Original subtitles

- [Macphee] 1939, under the cover of a forest

in Poland the Nazis constructed one

of the largest complexes of World War II.

- This is straight out of a Hollywood movie.

- [Macphee] Covering 10 square miles

the complex was built into the hills

and trees of Bydgoszcz,

transforming it into

a highly classified industrial compound.

- This site is enormous.

- Over a thousand structures.

- How they hid all that?

It's amazing, really.

- [Macphee] The workers had no idea

what they were manufacturing at Dag Bromberg

and the allies didn't discover it until the end of the war.

- This factory was one huge secret.

- The most evil things of the war were built

in Dag Bromberg.

- [Macphee] What dark mysteries lie inside?

- One mistake could blow this whole operation to pieces.

- It sends shock waves that will propagate

through anything they come in contact with.

- [Macphee] And how did the Nazis keep

it hidden for so long?

- They've made this complex as secret

as it could possibly be.

Why is that?

[dramatic music]

- [Macphee] January, 1945,

as the Soviet Red Army advanced through Poland

soldiers stumbled upon a massive abandoned Nazi complex

hidden in the Bydgoszcz Forest.

- When the Front rolls in the Russian soldiers

just find a series of abandoned buildings.

- [Macphee] The Germans had fled the facility

just five days earlier,

destroying everything they couldn't take with them.

- It was Nazi policy when they withdrew from any kind

of territory to carry out scorched earth policy.

That meant destroying every document,

every building, literally throwing everything into the fire.

- Unfortunately, the iron curtain was pulled

across the Soviet Union and the world did not learn

for decades what had happened there.

Perhaps the best kept secret of the Nazi regime

is this Dag Bromberg facility.

- [Macphee] 1939, after the annexation of Austria

and Czechoslovakia Hitler turns his attention on Poland.

- The first of September Hitler invades Poland.

Third of September Britain and France declare war

on Germany as a result of that.

- [Macphee] Two months after the invasion of Poland

the Germans begin construction on Dag Bromberg

in the forest near GdaDsk.

By 1942 it spans a vast 10 square miles.

- I live in Poland and I have to admit,

I'd never actually heard about the Dag Bromberg Complex.

- The camouflage and secrecy of this site was so good

that it was not discovered by the allies

until the end of the war.

- I'm standing, in fact,

in front of a building which is enormous

but you would never see this from the air

if you were an allied fighter pilot or bomber

or reconnaissance plane coming over.

It would be as if it really didn't exist.

And you get the sense of this place being so enormous

and yet so much of this is hidden under this undergrowth

which is pretty thick.

As I'm walking around the complex the guide I'm with,

Lukasz, is starting to tell me

that they don't actually know what's over here

or they haven't yet been over here.

I started to get a little bit disoriented

because you quickly lose yourself

in the undergrowth in these hills.

But they're not hills,

it's a tunnel leading from one part of the complex

to another and you didn't notice it.

The construction seems relatively simple.

You've got this main production facility here

and what are these?

These are tunnels that lead in?

- Yeah, these are tunnels that are connecting

those buildings and now we are going

to the next building that is just there.

Can you see him?

- Honestly speaking, you can barely see anything

through the tree.

- And this is the way that they wanted to just hide

this whole place here.

- And you can look up,

the foliage here,

these tree branches,

you wouldn't be able to see anything basically

if you were flying over 10, 15,000 feet.

- [Lukasz] Yes.

- Plus I'm assuming they've got camouflage nets

that are over here as well.

So with the trees,

the camouflage nets,

essentially this place is invisible.

- The overall aesthetic at Dag Bromberg

is very unlike what we've come to know

as big Nazi structures with a kind of brutalist form.

These all take more of a haphazard,

almost woodland kind of form.

- The other structures are out there, they're imposing.

They're an expression of power.

They are meant to be seen,

whereas this facility is very different.

It is so critical to their operations

that secrecy is the number one priority here.

- [Macphee] Construction of Dag Bromberg was continuous.

Anything this large would've been visible from the skies,

yet the ally's reconnaissance would never spot it. How?

Location was key to keeping Dag Bromberg a long term secret.

- There were three sites that were scouted as possibilities

but this site was chosen

because of its proximity to a river.

It had railways nearby but also it had a form

of natural camouflage from the undulating landscape

and the dense forest canopy.

- This forest was sparsely populated,

so dense, so thick that the Nazis could build

and operate there without the Allies discovering it.

- They used the forest as natural camouflage.

They created structures that were uneven,

that were not too tall,

that didn't stick above the treeline.

They landscaped the roofs and the sides of buildings.

They buried structures.

They back filled things.

They made it look like a natural landscape.

- [Macphee] The forest camouflage

would be effective once completed.

But to build such a huge and spread out complex

the Nazis had to employ large scale logistics

in and around the forest.

How did they manage such a massive undertaking

while remaining undetected?

- There's an interesting engineering dilemma that occurs

when you consider the construction of this site.

It's an enormous site and that's gonna need quite a lot

of engineering infrastructure to build.

That kind of thing is gonna be extremely visible from above.

But at the same time you need to keep all

of this secret so how did they manage it?

- You're bringing in concrete mixers.

You're bringing in dozers, excavators.

Typically in a situation like this we would pave a road.

We would clear cut the forest to knock down anything

that's in the way and we would build with that.

They didn't do that.

- The Germans undertook the most inefficient method possible

of bringing any equipment that they needed to this site.

They essentially utilized existing footpaths,

widened them ever so slightly and painted them khaki

and these other camouflage type colors

to maintain the secrecy of the site so that the allies

did not know that it was there.

[clock ticking]

- [Macphee] Even if the allies had discovered

the Dag Bromberg site,

how would they have known it was worth bombing?

- By having the Dag Bromberg site spread

over such a large area with so many buildings

if the allies did discover it through their reconnaissance

it would be very hard to determine which buildings

were a priority.

None of them were more conspicuous than any of the others.

Another advantage of having this site spread out

over such a large area is,

at the time,

aerial bombing was not an exact science.

- For the first few years of the war most of the bombers

that the Allied forces are using,

they're just guided by site, effectively.

It's no surprise really that the average hit rate

for bombing missions was very, very low.

You had no idea where those bombs were landing.

They were just as likely to murder sheep and cattle

as they were the enemy.

- [Macphee] Over the course of the war

the bombers improved and the RAF built

a sophisticated air intelligence operation,

but they didn't find Dag Bromberg.

- The Allies are flying non-stop reconnaissance

by the middle of the war, '43, '44,

to figure out what the Nazis are doing.

Despite the success elsewhere the Allies are unsuccessful

in finding Dag Bromberg.

This one stays a secret.

- It's almost unbelievable that the Allies didn't spot this.

They had planes travelling over Germany the whole time.

- Yes, they've made only one plane picture

in '44, that's all.

- It's phenomenal, you think about it,

you've got Allied planes scouring Germany,

literally photographing every square mile,

every square kilometer of the country.

They're probably flying over this complex every single day

and they never see it.

- That's the mysteries of this place

because it's one of the biggest mystery

in the Second World War.

- And of course if they had known what was here

they would've sent over several hundred planes to bomb it.

- I think so but not a single bomb was dropped here

in the Second World War.

- [Macphee] But not all of the risks came from the skies.

- So this is the most dangerous part in the whole complex.

- [Macphee] What mysterious terrors were created

under the forest canopy?

- It's very volatile and it's dangerous

to work with these things and they tried

to build an environment in which they can contain that.

- [Macphee] What nefarious plans

were the Germans engaged in at Dag Bromberg?

World War II,

the Nazis built a 10 square mile complex hidden

in a forest in Poland.

Thanks to a thick forest canopy

and the extraordinary efforts taken

to camouflage this site,

the allies couldn't spot it.

- They've made this complex as secret

as it could possibly be.

Now, they didn't do that for other places.

They did here.

Why is that?

- [Macphee] The occupation of Poland

was an essential component in Hitler's plan

for world domination.

- Hitler and some other Nazi leaders had this idea

of living space.

They wanted a greater German empire, a Reich,

and part of that was Poland.

It stems from the end of World War I.

With the Treaty of Versailles

it gives the city of GdaDsk to Poland.

Hitler had always seen GdaDsk as a Germanic city

and some of the lands he desperately wanted back

as part of this greater Reich.

- [Macphee] Confident in the superiority

of the German army,

the Nazis invaded Poland.

- Poland's defense essentially collapses

after four weeks of brutal fighting.

The western parts of Poland is incorporated

into the Third Reich.

The rest of Poland actually wiped off the map.

- [Macphee] Germany was successful in taking Poland

but to engage Britain and France would require

a whole new level of warfare.

- Hitler realizes he needs

to greatly expand the Wehrmacht, the military,

more men, more weapons,

bigger, better technology,

bigger, better weapon systems.

- When Hitler occupies Poland,

essentially, he's getting everything for free.

Any production facilities,

any logistics center,

any natural resource,

you name it,

the Nazis are gonna use it towards their warring.

- One problem for Hitler though

with all these newly acquired resources

is that these are very well established facilities.

And so the Allies will know exactly where they are

and exactly what they are.

The Nazi command the side that they're gonna need

somewhere they can produce what they need

in ultimate secrecy.

- [Macphee] In search of new solutions

the Nazis turn to a company

they have long collaborated with,

Dynamit Nobel AG or DAG,

built the Nazis one of the largest munitions factories

in their enterprise,

10 square miles.

- This site is enormous.

By the end of 1944 it contained 1,000 separate buildings.

It had 400 kilometers of roadway

and 40 kilometers of railway.

When you think of something that big

it's quite amazing they managed

to keep this whole thing a secret.

- [Macphee] Here, the Nazis would produce TNT,

smokeless powder and nitroglycerin,

enough to supply the German army the munitions

it would need for a multifront fight.

- If you're gonna be providing the majority

of the munitions from one facility

the scale of this operation needs to be significant.

It needs to be a well oiled machine.

It needs to be able to produce consistently

with quality and on time,

on budget and under duress.

How do you keep that secret?

It's amazing, really.

- The Nazis built

80 plus of these types

of munitions factories around their empire.

Almost all the major Nazi labs,

weapons facilities get bombed and bombed hard,

are rebuilt and bombed again,

taking them offline enough to delay the Nazis

from developing these horrifying weapons.

One of the few that remains a secret

is this Dag Bromberg facility.

- [Macphee] Protection from aerial bombing

was just the beginning of the incredible engineering plans

that went into construction at Dag Bromberg.

- You need to bear in mind the materials

that are being produced here.

This is explosive materials.

They are designed to do one thing,

to explode and cause damage.

The last thing you want is to have that happening

at a time that you hadn't planned for,

i.e. in the factories themselves.

Not only are you gonna lose all your materials,

you're gonna lose your labor force inside there.

But possibly more scary is the fact

that one explosion could set off a whole chain reaction

within the factory and you'd lose the lot.

- [Macphee] Determined to protect their endeavors

from the highly volatile nature

of the materials manufactured at Dag Bromberg

the Germans designed extraordinary systems into the site.

- The engineers thought about this.

They situated the buildings in certain ways

to prevent that chain reaction from happening.

- The Germans took the design to the extreme

by not building real houses

because an explosion in one could very quickly

be transferred to another.

They went to great lengths to ensure that the entrances

to different rooms were not directly across

from each other for fear that any blasts

could potentially set off a chain reaction

in another room.

- They wanted to accommodate the reaction

and so the mindset of these engineers and designers

was driven by that need.

And it's a really different approach

to engineering than we typically would deal with.

- [Macphee] The destructive impact of an explosion

was dependent on many factors.

The properties of various substances

required different safeguards.

In order to facilitate these factors

multiple structure designs were built.

- The Nazis were aware that there could be accidents,

that there could be explosions and in order to mitigate

that risk they designed their structures

and built them in a way

to control a release unexpectedly of energy.

- One of the explosive materials

that was being manufactured here

was something called smokeless powder.

When smokeless powder is ignited it turns

from its solid state into a gas at such a rapid rate

it sends shock waves that will propagate

through anything they come in contact with.

The other thing that makes it so dangerous

is that it doesn't need oxygen from the surrounding air

to detonate because it contains within itself

its own source of oxygen.

- You could actually cause an explosion

by compressing it and putting it in a confined space.

So they had to beal with that in a very different manner.

Throughout the facility there were structures

called light embankment buildings,

and they were built in a manner to control

a potential explosion.

They had built up embankments around the sides

of the buildings.

The earth would absorb energy and dissipate

that energy through the earth,

but the majority of the explosion would go upwards.

That would keep the explosion local to that

and hopefully not spread any sort of explosion

into adjacent structures.

- [Macphee] Alternately, production buildings

that held more equipment posed a greater risk of explosion.

They were designed to be bunker-style structures

that would direct the blast outward rather than up.

- They built rooms with a blast wall,

three sides being brick and a fourth side

a glass wall with an earthen wall behind,

so that in the event of an explosion

the glass would shatter and the shards of glass

would essentially be held by the earthen wall

allowing the energy generated within the room

to essentially escape without actually destroying

the rest of the structure.

- This is the blast wall you were talking about, yeah?

- Yes, this is the blast wall

and this is original from the Second World War,

made from wood and glass.

- Hang on a second, what's this?

This is an original piece of the window?

- [Lukasz] Yes.

This is the glass that were in those windows.

- Wow, so this has survived almost 80 years.

What's actually inside?

I can see some kind of wire netting.

- [Lukasz] Yes, it's a wire net

because if we get an explosion here

the glass will be thrown somewhere

and like knives will cut everything.

And with wire the glass won't be such a huge danger.

- So you've got reinforced glass.

You got the special wire netting that's been placed inside.

They're very obviously paying very careful attention

to detail to all the safety,

make sure this plant is as safe as possible

so it can carry on producing munitions.

- Yes. - And of course

the glass meant that no noise

would be coming from this production facility,

so again, anyone outside there wouldn't really know

what was actually going on here.

- Yes, exactly.

There was a road just behind this building

and workers walk on this road

and they didn't see this building.

It's impossible to see him because he's covered.

- They built this entire complex with creative ways

to prevent a chain reaction.

They didn't do it so much to direct the explosion

in a certain way or minimize the loss of life.

They did it so they could quickly rebuild

and get back online.

- While the workers may be expendable to Hitler,

the production is not.

This is key to the war.

- [Macphee] Nevertheless, escape tunnels were provided.

- [Lukasz] If something will go wrong in this building

the worker will just run through that tunnel,

went here, hold those supports that you can see on the wall

so the blast won't throw him somewhere

in the forest and he will be survive.

- If an explosion takes place this kind

of little shelters offers you no protection whatsoever.

- It's impossible because you need to see

that something will go wrong,

just run through that tunnel,

run here to this chamber

and the chamber will be destroyed after the explosion.

- And we're talking about explosions that happen like.

[Ney clicking his fingers]

- Yeah, in one second,

not even in one second.

So it's only for the psychology.

And if at the start of your job someone will tell you,

if something will go wrong you will die,

you don't wanna came here to the next day.

- Yeah and of course your death would be lost

in this huge secret complex.

- Yes, of course.

- [Macphee] Some of these structures would produce

a product so dangerous it could only

be contained entirely underground.

- Even shaking this compound could set off an explosion.

- [Macphee] This material was the key to terrible munitions.

- The Nazis weren't producing just one type of explosive,

they were producing every type and variety imaginable.

- [Macphee] What secrets did the Red Army discover here?

Early in World War II the Nazis constructed

the secret Dag Bromberg base,

a massive munitions factory under the cover

of a dense Polish forest.

Because it provided a third of the munitions

for the Eastern Front,

the Nazis put extraordinary effort into keeping it secret

from the Allies and safe from its own lethal products.

- So the whole time you're living under this threat

that a single spark would literally set this place alight.

- [Macphee] But there was one ingredient made here

that required extraordinary precautions.

- This leads through to the main production facility?

- [Lukasz] Yes, to the main building here.

- [Macphee] With no facade the nitroglycerin facility

could only be accessed via the service

and emergency tunnels.

- This is the place where they made nitroglycerin.

There was a machine called nitrater

and to this machine they input sulfuric and nitric acid,

also glycerin and they mix those.

And we've got nitroglycerin,

really unstable, dangerous explosive material.

- Nitroglycerin is one of the most explosive substances

that Hitler would've had access to.

Merely shaking it would be sufficient

to create an explosion.

- And so in order to keep it stable

one of the conditions is to make sure that the temperature

around it is kept at 20 degrees Celsius or lower.

That's another engineering challenge

that had to be incorporated into this base.

- [Macphee] In addition to temperature being a concern,

static electricity was also an issue.

- In the buildings in which nitroglycerin was produced

the floors were made out of lead.

Lead is a conductive material.

It will prevent any sparks from forming

that could potentially ignite the nitroglycerin.

- You can just imagine the meticulous care

that was taken day by day,

building by building to make sure

there were no unintended explosions.

- So here exactly we are staying

there was a blast wall made from wood and glass.

Also, the ceiling is rounded,

as you can see and all walls are painted in white,

because if some acid or nitroglycerin will go out

from the machine you can see it on the walls,

because the color of the wall will changed.

- And how many people are working

in this particular part of the facility?

- Only three of them.

- Just three people

in this room? - Yes, exactly.

- [Macphee] Accidents an sabotage were always a concern.

Laborers would be subject to strict precautions.

- There were regular contraband inspections

on this site to make sure that nobody was bringing in,

either intentionally or not,

anything like matches or cigarettes or metal

that could cause a spark.

And whether it was intentional or not,

whatever you had that could possibly cause a danger

would immediately be confiscated.

- [Macphee] In order to achieve production

with maximum secrecy and further reduce

the risk of explosion,

materials were transported between structures

in either overhead pipes or underground tunnels.

- Typically, if we have a liquid we would pump it

using mechanical pumps across great distances.

The danger of using mechanical pumps

is the potential for electrical shocks

to ignite the nitroglycerin.

In order to avoid this they used a gravity fed system.

- This is one of these tunnels that was used

for transporting chemicals?

- They used pipes and gravity force.

But nitroglycerin in the Second World War

can sometimes be thinner or thicker.

So the speed of the swimming nitroglycerin

can be different so they need

to have exactly the wanted speed.

And this support that you can see is inputted by two screws.

You take out the screws

and change the height of its support,

changing the angle of pipes and you've got

the wanted speed of nitroglycerin that will swim

to the next building.

- So it's quite a genius solution, really.

They're using gravity.

- Yes. - It doesn't cost

very much but it can be individually calibrated

for the production that they've got.

What happens if they get it wrong?

- Explosion.

Every single mistake makes an explosion.

- Okay, so it's kind of a little bit ghoulish, isn't it?

Was it this dark at the time or did they

have electrical lights here? - No, they've got

here the electricity,

you can see it.

Here's the supplies where the lamp was.

And wires, plenty of wires here.

- But this be quite constricted here

'cause you got this tunnel here,

you got the tunnel,

you got the pipe, lighting,

it's pretty tight.

- Yes, but workers couldn't move through those tunnels.

It was forbidden.

That's why they transport those ingredients

in tunnels so a worker that is working

in a different building,

he won't see anything.

- [Ney] So you've got this production that's so critical

but they're also trying to think about

how they can make sure that anybody here

doesn't actually really know what they're doing.

- [Lukasz] If you don't know what you are doing

you don't have ideas to sabotage it.

And here we are in the next building

where the nitroglycerin was cleaned

with new water.

- Because the water was dangerous,

it had chemicals from the actual production.

- Yes. - And they couldn't afford

to have them there.

Wait a second, Lukasz,

what does this word say?

- [Lukasz] It's bath.

- So the pipe is coming down here

and it's going through this?

- [Lukasz] Yes.

- And this is original lettering?

- [Lukasz] Yes, from the Second World War,

made by Nazi Germans.

- So I see this isn't the only stamp you got here,

you got one here.

And what's this here?

- Here was a switch that was changing the nitroglycerin

that was incoming to this building here

and you can change the switch to put this nitroglycerin

that is with new,

clean water, to the next building.

And you've got numbers of the buildings

as you can see here.

- So and engineers would be standing right here.

- Yes, exactly - 80 years

ago controlling the flow

of one third of all of the production

of German military explosive materials?

- For the Eastern Front, of course.

- From this very spot?

- Yes.

For the nitroglycerin that went

and killed people on the Front.

- I mean, it's a production facility

and it is so important

to what the Germans are doing in the war.

It kind of brings a chill to your bones.

- It's a little,

I guess, goosebumps here.

- So where are we now, Lukasz?

- Here was the mixing nitroglycerin with new, clean water.

- We've actually got a larger piece of the glass here.

So you can see it's pretty thick.

You got the

lacing of the wire. - And the wires, yeah.

- But of course we're talking about explosive power

that was so strong that this glass

would've been smashed to pieces.

- Yes, but with wire,

huge pieces not small ones.

- And of course it's through this glass that these workers

would've been looking out with the only access they had

to the outside world once they were working here,

cut off really from the rest of life.

I don't know why but just the cut of the glass

or the light or just the sheer fact

that it's still here after so long,

it makes me feel kind of uncomfortable.

It's almost like this glass is a symbol of something bigger.

- [Macphee] By 1944, it was estimated

that Dag Bromberg produced 30% of the demand

for munitions on the Eastern Front.

- This is a critical base for Hitler and the Nazis.

It's pivotal to success in the war.

- [Macphee] But despite the extraordinary precautions

the Germans took there was always a threat

to the secrecy of the complex.

- Poland has the biggest underground resistance

of the entire Second World War.

They've got eyes everywhere.

- [Macphee] One event jeopardized this critical complex

and even threatened the entire Nazi effort.

Dag Bromberg was a secret 10 square mile munitions factory

camouflaged by the Nazis in a forest in Poland.

The Germans took extraordinary precautions

to keep the complex hidden from the allies

and safe from its explosive products.

But despite that threats lurked

in the forest around the base.

- Just one sliver of information about what was going

on there in that camp from the local resistance,

just one act of sabotage could've blown the cover

of the entire complex

and led to its downfall and destruction.

- Standard procedure for a facility like this

would be to pat down and check everybody

on the way in,

check everybody on the way out,

limit the access of anybody working there

and station guards at every entrance, every exit.

- [Macphee] To keep factory operations secret

construction of the base minimized any contact

with the nearby population.

- This is a critical base for Hitler and the Nazis.

It's pivotal to success in the war.

They were gonna go to great lengths to make sure

that this facility wasn't discoverable.

They needed to make it completely independent

from any surrounding communities,

any outside influence.

And so they essentially built a town,

a place to live and work all under the cover of the forest.

- So that everything it needed it had to provide on site.

So we're talking about electricity,

its own sewage system,

its own water supply.

It had its own laboratories,

its own medical facilities.

This base was so highly classified

that even the workforce had no idea

what it was they were building.

- The Nazis were very good at propaganda,

at deceit and they used it non-stop throughout the war.

But one of the more absurd examples

is this massive concrete hardened facility and tunnels.

They announced that it was to make food,

including chocolate and fruitcakes.

At least come up with a better lie.

This is laughably absurd.

- [Macphee] The Germans were taking no chances

with the secrets and safety of the base

and created separate production lines and workforce.

The two halves of the factory would be separated

by the railway used for production,

reloading and importing raw materials.

- The western half of the complex was codenamed Coal.

It contained production lines for nitroglycerin,

for smokeless powder and for nitrocellulose.

It also had a testing facility for each of those.

And in the eastern half of the complex codenamed

[speaking in foreign language] or heat in English,

they had production lines

for trinitrotoluene and dinitrobenzene.

- To ensure further secrecy at this site

the areas were color coded.

Therefore the person that worked in the nitroglycerin area

would have had a completely different color code

from someone who worked with smokeless powder.

- Making this all the more remarkable

how the Nazis kept this a secret,

the Poles had an act of resistance.

They do manage to set off a small explosion.

- [Macphee] In 1944, Operation Cream resulted

in an explosion in the nitration building

that destroyed rare factory equipment

and killed German engineers.

- You can see that the whole tower is damaged.

This concrete is like house of cards.

- It's quite literally snapped off

by the force of the explosion.

And this is exactly as it was when the bomb blast went off?

- Yes.

- [Macphee] The roof of the building was so badly damaged

it had to be removed and replaced.

But production was back up and running in just three months.

- So the explosion was violent.

These chemicals are hugely volatile.

It killed everybody

in there. - Inside the building.

- And yet the Germans are immediately repairing it

and immediately getting that production

facility back. - Yes.

- And three months is a pretty short amount of time.

- Yes definitely, they need this nitroglycerin.

It was one of the most important things here.

- And so this is the concrete you're talking about?

- Yes, this is the rooftop that was exactly here

in this building just behind us.

So they just put this whole concrete here,

rebuilt the building and start again with the production.

- So you got this enormously thick concrete slabs

that just been literally exploded in the air

and torn up like paper.

And then you've got this concrete rebar here,

also really solid and that's just,

in the case of these explosions here,

has been snapped in half like a stick.

- It's really damaged because of one,

only one explosion here.

- Again, it just goes to show just how fragile

this production facility was to explosions,

bombs from the air,

all the kind of mistakes that led to this blast.

- [Macphee] The explosion was so powerful

it even caused damaged to underground tunnels nearby.

- [Lukasz] Here you can see again the collapses

after the explosion.

- Wait a second,

so the blast from the production facility over there

has actually caused this tunnel here to crack?

- Yeah. - Man, that's

actually surprisingly powerful,

'cause this is rendered the two walls apart.

They've literally cracked.

- Yes, because here we've got the angle,

so the blast went inside the angle

and just separate those parts of the tunnel.

- So I imagine the German military commanders

and the engineers here must've been quite worried

when the explosion went off,

because they're thinking. - Yes.

- Have they really done everything

to prevent a chain reaction explosion?

- The Polish resistance is impressive

and it's reasonably well organized

and it's pretty substantive.

But how they were not able to penetrate this facility,

how they were not able to notify the Allies

of precise maps, coordinates,

so that it could be bombed is a disturbing reality

of this facet of the war.

- [Macphee] As the tides of the war began to turn

the factory grew in size and importance.

- The Allied forces would have given their right arm

to know what was happening there?

- [Macphee] What mysteries were concealed in Dag Bromberg?

- So the secrecy is not just from building to building,

it's quite literally from room to room.

[clock ticking]

- [Macphee] Dag Bromberg was a secret Nazi factory

hidden in a forest in occupied Poland.

- The Germans aren't putting this kind of effort

into every single munitions factory plant they've got.

In fact, this place is really quite special.

- [Macphee] The factory was critical to the war effort,

providing one third of the total munitions

for the Eastern Front.

In 1942, Dag Bromberg begins

its mass production of gunpowder.

- Okay, we're coming through the trees and I can see

what looks to be the biggest building we've seen so far.

- [Lukasz] Yes, this is the building where the nitroglycerin

was mixed with nitrocellulose

and there you've got gunpowder.

- The building is absolutely enormous here.

How many people worked here?

- One people on each floor and the third part of it

on the right side that you can see was closed.

- I mean, the structure itself is very imposing.

It's a little ominous looking.

I don't know,

maybe it's because it's been damaged

and destroyed but it's kind of,

I don't know,

I get the sense of evil lurking here.

- [Macphee] The secrecy of the factory was so strict

even the workers had no idea what they were making.

- We found a few years ago a woman that worked here

in this building exactly.

She didn't knew anything about the third part

of this building that she was working in

because it was forbidden to go

across those stairs here to the second part

and what about the third part that was closed?

- So the secrecy is not just from building to building

or from complex to complex,

it's quite literally from room to room?

- Yes, exactly.

- So I mean you can definitely say

that the whole secrecy operation that the Germans had here,

the Nazis put in place, worked,

because people who were even in this complex

don't really know what's going on here.

- Yes, exactly.

You knew only the place and the knowledge

about the place where you are working and that's all.

- [Macphee] In its first year the factory produced

over a thousand tons of gunpowder.

By 1944, Dag Bromberg would produce 13,700 tons

of the material.

- [Ney] What on earth are we looking at?

- Here there was something

like a huge bath with nitrocellulose.

And here on the wall you've got switches.

The letter A is for open.

The Z is for closed.

So you open those baths and the nitrocellulose

is falling to the mixers.

- So the Nazis have taken the secrecy so seriously

that even that person doesn't know what they're doing

and you've got thousands of people working here,

relatively small workforce,

but they don't know what they're doing.

- Yes.

- The only people who do know are the people

who are getting shot at and explosive artillery rounds

coming flying at them 'cause they're the ones

who are at the receiving end of this production line.

- Yes, that's right.

Here is the hole that we saw from the higher level

for throwing the nitrocellulose.

Here

were huge mixers

that makes nitroglycerine and nitrocellulose

and there was only one person working here.

And in those pipes that you can see here,

nitroglycerin came here with water to those two mixers.

And because nitroglycerin came here with water

the gunpowder was really, really wet.

So it fall down to the next level

where we have a huge centrifuge machines.

On a huge speed the water was thrown out

and the gunpowder went to the next floor.

And again, this building is a dangerous one

so the ceiling is going up.

This corner, as you can see,

is rounded and we've got the blast wall from wood

and glass of course.

- You've got all of this material coming flying through.

You've got lorries and trucks coming out

taking the material out.

Is it round the clock, yeah?

- Yeah.

In the night time they use curtains here

to just black out the factory because it's really easy

to see in the middle of forest plenty of light.

- So the war is going on 24/7,

so the Germans can't afford to let production stop.

It has to keep on going.

- It's impossible to stop the production

because the explosive materials all the time need

to be mixed and it's impossible to stop the mixers

because there's a huge chance for explosion.

- Again, when you just think about the magnitude

of what was produced here,

not just for one month or one year,

but throughout the course of the Second World War

to kill millions of people.

It's eerie being here.

- Yeah.

- When you talk to the survivors,

people who worked here,

do they mention that?

- No, maybe they don't wanna think about it.

You are making ammunition to kill your own friends.

After the centrifuging the gunpowder

was a little bit wet so it was put it into bags

and transport it to the next building.

This is the place where the hooks were for the bags.

And here on the,

let's say roof,

of this tunnel the bags were transported

to the next building where they used rollers

to just take the last of water from the gunpowder.

Then it was cut it into smaller parts,

put into crates and transport it by train

to the next zone of the factory.

- Just give me a sense of the scale here,

how many bags are coming out of this production facility

on a daily basis?

- [Lukasz] To be honest,

we don't have any information about that

but millions of ammunition were made here,

so it was plenty of bags.

- [Macphee] Despite this massive output of explosives

and gunpowder the German army was losing ground.

- As the tide of the war turns the Germans

become ever more reliant on the Wunderwaffe,

these wonder weapons that Hitler believes

are gonna transform his chances and actually lead him

to winning the war.

- [Macphee] The work at Dag Bromberg shifted

to secret weapons the Nazis needed to give them an edge.

- One can only imagine what they were working on.

- [Macphee] What secrets fell hidden

behind the iron curtain?

World War II.

The mysterious Dag Bromberg Factory in Poland

was producing a huge portion of the Nazi's munitions

all while avoiding Allied discovery.

- The efficiency of this complex is absolutely spectacular.

They're producing 600 kilograms of nitroglycerin an hour.

20 million riffle shells every day.

- [Macphee] Between 1940 and 1944

the Nazis dropped 650,000 tons of bombs on the world

but the Germans struggled to match Soviet tank production.

Desperate to find an edge the Germans developed

never before seen technology or wonder weapons.

- At Dag Bromberg the German scientists

were manufacturing a bewildering array

of explosive materials and weaponry.

One can only imagine what they were working on

in terms of for future projects,

what kind of research was going on?

And yet again,

this is possibly another secret

of the Nazi regime that eludes us.

- [Macphee] In 1943, a testing range for ammunition

and rocket weapons was established at the factory.

This would be the training ground

for secret new weaponry built in the factory.

- One of the secret projects that was developed

and tested at Bromberg was the 250

and a 500 kilogram aerial bombs that were fitted

to the Nebelwerfer.

- The Nebelwerfer is a multiple rocket launching device,

the likes of which no army had ever used before

and it was gonna be a game changer

on both the Eastern and the Western Front.

Now, I've spoken to people who have been hit

by Nebelwerfer attacks and it's a terrifying experience.

They were loaded with gasoline.

They would burn everything around them.

- [Macphee] Among the many secret developments

in the last years one in particular,

an experimental weapon codenamed Rheinbote

could've changed the war.

- The Rheinbote was a V4 ballistics rocket

which unlike its sisters,

the V1 and V2,

had the added benefit of being highly transportable.

- Now ultimately, the V4 is not a successful weapon.

It can only carry a 40 kilo warhead,

which is just 6% of the entire weapon compared to,

say, 90% of the V2 rocket,

but it can travel a at 6,800 kilometers an hour.

- [Macphee] It was the only long range ballistic missile

to enter into World War II,

but the Rheinbote would never be used for its intended role.

- It's accuracy was found to be wanting

and there were troubles with its fuse system

whereby it would bury itself in the ground

before the payload detonated.

And with such a small payload onboard

that never caused any huge damage.

[clock ticking]

- [Macphee] Production at Dag Bromberg

would carry on almost to the last minute

when it became clear that the Germans were facing defeat.

- On their departure the Nazis destroyed any evidence

and documentation about what had been developed there.

You can imagine that this was an entity

the Allied forces would have given their right arm

to know what was happening there

and get hold of that technology for themselves.

- [Macphee] The Germans evacuated only five days

before the Soviets reached the city of Bydgoszcz.

- So the Red Army hordes roll over the Polish border.

They've come to liberate Poland,

or have they?

Actually, it's the next occupying force.

The Soviets replace

one totalitarian dictatorship with another.

They're here to stay and they're here

to take away anything they want.

- Remarkably, over one thousand trucks and wagons

were used to transport everything back to the Soviet Union.

Over one thousand,

that's how many secrets,

that's how many materials were inside Dag Bromberg.

Only after the collapse of the Soviet Union

is the world given just a glimpse behind the curtain

of what was happening there.

But by then the ghosts of Dag Bromberg are long, long gone.

- It's amazing, really.

The distribution of this product across this stretch

for years is an amazing logistical feat.

And to have this all done in secrecy is a testament

to the engineers and the people

behind the design of these structures.

- It's amazing, we've been walking for half a day

and we've still only seen a tiny spec

of the Dag Bromberg Complex.

- Yes, 1% of 25 kilometers square area.

- The Nazis managed to build a city in a forest.

This is straight out of a Hollywood movie

or some fantasy novel or something.

An entire city with tens of thousands of workers.

Scientists building massive amounts of armaments.

The most evil things of the war

were built in Dag Bromberg,

many of them.

And yet, it was a secret not for a year,

not for two,

not for three,

but the duration of the war when the world's attention

is turned toward the Nazi empire.

This is absolutely one

of the most remarkable accomplishments,

unfortunately, of the Nazi regime.

- [Macphee] At Dag Bromberg the Soviets took everything.

The equipment, materials and technology

would be a major factor in the development

of modern Russian aircraft at the end of World War II.

- Now, we know of course that the Americans

under Operation Paperclip were getting the brightest brains

and the best technology back to America

to begin eventually the Space Race.

Is it possible that the Soviets got their hands

on the V4 Rheinbote technology at Dag Bromberg?

And if so,

what other Nazi secrets did they get their hands on?

- [Macphee] Today, the ruins of the Dag Bromberg Factory

stand as an eerie reminder of the havoc

and destruction its products unleashed on the world.

But some of the site's past experiments

and secrets may be buried forever.

[dramatic music]