Original subtitles

The 3rd of September 1939, a British passenger ship, the SS Athenia,

steams across the Atlantic Ocean, bound for Canada.

Back home, Britain has just declared war on Nazi Germany.

Now a predator hunts the Athenia beneath the waves,

a Nazi U-boat submarine.

At 7.40pm, a torpedo slams into the liner and sinks it.

117 people on board lose their lives.

These are the first shots fired in the Battle of the Atlantic,

a ruthless six-year campaign to bring Britain

to its knees and force its people to surrender.

In this series, we investigate the most extraordinary events

of World War II from a brand-new perspective, matching rarely-seen archive film...

..photography from the front line...

..and declassified aerial reconnaissance images... ..to their original locations.

We reconstruct the crucial battles... ..daring bombing raids...

..and deadly terror weapons, which change the course of history.

Soaring over the battlefields, we reveal the secrets of World War II from above.

In 1939, a German reconnaissance aircraft

crosses the English Channel.

Its target is the city of Portsmouth on England's south coast.

As the Nazi pilot soars over its dockyards,

the on-board camera reveals a remarkable sight.

Below, ships of the Royal Navy.

This is the most powerful fleet on the planet.

At the start of the Second World War,

the battle for the oceans looks like a one-sided contest.

On paper, Britain has the largest navy and the biggest empire in the world.

The British have more than 400 vessels.

That's more than four times as many ships as the German fleet.

But as an island nation, the United Kingdom is also vulnerable.

It relies heavily on imported goods,

most of which come from across the Atlantic Ocean.

Merchant ships bring in a million tonnes of

critical fuel and food supplies every year.

Wheat from Canada, oil from the United States,

and fruit and sugar from the Caribbean.

Germany knows that if it can attack these shipping routes

and sever Britain's lifeline, the war in Europe will be over...

..before it has barely begun.

A clue to how the Nazis plan to win the Battle of the Atlantic

lies here in the city of Kiel.

In the 1930s, Kiel is the headquarters of the German navy, the Kriegsmarine,

and where the Nazis embark on a vast naval rearmament programme.

This aerial photograph reveals how Kiel's immense dockyards

churn out huge new battleships.

These Nazi combat vessels are the most advanced in the world.

They are designed specifically to hunt down and destroy

British merchant ships.

The Nazi dictator Adolf Hitler revels in the prestige of these new warships.

But in 1939, Germany's admirals know it will be another five years

until their fleet can challenge Britain's Royal Navy.

The British Home Fleet patrols the North Sea from their base at Scapa Flow in Orkney.

Their aim is to prevent the German navy from reaching the Atlantic Ocean.

So to even the odds, the Nazis turn to a relic of

the First World War - the legendary U-boat submarine.

Naval historian Jan Witt explores U-995

that now lies on the icy shoreline of the Baltic Sea.

The U-boats became almost a matter of myth

because on the German side, they were presented by

the propaganda as the grey wolves of the Atlantic.

The invincible U-boats that had so many successes in sinking ships.

The U-boat is the ultimate underwater stealth weapon.

With two supercharged engines running on 114 tonnes of diesel...

..this submarine can cover over 18,000km, enough to reach New York and back.

U-995 can carry 14 deadly torpedoes.

In battle, it can fire one from its tail and four from its nose.

At 50km/h, these subaquatic missiles swim faster than any merchant ship.

Jan investigates U-995's cramped interior

to discover how everything about this submarine is purpose-built for war.

I'm standing now in the bow torpedo room,

which was also the living quarter for the main crew.

You press in everything you need for a U-boat to function -

engines, controls, weaponry - and the little space that is left is for the crew.

The 27 men on board this submarine could stay submerged for up to 72 hours at a time,

waiting for the moment to strike.

A U-boat could appear out of nowhere, sink ships in the

blink of an eye and vanish as quickly as it had appeared.

Fortunately for the British, the Nazis had fewer than 50 operational U-boats in 1939.

Many in the German navy still see battleships as the key to victory.

But an ambitious naval officer, Admiral Karl Dönitz,

has a secret plan to boost U-boat production on a massive scale.

He's the head of Germany's submarine service

and believes that with 300 U-boats,

he can single-handedly win the war.

Dönitz devises a daring raid to prove his point.

He plans to attack the Royal Navy on home turf,

here at Scapa Flow in Orkney, off the coast of Scotland.

Emily Turton is an expert diver and skipper.

She investigates how Dönitz plans to pull off this near-suicidal mission.

The British naval base at Scapa Flow is supposed to be impregnable.

So Scapa Flow is a natural harbour.

It's strategically really well placed for us

to be in both the Atlantic and the North Sea.

So easy to protect in that we have narrow entrances

into this big bit of water where we can house an entire fleet.

The British have fortified Scapa Flow's natural geography.

They have sunk derelict vessels called block ships

at strategic locations to control the ways in and out.

But a German scouting mission reveals that the block ships

in one of the narrow channels have not been maintained.

Dönitz thinks there is a wide enough gap between them for a U-boat to sneak through.

On the night of 13 October 1939,

he seizes the opportunity to strike.

Under a new moon, U-47 approaches Scapa Flow and heads for Kirk Sound.

Once inside, it spots a warship, HMS Royal Oak, at anchor.

It's a sitting duck.

Just before 1am, the U-boat fires three torpedoes at the unsuspecting battleship.

One torpedo hits the vessel, causing only minor damage.

The Germans reload and fire again.

This time, they blow two holes in the stricken vessel.

As HMS Royal Oak rolls over and capsizes, U-47 slips away in the dead of night.

Today, this buoy marks the spot

where 835 British sailors lose their lives in the raid.

The German navy has shown it is capable of bringing war home to Britain.

- The actual sinking of the Royal Oak would have been quite brutal.

There would have been a lot of flash and

blasts caused by the torpedoes inside the ship.

So it's a huge blow to morale for a U-boat

to successfully navigate their way into Scapa Flow,

and yes, under the cover of darkness, but still, to get

into Scapa Flow and then manage to sink a capital warship.

It's a massive disaster at the beginning of World War II.

- The Royal Navy upgrades Scapa Flow's defences after the attack.

Engineers build huge new causeways

called Churchill Barriers to protect anchored ships.

They lay six miles of anti-torpedo nets across

the water to prevent future submarine attacks.

After the raid at Scapa Flow, U-47 returns home to a hero's welcome.

Its captain, Gunther Prien, receives Germany's highest military decoration,

the Knight's Cross of the Iron Cross with Oak Leaves.

At Kiel, the German Navy ramps up submarine production.

From this moment on, the Battle of the Atlantic will be a war against the U-boats.

Britain must find a way to defeat these silent hunters before it is too late.

In the first month of World War II, Nazi U-boats

sink 30 merchant ships alone in the Atlantic Ocean.

Among them is the SS Kensington Court, torpedoed on the

18th of September, 1939, while carrying a shipment of grain.

Many of the cargo vessels that do survive the perilous Atlantic voyage

unload their critical supplies here in the northern city of Liverpool.

In 1939, this port on the banks of the River Mersey is one of the busiest in the UK.

World War II expert Lauren Shacklady investigates how Liverpool now plays a key role

in fighting the U-boats and keeping Britain alive.

- All of this view that we have now

would have been completely different during the war.

At the start of the war, we only had a few weeks worth

of supplies in the country at any one given time.

There would have been tons of ships coming into the city every day, every week.

- The Admiralty knows that it must protect its ships

heading for ports like Liverpool, so it reactivates an

old World War I countermeasure called the convoy system.

A convoy is a group of 30 to 40 merchant ships that sail together.

The vessels are spaced up to one kilometer apart.

Warships from the Royal Navy surround the

cargo vessels as they travel across the ocean.

Most convoys sail from North America,

bringing vital war supplies to ports like Liverpool.

Ships heading the other way are less critical.

Some are only escorted for a few days.

The Royal Navy controls the entire operation from its naval base in Plymouth.

The convoy system can't protect every vessel.

In the first seven months of the war, Germany sinks a staggering

222 merchant ships, but it loses 18 U boats in return.

The tiny German Navy can't sustain these losses.

By April 1940, the Kriegsmarine has only 31 operational

submarines left until the German army comes to its rescue.

In May 1940, Hitler's Blitzkrieg knocks France out of the war.

At Dunkirk, the Germans force the British to retreat back across the Channel.

Nazi troops now occupy all of France.

The French ports they now control give the Nazis direct

access to the Atlantic Ocean for the very first time.

With the fall of France quite early on in the war, not only did Germany gain access

to all of the ports along the French Atlantic coast,

they also mined the English Channel with U-boats

and basically blockaded all of our supply lines down there.

The Admiralty realizes that its HQ in Plymouth is vulnerable

so it moves command of the entire convoy system north to Liverpool.

They set up a top secret base

deep beneath this seemingly ordinary office block.

Naval personnel build a huge map room to track every convoy

crossing the Atlantic and alert them to any U-boat sightings.

The Royal Navy hopes that the team in Liverpool will keep the supplies flowing.

Unfortunately for them, the Nazis set up a rival

operation at their bases in occupied France.

Germany's Admiral Dönitz is a former World War I U-boat commander.

He has spent years plotting how to defeat

the hated British convoys and comes up with a new strategy.

Instead of a single U-boat searching for a lone convoy,

Dönitz orders his U-boats to hunt together in packs to coordinate their attacks.

The U-boats sail into the Atlantic side by side, 16 kilometers apart.

The group stretches north to south like an invisible net, cast out to snare a convoy.

When one submarine spots the target, it doesn't

attack immediately, but radios the others to join it.

All the U-boats then converge on their prey like a pack of wolves.

The Atlantic Ocean now turns into a gigantic chessboard,

one where thousands of lives are at stake.

Inside Liverpool's command centre, naval personnel work

day and night trying to guide the convoys to safety.

Eight out of 10 are women, with so many young men away at sea.

- The people in here knew that they had loved ones,

family members, partners who were out on the ocean

and they're seeing all of these ships sinking in real time.

It must have been a really difficult job to do in that sense.

- The Allies try different ruses to outwit the wolf packs.

They order their convoys to change course frequently,

but Germany's new tactics are a stunning success.

In 1940, the U-boats sink 471 merchant ships,

most of them off the west coast of the British Isles.

And the following year, another 432, largely along the convoy routes.

As Allied losses mount, U-boat commanders call this the happy time.

By mid-1942, the U-boats appear to have free reign

over the Atlantic, striking down more than 1,000 ships, some as far as Mexico.

Britain seems powerless to stop Hitler's U-boats,

but British Prime Minister Winston Churchill is determined to strike back.

He plans a daring raid on the German Navy deep in occupied France.

By January 1942, the war at sea is firmly in Germany's favor.

Hitler's U-boat fleet has grown to nearly 200 vessels.

Many of them sail south to ports like San Nazaire in occupied France,

where the Nazis now tighten their grip on power.

In March 1942, an Allied reconnaissance plane takes this photograph of San Nazaire.

It reveals how the Nazis totally transform this once peaceful port.

The Germans build a gigantic concrete bunker called a U-boat pen in just 16 months.

Structural engineer Zeynab Adegan investigates its secrets.

The San Nazaire base is a huge engineering project.

Just the sheer magnitude of people you would need

to be able to construct such a large structure

in the space of time that they had is absolutely mind-blowing.

To protect their submarine base from bomb raids,

the Nazis build an eight-metre-thick concrete roof.

Up to 20 U-boats can shelter here, along with hundreds of crewmen.

They defend this concrete megastructure with anti-aircraft guns.

This submarine fortress is the perfect safe haven for a U-boat wolf pack.

Inside, Zeynab finds clues which reveal

how the Germans build this enormous structure.

It's massive, isn't it?

It's almost intimidating, actually.

It's like a tiny little speck.

And just imagine, this was only one of the rooms.

These faint lines on the wall are the imprints of wooden panels.

They reveal that German engineers build the bunker

by pouring the concrete layer by layer into a wooden framework.

The construction technique used is actually very similar to baking,

where you've got your mould around it, which is the formwork.

Once the concrete is dry, you take away the wooden panels

and you're left with a really, really thick wall.

Saint Nazaire is one of five enormous U-boat bases on the Atlantic coast.

But the Allies fear the Nazis have even bigger plans for it.

The port's massive dry dock is the only one big enough

on the Atlantic coast to house Germany's new super battleship.

The Tirpitz.

In 1942, this giant is the most powerful warship in the world.

The vessel is currently moored in occupied Norway.

But the Allies are terrified that if the Tirpitz reaches Saint Nazaire,

it will wreak more damage to the convoys than an entire wolfpack of U-boats.

So they plan a daring commando raid to put Saint Nazaire's dry dock out of action.

In the early hours of March 28th,

a small flotilla of Allied ships heads for Saint Nazaire.

Among them is an old destroyer, HMS Campbelltown, disguised as a German vessel.

Campbelltown's mission is to ram the dry dock doors.

The Germans rain down heavy fire onto the ship once they realise what's happening.

Against all odds, the Campbelltown stays on course.

and at 1:34am smashes into the dry dock gate at 35km/h.

British commandos now leap out of the ship and wreak havoc.

They inflict even further damage on the dockyard.

Over half of the commandos are captured or killed.

But the dock is severely damaged.

The next day, the Germans inspect the wreck of

the Campbelltown to figure out how to remove it.

What they don't realise is that the ship is a ticking time bomb.

Hidden below deck, commandos have rigged over four tonnes of explosives to a timer.

At 10:30am, they detonate, sinking the ship and making it impossible to tow away.

The raid is a spectacular success and a huge psychological boost to the Allies.

The Tirpitz now has no base on the Atlantic coast.

It never reaches the Allied convoys.

But the submarine pens remain undamaged.

The dwarf packs are still free to hunt the convoys.

The Allies desperately need a way to defeat the U-boats.

The solution lies not out at sea,

but right on their doorstep in a quiet corner of southern England.

In 1938, the UK's Secret Intelligence Service

takes control of a large house deep in the English countryside - Bletchley Park.

Historian Tom Cheetham investigates how this seemingly ordinary building

plays a game-changing role in the Battle of the Atlantic.

It doesn't look like an ideal wartime secret base

but this mansion and the grounds around it,

they proved to be a suitable nucleus for an organisation

which would grow many times over during the war

and play an invaluable role in Allied victory.

The German wolfpack strategy relies on communication

between U-boats to coordinate their deadly attacks.

From 1939, Bletchley becomes home to hundreds of intelligence personnel.

They are all working towards a single goal - cracking top-secret Nazi communications.

The German navy possesses what it confidently believes

is an unbreakable code...

..thanks to an innovative device that looks like an old-fashioned typewriter.

The Enigma machine.

This is an M4 Enigma machine used by the German navy during the Second World War.

Every U-boat would have had one of these machines.

Enigma codes messages in an ingenious way.

Every time the Enigma operator punches in a key,

one or more of its four rotors spin to scramble the electric signals.

The chosen key gets encrypted to a different letter every single time it is pressed.

The variables add up to create a code

with more than 31 trillion trillion different configurations.

The Germans also change the machine settings

every day to make the coding even harder to crack.

One man is determined to break the Enigma's unbreakable code -

British mathematician Alan Turing.

Turing decides to take on the naval Enigma

because he's a natural problem solver, he loves a challenge,

and it was a problem where nobody else was doing

anything about it and he could have it to himself.

Turing and the code-breaking teams work around

the clock to decipher intercepted U-boat messages.

They use a combination of elaborate code-breaking techniques

and lucky guesses at commonly used phrases such as "Heil Hitler".

The team breaks the German army Enigma code in 1941,

but the naval version, code-named "Dolphin", proves far harder to crack.

Turing designs a machine to churn through potential

Enigma setting combinations to speed up the process.

It is a forerunner to the world's first computer.

Dozens of these machines fill ever-expanding rows of huts.

Even with their mechanical help, the code remains stubbornly hard to crack.

What the Allies need is a lucky break.

It is not until the spring of 1941 that they finally get one.

On 9th May that year, three British destroyers

damage a U-boat just off the coast of Greenland.

The German captain tries to sink his own ship, but bungles the task.

The British sailors manage to climb on board,

where they find a working Enigma machine

and a code book.

What the German cipher operators are meant to do is destroy everything,

throw the machine into the sea and also all of their secret papers.

Everything is printed in water-soluble ink so that it

will dissolve when it comes into contact with water.

In the case of U-110, they fail to do this.

The team at Bletchley Park pounces on the unexpected bounty.

Now that they have the German code books,

the code breakers can more easily decrypt the top-secret messages.

The translated codes give them valuable insight into the U-boat movements,

removing the key element of surprise from the wolfpack hunting strategy.

The code breakers at Bletchley alert the command centre up in Liverpool,

where the team is able to divert the convoys away from the U-boats

and send British forces to intercept the enemy.

Thanks to the work at Bletchley Park,

the balance of power in the Battle of the Atlantic begins to shift.

The German navy never realises the extent to which its secrets are exposed.

By the end of 1942, the Royal Navy finally gains an edge over the Kriegsmarine.

Now they plan to take the fight to the Germans by hunting down the U-boats

and destroying them.

RAF Coastal Command faces the difficult

challenge of hunting for U-boats from the air.

At the start of the war, it has to make do with lightly armed aircraft,

like the Avro Anson, which only have a limited range.

The Atlantic is so vast and a U-boat so small,

it is like looking for a needle in a haystack.

But by 1942, a new aircraft promises to transform the battlefield.

It is engineered here, at the world's first purpose-built motor racing track.

This is Brooklands in Surrey, one of the most famous names in British motorsport.

In the 1920s, its huge oval circuit hosts the first ever British Grand Prix.

But Brooklands is also home to one of Britain's oldest aerodromes.

Former Royal Marine Commando and pilot Arthur Williams

discovers how the racing stops and Brooklands prepares for war.

I love Brooklands. Brooklands is a magical place.

In 1939, it was requisitioned by the Air Ministry and it

really upped its gear in terms of aircraft manufacturing

and it became a super powerhouse for churning out

aircraft that we desperately needed during the war.

Aviation engineers now replace spectators at Brooklands' famous track.

They become famous for producing one pioneering plane in particular.

Inside this huge hangar is an aircraft that makes its mark as a U-boat killer.

The Wellington bomber.

It's fascinating to see the Wellington here as a whole.

Normally you're used to just seeing component parts of this aeroplane

because it's one of only two that are still remaining in this sort of state.

This aeroplane doesn't look like other aircraft.

It's not designed in the same way.

The Vickers Wellington is designed at Brooklands as a long-range medium bomber.

One of the brains behind it is the maverick inventor Barnes Wallace,

who also devised the Dam Busters bouncing bomb.

He engineers the Wellington with a revolutionary

lattice framework called a geodetic structure.

This makes it light but incredibly strong. It can still fly with even major damage.

To keep its weight to a minimum, engineers cover the alloy frame with linen fabric.

Its 19-metre-long fuselage is big enough to hold eight lethal depth charges,

a type of bomb designed to detonate underwater.

The Wellington has a disappointing start to the war as a conventional bomber.

Its limited defences make it vulnerable to enemy fighters.

But by 1942, it is transferred to RAF Coastal Command,

where its superior range is ideal for anti-submarine duties.

The Wellingtons are able to patrol the Bay of Biscay off the coast of France.

German U-boats, leaving their bases in ports like Saint-Nazaire,

have to pass through here on their way to the Atlantic.

The British bombers have a range of around 1,000 kilometres,

which allows them to spend hours hunting for the

enemy and still return home with fuel to spare.

In an aerial attack, the bomber uses a searchlight to hunt for the submarine

and drops two depth charges, ideally within six metres of the target.

These detonate underwater, the shock waves damaging

the U-boat before it can escape beneath the waves.

The Wellington is also able to carry an ingenious

bit of kit packed into its nose - radar.

The Wellington bomber was an incredibly versatile aircraft.

As radar started to be developed, it proved itself really

useful in Coastal Command, searching down the wolf packs.

The radar gave the crew the ability to see the U-boats beyond visual range,

and then when the crew knew where the U-boats

were, they could press home their attack.

The Wellington excels as a U-boat hunter.

It scores its first confirmed kill on 5th July 1942.

The factory at Brooklands builds more than 2,500 of these aircraft during the war.

But even the mighty Wellington has its limitations.

The Wellington can only protect convoys effectively

for around 640 kilometres from the coast

before it runs low on fuel.

Beyond this, the Allied ships enter the so-called 'Mid-Atlantic Gap'.

In this Nazi-infested danger zone, the convoys have to sail without air cover,

placing them at the mercy of the wolf packs.

In 1943, a new American bomber, the B-24 Liberator, is a game-changer.

Now the Navy finally has an escort plane with a long enough range to close the gap

and protect the convoys across the Atlantic.

The new Allied planes have enough fuel to

spend around a third of their time in the air

hunting for U-boats threatening the convoys.

Hitler's submarines are now caught between the

Liberators in the Atlantic and the Wellingtons back home.

They suffer appalling losses.

Liberators alone sink 70 German subs.

By 1943, the tables have turned decisively

and the Allies begin to dominate the battle for the Atlantic.

But the Nazis aren't finished yet.

They plan one last throw of the dice, which they hope will finally bring them victory.

In 1944, the Nazis rush out new weapons in a

desperate attempt to halt the Allied advance.

They develop terrifying devices, like the fearsome

V-rockets, to swing the war in their favour.

The German Navy also tries to upgrade its increasingly obsolete U-boat fleet.

Shipyards like Kiel build new submarines to

counter the threat from anti-submarine aircraft.

The chief drawback of the second World War German

U-boats was they were based on World War I technology.

They could spend only a very limited time submerged.

After the Allied had closed the air reconnaissance gap in the Atlantic,

the U-boats had no place anymore to retreat.

So the Germans had to come up with new ideas.

And this resulted in the construction of the

so-called Elektroboote, or electric U-boats.

The Elektroboote is powered by diesel-electric engines.

It could run underwater at an incredible 17

knots, 10 knots faster than an existing U-boat,

reducing time spent above the waves.

The U-boats also get new weapons.

Commanders equip their submarines with heavier anti-aircraft guns,

as well as radar and decoy balloons to fool the bombers.

But it's not enough.

By May 1945, it's clear that Nazi Germany has lost the war.

In Berlin, Hitler names Admiral Dönitz, a committed Nazi,

as his successor before his own death by suicide.

On 7th May 1945, Germany surrenders,

and all U-boats are ordered to hand themselves in.

One week later, eight German U-boats enter

Loch Foyle on the coast of Northern Ireland.

They head for the Royal Navy base at Lysa Halley.

It is an extraordinary moment.

Allied warships keep watch, while fighter planes circle overhead.

But this is no attack.

The Kriegsmarine is officially surrendering,

bringing the Battle of the Atlantic to a symbolic end.

Newsreel footage catches the German crews smiling and joking with each other on deck.

This incredible aerial photograph reveals rows

of submarines tied up alongside each other.

By the end of the month, nearly 40 have arrived in this quiet Northern Ireland port.

The war at sea, which has cost so many lives, is over.

British demolition experts now rig the German submarine fleet with explosives.

Many are in poor condition.

Royal Navy ships tow them out to sea and sink them.

For Britain, the Battle of the Atlantic was the longest campaign of World War II.

Fought from the very first day until the last.

Hundreds of ships have been lost on both sides.

The Kriegsmarine loses 30,000 submariners,

the highest casualty rate of the German armed forces in World War II.

Dönitz is convicted after the war, but escapes execution.

He remains an unrepentant Nazi for the rest of his life.

Today, the wrecks of many ships and U-boats still

lie in the vast ocean, silent and unknown war graves.

Testament to the savagery and the courage of the Battle of the Atlantic.