Boats.

He believed that if he had 300 U-Boats, with a third of the force at sea at any one time, he would actually be able to cut off Britain from its overseas supplies.

But at the start of the war, Germany, and specifically Admiral Doenitz, only had 22 ocean-going U-Boats to fight his campaign against Britain.

BUT DOENITZ'S BELIEF THAT U-BOATS HOLD THE KEY TO WINNING THE WAR IS NOT SHARED BY HITLER.

The way Hitler understood naval power, was as the battleship.

Because these were prestige vessels.

Vessels that projected the image of Nazi power abroad.

This was a man who used to sketch images of battleships in his spare time.

He thought naval power was big-surface ships with big guns, not tiny U-Boats under the surface.

TO CHANGE HITLER'S MIND, DOENITZ IS RELYING ON U-BOAT COMMANDERS LIKE GUNTHER PRIEN TO PROVE THAT HIS SUBMARINES ARE WAR WINNING WEAPONS.

AND ON 19TH OCTOBER 1940 PRIEN DOES JUST THAT.

No markings.

Looks like a British supply ship.

Alarm!

[Speaking German] Enemy speed 12.

[Speaks German] Angle of the bow, 60.

Keep it steady.

Torpedo, 25 knots.

Torpedo, 25 knots.

[Speaks German] Range, 500.

Range, 500.

[Speaks German] Tube 1 is ready.

Stand by.

- Fire!

- Fire!

[Shouts in German] [Speaks German] Torpedo running.

IN JUST 48 HOURS, PRIEN AND SIX OTHER U-BOATS ATTACKED TWO CONVOYS, SINKING 34 ALLIED SHIPS.

These were exactly sort of successes that Doenitz needed.

All that tonnage that went to the bottom was a ship lost and resources lost.

In other words, they were sapping Britain of its strength.

BUT WOULD THESE SUCCESSES BE ENOUGH TO GET DOENITZ WHAT HE WANTS?

ON THE 25 OCTOBER 1940, DOENITZ GOES TO SEE HITLER, DETERMINED TO CHANGE HIS MIND ABOUT U-BOATS.

But we have a weapon capable of dealing Britain a mortal blow at her most vulnerable spot.

The U-Boat war, however, can only be successfully waged if we have sufficient numbers available.

The minimum requisite total is 300 U-Boats.

300?

[Speaks German] You need 300 new U-Boats?

Now tell me about these concrete pens.

I am assured they will be completely bomb proof, my Fuhrer.

You'll have your U-Boats.

And the concrete pens.

I'll see to it.

You need take no further steps.

HITLER COULD NO LONGER IGNORE THE SUCCESS OF DOENITZ'S U-BOATS.

NOT LONG AFTER THE MEETING HE ORDERS AN INCREASE IN U-BOAT PRODUCTION AND CONSTRUCTION OF THE INDESTRUCTIBLE BASE.

A 50 ACRE SITE IS CHOSEN ON THE KEROMAN PENINSULA, OPPOSITE DOENITZ'S HEADQUARTERS.

DANIEL GUYADER HAS BEEN A GUIDE AT THE BASE FOR THE LAST 15 YEARS.

The Keroman base, the collection of bunkers, represents around a million cubic meters, in simple terms.

The Keroman base is the most important construction by the Nazis in all the occupied countries.

WORK BEGINS ON THE BASE WITH THE FIRST OF TWO BUNKERS, KEROMAN OR K1 IN FEBRUARY 1941.

THEIR CONSTRUCTION WILL EVENTUALLY CONSUME A QUARTER OF ALL THE CONCRETE USED BY THE NAZIS IN FRANCE AT A COST OF 2.6 BILLION DOLLARS IN TODAY'S MONEY.

BUT THE TECHNICAL CHALLENGES ARE IMMENSE.

AS STRUCTURAL ENGINEERING LECTURER MATT DE JONG KNOWS ONLY TOO WELL.

So, these buildings were built over 100 meters long and 100 meters wide, and 18 meters high.

It's just a huge amount of mass which a foundation needs to resist.

TO ERECT THE PENS AS QUICKLY AS POSSIBLE, THE NAZIS HAD DECIDE TO BUILD THEM ON DRY LAND.

BUT THERE'S A PROBLEM.

EXCAVATION OF THE FOUNDATIONS REVEALS THAT THE SOIL IS TOO UNSTABLE TO TAKE THE WEIGHT OF THESE MASSIVE STRUCTURES.

Engineers, I think, were quite worried about the building actually settling down and sinking into the ground, so they built the structure on pile foundations.

Basically, long steel beams inserted vertically... pounded vertically into the ground, and that gives you the ability to resist sinking of the building.

WITH THE FOUNDATIONS IN PLACE, BOATS, TRUCKS AND RAILWAY WAGONS BRING 167,000 TONS OF RAW MATERIAL INTO LORIENT EVERY MONTH TO FEED THE CONSTRUCTION.

ON TOP OF THAT, THOUSANDS OF WORKERS FROM ALL OVER THE NAZI EMPIRE ARE DRAFTED INTO LORIENT TO BUILD THE PENS.

ALONGSIDE THEM, A HANDFUL OF SKILLED FRENCH ENGINEERS OCCUPY SENIOR POSITIONS.

ONE OF THEM, THE DEPUTY DIRECTOR OF NAVAL CONSTRUCTION, IS CALLED JACQUES STOSSKOPF.

I need the estimates from the steelyard for the tonnage.

Stosskopf was one of a number of highly skilled French engineers who worked here at the base.

His main role was at the arsenal, the storage facility for torpedoes, which was a little way away from the pens themselves.

You can provide before the end of the month?

End of the month.

Is that all, sir?

No.

Excuse me.

What made Stosskopf different was that he was from Alsace, the border region between France and Germany, and that meant that he spoke excellent German.

That made him particularly useful to the German hierarchy.

When you all have the figures, let me have your reports.

Gradually his responsibilities increased, and he supervised French workers all across the base.

He wasn't particularly well-liked by his compatriots, partly because he was a rather serious character, but also because of his close associations with the Germans.

But men like Stosskopf were essential to the Germans in keeping the construction and the operation of this base on track.

AS CONSTRUCTION PROGRESSES, U-BOATS CONTINUED TO SAIL FROM LORIENT.

IT RAPIDLY BECOMES THE MOST IMPORTANT BASE IN FRANCE, KNOWN AS THE 'BASE OF ACES', AS ALL THE U-BOAT TOP GUNS DOCK HERE.

MEN LIKE GUNTHER PRIEN.

HAVING ALREADY SUNK 27 ALLIED SHIPS, AT THE END OF FEBRUARY 1941, HE SAILS OUT OF LORIENT ON A NEW MISSION.

Keep it steady!

HUNTING A CONVOY OF 37 SHIPS, PRIEN AND THREE OTHER BOATS ATTACK.

- Fire!

- Fire!

[Shouts in German] BUT THEY'RE SPOTTED BY ALLIED DESTROYERS AND FORCED TO DIVE FOR SAFETY.

It's coming straight at us!

Dive!

90 meters!

Alarm!

[Indistinct shouting] PRIEN'S CREW RUSH TO THE FRONT OF THE U-BOAT TO SPEED THE SUB'S DIVE.

REMAINING TOTALLY SILENT IS VITAL AS THE DESTROYER'S SONAR LISTENS FOR ANY NOISE FROM BELOW IN THE SEARCH FOR THE U-BOATS.

[Screams] ONCE DETECTED, THE ALLIED ESCORTS UNLEASH HUNDREDS OF DEPTH CHARGES.

No survivors?

PRIEN'S DEATH HITS DOENITZ HARD.

IT CONFIRMS HOW VULNERABLE HIS SUBS ARE, AND HOW VITAL IT IS TO COMPLETE THE NEW PENS.

AS CONSTRUCTION STEPS UP, SO DOES THE INFLUENCE OF FRENCHMAN JACQUES STOSSKOPF.

He was a rather serious character.

And his close associations with the Germans meant that he developed a reputation as a collaborator.

On one occasion, he was responsible for organizing 250 Frenchmen to be sent to Germany to work.

As the train left the station, some people in the crowd were heard to shout, "Death to Stosskopf!"

MEANWHILE, THE SUBMARINE BUNKERS K1 AND K2 CONTINUE TO RISE AT A PHENOMENAL RATE.

BY 20 DECEMBER 1941, AFTER JUST 10 MONTHS OF WORK, THE PENS ARE COMPLETED, AND NOT A MOMENT TOO SOON.

JUST NINE DAYS EARLIER, HITLER HAD DECLARED WAR ON THE UNITED STATES.

DESPITE FACING TWO OF THE MOST POWERFUL NAVIES ON EARTH, NAZI U-BOATS CONTINUE CAUSING HAVOC IN THE ATLANTIC, SINKING 27 SHIPS IN DECEMBER ALONE.

THE U-BOATS ARE USING UP THEIR STOCKS OF TORPEDOES ALMOST AS FAST AS THEY CAN REPLENISH THEM.

BUT DOENITZ'S NEW U-BOAT BUNKERS ARE READY, AND THE ENTRANCE TOWERS OVER THE HARBOR AT LORIENT.

K1 and K2 were unique in that there was only one access point from the water.

Once they had entered the dock, the gates would have been closed, and the water gradually pumped out.

BENEATH THE WATER IS A CRADLE SITTING ON A 10 DEGREE RAMP.

THE SUBMARINE SETTLES IN THE CRADLE AND IS WINCHED UP THE RAMP TOWARDS THE DOCK FACILITIES.

U-BOATS ARE THEN PULLED OUT OF THE SAFETY OF K1 AND ONTO A TRANSPORTER UNIT BETWEEN THE TWO PENS.

This is a post-war French submarine, but the unit itself is the original piece.

It's absolutely enormous.

It had 32 wheels running along various rails, left to right in parallel with the pens themselves.

That meant that the U-Boat could be shunted into position next to whichever dock was ready to receive it.

This system is unique.

There's nothing like it anywhere along the Atlantic coastline.

But it's also quite vulnerable.

The U-Boats are exposed out in the open air.

But most importantly, the system itself is comparatively fragile.

Just one bomb strike here would render the entire system completely useless, and any U-Boats that were being repaired wouldn't be able to get back out to sea.

And of course it wouldn't be able to receive any new U-Boats coming in through K1.

THE WHOLE PROCESS TAKES AROUND 90 MINUTES BEFORE THE U-BOATS ARE SAFELY UNDER COVER AGAIN.

It's hard to describe just how enormous a space this is, and there were 13 of these all around the base in K1 and K2.

It's so tall that an overhead crane could come over the submarine and remove the periscope completely.

EXTENDING 16 FEET FROM THE TOP OF THE SUBMARINE, THAT'S NO MEAN FEAT, BUT IT'S JUST ONE OF A NUMBER OF REPAIRS AND CHECKS TO DO.

In fact, almost half of all the repairs and refits that happened to U-Boats on the French Atlantic coast were done here in Lorient.

That's what made it so special and so critical to the German U-Boat campaign.

LORIENT'S TRANSFORMATION FROM FISHING PORT TO DEADLY U-BOAT BASE ENABLES ADMIRAL DOENITZ TO HIT 500 ALLIED SHIPS IN 1941, AND FORCE BRITAIN TO THE BRINK OF DEFEAT.

The U-Boat war was the only thing that frightened Churchill, as he later admitted in his memoirs, about the war.

The possibility of Britain's resources being cut off at sea.

And it prompted him to focus attention of the British armed forces and the allies in general on winning the battle of the Atlantic.

He directed them to hunt the U-Boats at sea... to strike them at their bases.

BUILT TO SHELTER ADMIRAL DOENITZ'S FLEET FROM ALLIED BOMBS, LORIENT IS NOW BRITISH PRIME MINISTER WINSTON CHURCHILL'S NUMBER ONE TARGET.

THIS LEADS THE NAZI PLANNERS TO FEAR THAT THEIR CONCRETE GIANTS MAY NOT BE TOUGH ENOUGH TO SURVIVE.

So, at Keroman 2, the roof was roughly 3.5 meters thick.

That's even thicker than the walls of Fort Knox.

Originally, I think, they wanted something thicker than 3.5 meters, but they were concerned at this location that the foundation wasn't strong enough to support it.

AND SO THE LORIENT BASE HAS TO EVOLVE AGAIN.

THE NAZIS BEGIN BUILDING AN EVEN BIGGER PEN, WHICH THEY HOPE WILL BE TRULY IMPREGNABLE - K3.

THE NEW PEN REQUIRES STRONGER FOUNDATIONS.

BUILT BEHIND GIANT DAMS WHICH HOLD BACK THE SEA, IT IS FORGED FROM THE BEDROCK ITSELF.

So the building work was much more difficult.

There was considerably more construction done on Keroman 3 than on Keroman 1 and Keroman 2.

But they had a perfect structure as in certain parts the walls holding up the upper levels were up to 20 feet thick at the base, and the Germans could reinforce the upper level without destabilizing the building.

THE BUILDING ABOVE IS ALSO HEAVILY REINFORCED.

THOUSANDS OF TONS OF STEEL BARS ARE LINKED TOGETHER AS A FRAMEWORK.

AROUND THIS, AN OUTER CASING OF WOODEN SHUTTERS IS BUILT.

WET CEMENT IS THEN POURED INSIDE, AND THE PENS START TO RISE.

Concrete itself is really strong in compression, when a pushing force is applied, but in tension it's very weak, and it's brittle, and it cracks.

That's basically why we put steel in concrete structures, to prevent them from cracking when tension is applied.

When a bomb explodes, a huge shockwave or a pressure wave results, and when that hits the structure, the walls want to bend, and essentially the back side of the wall wants to spread, it wants to open in tension, and so the concrete would just crack at that point, and the wall would collapse inwards.

But that steel actually gives the strength to that tension side of the wall to prevent it from just collapsing inwards.

CONSTRUCTION OF K3 MOVES AT BREATHTAKING SPEED WITH CONCRETE POURED 24 HOURS A DAY TO GET THE PEN BUILT BEFORE AN ALLIED ATTACK COMES.

BUT DESPITE CHURCHILL'S DECREE, THE SKIES REMAIN EMPTY.

We are fortunate the British do not attack these pens from the air while they are under construction... DOENITZ BELIEVES LORIENT IS SAFE.

U-111.

Captain Kleinschmidt.

WHAT HE DOESN'T KNOW IS THAT THE FRENCH RESISTANCE HAVE A MAN ON THE INSIDE.

JACQUES STOSSKOPF.

Jacques Stosskopf was known as an active collaborator.

But nothing could have been further from the truth.

His increasing access to the U-Boat pens meant that he could keep a daily watch over the comings and goings here.

He had a formidable memory, almost photographic, and by identifying the emblems on the sides of the U-Boats he was able to build up a detailed picture of the daily operations here.

He would meet in secret with a contact from the French resistance and pass on his information.

Captain Lehm departed at 0500 hours on 20 February.

U-106, Captain Rush.

Reciting it twice, to make sure he had the details correct.

U-103, Captain Schulz.

Arrived at 2300 hours, 24 February.

Three boats sunk.

Again.

HE PROVIDES INFORMATION ON WOLFPACK NUMBERS AND U-BOAT SUCCESSES.

[Speaks French] STOSSKOPF ALSO PASSES ON PLANS AND TECHNICAL INFORMATION, THAT MIGHT BE USED TO DEFEAT THE U-BOATS.

This information would then be sent back to the British.

And in the battle of the Atlantic, information could be a crucial weapon.

BUT IN HIS SECRET BATTLE AGAINST DOENITZ'S FLEET, STOSSKOPF IS RISKING EVERYTHING.

THROUGH 1942, CONSTRUCTION ON THE K3 BUNKER IS CONSUMING VAST AMOUNTS OF GERMAN RESOURCES.

HUNDREDS OF CONCRETE MIXERS WORKING DAY AND NIGHT EXHAUST THE AVAILABLE SUPPLY OF SAND.

AND SO SAND IS TAKEN FROM THE BEACHES.

Notably sand for cement and reinforced concrete, this came from the seaside, the coast.

So it contained salt, and if you mix it with cement it doesn't bond well, and as there was an extremely high iron content this would accelerate the corrosion, making the building more fragile.

So the Germans had all the sand washed in fresh water in these giant sieves.

IN A BID TO BUILD AN INDESTRUCTIBLE SUBMARINE BASE, THIS ATTENTION TO DETAIL IS CRUCIAL.

TOWARDS THE END OF 1942, THE VAST K3 BUNKER NEARS COMPLETION.

IT'S A CONCRETE FORTRESS WITH HUGE STEEL GATES GUARDING THE ENTRANCE TO DOCKS THAT CAN BE DRAINED OF WATER IN JUST A FEW HOURS.

BUT THE ROOF COVERING THE COMPLEX IS THE NAZI ENGINEERS' GREATEST ACHIEVEMENT.

So, at Keroman 3 they weren't even satisfied with a 3.5 meter thick roof.

They wanted something thicker.

And so the revised the design, and what they came up with was on top of the roof build two pre-cast beams in a triangular form to create an air-gap of roughly half a meter.

On top of the air-gap they built another 3.5 meter roof.

So, in total you would end up with a roof of 7 to 7.5 meters thickness.

That's the thickness of two double-decker buses or of three stories almost of solid concrete just above the structure beneath.

BUT THEY DON'T STOP THERE.

THEY ALSO BUILD LIGHTWEIGHT BOMB-CATCHING BEAMS ON TOP OF ALL OF THE BUNKER ROOFS.

THIS ADDITIONAL ENGINEERING MAKES THE SUBMARINE PENS EVEN TOUGHER TO DESTROY.

Above you can see reinforced concrete beams for the entire length of the building.

The bombs were dropped from planes and the first thing they hit before exploding were the beams.

They absorbed the destructive energy of the bombs and limited the effects of the shock wave to the building.

And these chambers also allow the blast to be dissipated along here, so there's no compression.

DOENITZ HAD ASKED FOR AN UNSMASHABLE LAIR, AND IT SEEMS THAT THE GERMAN ENGINEERS HAVE DELIVERED.

[Speaks German] IN JANUARY 1943, THE DAMS BUILT TO KEEP THE SEA OUT DURING CONSTRUCTION ARE BLOWN UP, AND THE K3 PENS FINALLY OPEN.

BUT IT'S MUCH MORE THAN JUST A MILITARY INSTALLATION.

FOR THOUSANDS OF PEOPLE IT BECOMES HOME.

When you imagine that there were 15,000 people here, you realize that not only did the Germans need to bring in everything that the U-Boats needed, but they also needed to bring in everything the people needed.

So, this base was full of power stations, a water purification unit, storage facilities, extra railway tracks had to be built to bring in all of these supplies.

And as the base developed, as its importance developed, more and more needed to be built here to sustain the workforce.

This isn't so much a workshop as a small town.

IN THE ABANDONED OFFICES, STORE ROOMS, AND ACCOMMODATION, SOME OF WHICH HAVEN'T CHANGED SINCE THE WAR, YOU CAN STILL SEE FLASHES OF THE PEOPLE WHO LIVED AND WORKED HERE.

All over the place there are relics from the previous life of this room.

We can still make out some German writing here.

After the war, the French would have taken down any Nazi insignia, painting over any German writing.

You can still make out some of the original details.

You can still see up here some of the original paintwork.

At the time they would have tried to make it as homely and as comfortable as possible.

It's quite eerie, it's quite spooky.

This is obviously a bedframe.

So, this would have been someone's bedroom, as well as their workplace.

And then just through this window that's been boarded up is the pen itself and all the work going on.

So, there's no escaping it, this would have been a 24-hour-a-day operation.

And it needed to be, because Lorient was absolutely vital to the U-Boat campaign.

They needed to get the work done quickly and efficiently and get the U-Boats out back into the ocean.

WITH SERVICING WORK GOING ON ROUND THE CLOCK, THESE CHAMBERS WOULD HAVE BEEN HIVES OF ACTIVITY.

BUT FOR THE RETURNING CREWS, LORIENT OFFERS MORE THAN JUST A BASE.

IT IS A RELEASE.

SINCE THE START OF THE WAR, 155 U-BOATS HAD BEEN LOST, NEARLY HALF OF THEM IN THE SECOND PART OF 1942.

BUT THEIR SACRIFICES THAT YEAR ALSO RESULT IN A DRAMATIC INCREASE IN ALLIED LOSSES.

OVER 1600 SHIPS ARE SENT TO THE BOTTOM BY THESE SUBMARINERS.

AMONGST THEIR NUMBER IS U954'S NEW 2ND WATCH OFFICER.

PETER DOENITZ JUST 21 YEARS OLD, THE ADMIRAL'S SON IS PART OF A GROWING NUMBER OF INEXPERIENCED U-BOAT CREWS BROUGHT IN TO REPLACE THOSE KILLED AT SEA.

Peter?

BUT CHURCHILL IS STILL DETERMINED TO STOP THE U-BOATS PUTTING TO SEA AT ALL.

BRITISH AND AMERICAN BOMBERS LAUNCH RAID AFTER RAID AGAINST THE BASE.

BUT THIS HAS LITTLE EFFECT, AND SO IN JANUARY 1943, CHURCHILL CHANGES TACTICS.

IF HE CAN'T DESTROY THE PENS, HE'LL DESTROY THE CITY OF LORIENT ITSELF.

The allies had failed to destroy the U-Boat pens themselves, so they turned to the idea of devastating the towns around them.

The idea was, if you could destroy the town, the civilian labor force, the infrastructure, the railways, all the communications links, that would at least hamper operations out of those U-Boat pens, or maybe even render them useless.

NIGHT AFTER NIGHT, DAY AFTER DAY, THE BOMBERS RETURNED, DROPPING 60,000 INCENDIARIES AND 500 HIGH EXPLOSIVE BOMBS.

AFTER ONE MONTH OF NEAR-CONSTANT RAIDS, THE CITY OF LORIENT VIRTUALLY CEASES TO EXIST.

THREE QUARTERS OF THE BUILDINGS IN THE CITY ARE DESTROYED.

Contact the base.

I want a detailed report of any damage as soon as possible.

Yes, sir.

AMIDST THE DEVASTATION, THE U-BOAT PENS STILL STAND.

DOENITZ'S DETERMINATION TO PROVIDE HIS BOAT FLEET WITH THE BEST POSSIBLE PROTECTION HAS PAID OFF.

BUT THE ALLIES ARE STILL DETERMINED TO CRACK THE PENS, AND SO THEY CONSTRUCT THE BIGGEST BOMB YET MADE.

THE TALLBOY.

The Tallboy was a special bomb.

A bomb that that weighed over five tons, carrying two tons of explosives.

It was dropped at 6,000 meters, so by the time its reached the target, it would be travelling at the speed of sound in the hope that it would penetrate the reinforced roofing of the U-Boat pens.

11 OF THESE GIGANTIC BOMBS ARE DROPPED ON THE CITY.

ONLY ONE MAKES A DIRECT HIT ON THE PENS.

BUT WITH THE TALLBOY... ONE MIGHT BE ENOUGH.

THIS IS THE INDESTRUCTIBLE PEN'S TOUGHEST EVER TEST.

NEARLY THREE MILLION TONS OF THICK, REINFORCED CONCRETE FACES A DIRECT HIT FROM AN EARTHQUAKE BOMB DESIGNED TO SMASH IT.

This is absolutely incredible.

This is where the Tallboy actually hit the bunker.

You can still see clearly where the original line of the roof would have been.

And remember, that's 3.5 meters of concrete that the bomb has penetrated straight through, literally obliterated.

But you can also see the gap between the roofs, and then through that you can quite clearly see the second layer.

And what's most impressive is that there is barely a scratch on that layer of roof.

From underneath, for the people working in the bunker itself, it must have sounded quite horrendous.

But crucially, they would have sustained no damage at all.

Whichever way you look at it, this is an absolutely incredible feat of defensive construction.

THE BASE HAS PROVED INDESTRUCTIBLE, AND DOENITZ CONTINUES TO PERSONALLY DIRECT THE U-BOAT BATTLES OUT AT SEA.

Here are my orders.

The Meise group are to proceed northwest at high speed.

Understood?

Good.

BUT BY APRIL 1943, JUST AS THE U-BOAT FLEET REACHES ITS STRONGEST, WITH OVER 400 SUBMARINES...

THE WAR ON LAND TURNS AGAINST GERMANY, FOLLOWING DEFEATS AT STALINGRAD AND IN NORTH AFRICA.

AND THEN, JUST AS IT LOOKS LIKE ADMIRAL DOENITZ MIGHT HAVE TO PROVE HIS BOAST THAT HE COULD WIN THE WAR FOR GERMANY, THE TIDE TURNS AGAINST THE U-BOATS, TOO.

THE ALLIES REALIZED THEY WILL NEVER DEFEAT THE U-BOATS IN THEIR INDESTRUCTIBLE BASE AT LORIENT, SO THEY DEVELOP TECHNOLOGY TO GO AFTER THEM AT SEA INSTEAD.

AND IT'S DOENITZ'S OWN COMMUNICATIONS THAT GIVE THE ALLIES THE CHANCE THEY NEED.

Allied intelligence were able to penetrate German high-level communications.

Doenitz had to stay in constant communications with his U-Boats to co-ordinate their attacks, and the U-Boats had to communicate with each other.

That gave the allies a way of locating them and ultimately destroying them.

THE HUNTERS ARE ABOUT TO BECOME THE HUNTED.

THE ALLIES DEVELOP EQUIPMENT TO DETECT U-BOAT RADIO SIGNALS, AND THEY BREAK DOENITZ'S SECRET ENIGMA CODE, ENABLING THEM TO READ HIS MESSAGES TO THE U-BOATS.

OBLIVIOUS TO THIS, ON 17 MAY 1943, DOENITZ ORDERS A WOLF PACK TO ATTACK A CONVOY OF 37 SHIPS SAILING FROM CANADA.

Which boats are closest to the sector?

U-544 and U-954, Admiral.

Order them to close immediately.

U-BOAT U-954 IS CARRYING DOENITZ'S SON, PETER.

THE 21-YEAR-OLD OFFICER IS EAGER TO EXPERIENCE HIS FIRST MAJOR ACTION.

Order, sir.

A nice, fat convoy heading for us.

At least 14 heavy ships and a light ring destroyers.

FROM CRACKED ENIGMA MESSAGES THE BRITISH CONVOY COMMANDER KNOWS EXACTLY WHERE THE U-BOATS WILL BE.

AND THE ESCORT DESTROYERS AND BOMBERS FROM ICELAND SPRING THEIR OWN TRAP.

My God!

Alarm!

Aircraft attacking on the port bow.

- Thank you.

- You're welcome.

U-954, CARRYING DOENITZ'S SON, IS LOST WITH ALL HANDS.

THE ALLIES' TECHNOLOGICAL ADVANTAGE PROVES THE DECISIVE BLOW IN THE BATTLE FOR THE ATLANTIC.

243 U-BOATS ARE SUNK IN 1943.

My U-Boat men.

You have fought like lions.

We have been driven... into a tight corner from which it is no longer possible... to continue the war.

JUST FIVE DAYS AFTER THE LOSS OF HIS SON, DOENITZ ORDERS ALL HIS CREWS BACK TO BASE AND SCALES DOWN OPERATIONS.

Doenitz's campaign had failed.

The balance in the battle of the Atlantic had shifted dramatically in favor of the allies.

The allies had defeated the U-Boat threat at sea, and in 1944-45, Hitler's armies were being pushed back on all fronts.

Defeat was now inevitable.

BUT AT LORIENT, ONE MAN WON'T LIVE TO SEE THE END OF THE WAR.

In 1944, Jacques Stosskopf vanished.

He wasn't particularly missed here in Lorient.

He still had a reputation as a collaborator.

But in fact, his resistance cell had been broken... and his name given up.

He was arrested by the Gestapo.

And then taken to a concentration camp... where he was interrogated and tortured and finally executed with a single shot.

IN 1945, HITLER AND HIS EMPIRE FALLS TO THE ALLIES.

The war was over.

Even as Berlin was falling, the indestructible U-Boat pens proved to be just that.

Indestructible.

THEY WERE ONE OF THE LAST BASTIONS OF NAZI RESISTANCE, ONLY SURRENDERING TO THE ALLIES 2 DAYS AFTER THE END OF THE WAR.

TODAY THE PENS STILL STAND, SERVING AS A POWERFUL REMINDER OF THE NAZI QUEST TO DOMINATE THE OCEANS FROM THIS CONCRETE LAIR.

AND, IN A FITTING TRIBUTE, THESE OVERWHELMING STRUCTURES ARE NOW DEDICATED TO A MAN WHO SOUGHT TO DESTROY THE U-BOAT FLEET.

IN 1946, THE NAVAL BASE AT LORIENT WAS RENAMED AFTER JACQUES STOSSKOPF.

NEXT TIME ON NAZI MEGA WEAPONS: It's an incredible structure.

This place was groundbreaking.

There was a smell.

The smell of death.

This changes the face of warfare.

This changes the history of the modern age.

[Speaks German] THIS IS THE STORY OF HITLER'S ROCKET PROGRAM.