Original subtitles

- [Narrator] In total secret,

Germany rapidly builds a series of complex hydro works

in its eastern territories.

- This is an immense infrastructure project.

It really is on a grand scale.

- [Narrator] Damns, canals,

and swinging bridges are erected.

- Germany created it only in two years' time.

- [Narrator] But this is only the beginning.

Construction morphs into massive tunnels,

bunkers, and concrete teeth built into the earth.

- You look at it, it just looks [pausing] evil.

- [Narrator] But what's beneath these structures

is something else entirely.

- It's an underground city down there.

- [Narrator] Allies are unaware

of Germany's true intentions.

- It would take a lot of effort to hide all of that material

that's coming on to site for how many years?

- [Narrator] What could the purpose be

for such an elaborate construction project?

- You have to understand, this is a project

costing hundreds of millions of Reichsmarks.

This is one of the biggest things they'll do

in 1930s Germany.

- [Narrator] The Nazis have frightening plans

for this region and intend to protect it at all costs.

[suspenseful orchestral music]

[door slamming]

1932.

Post-World War I.

Germany is forbidden from rearming,

but a lot has changed since the '20s.

The Nazis are coming to power and they have ambitious plans.

But unbeknownst to the Allied powers,

construction on a vast project begins in total secrecy.

Under the guise of a highway being improved,

massive construction

in the eastern region of Germany begins.

Forests are cleared,

canals carved into the terrain,

and secret tunnel construction begins,

as an ominous project slowly takes shape.

- The Nazis were obsessed with secrecy,

and in all respects they went to great lengths

to try to deny realities of what was going on.

- The simple truth is if you talk,

you're imprisoned [pausing] or worse.

- Amazingly, they were able to do a lotta this

straight under the noses of the world and get away with it.

- [Narrator] Germany sets to altering the landscape forever

with an expensive hydro system.

- If you are today in the area of Festung,

you see beautiful nature, full of rivers, forests.

But if you realize that there is only one river

and hundreds of the fortified canals,

very artificial, built by Germans in the '30s,

you realize how this huge system was extent,

and Germany created it only in two years' time.

- [Narrator] One of these secret construction projects

is a massive bridge.

- Water's a big force.

It can change the landscape.

It can be unpredictable depending on its source.

And building in water is really hard as well.

Think of building a bridge.

You have to span over this length of river

and build abutments and then build the structure

to bridge those abutments.

It's not an easy task.

- [Narrator] But the complexity of this build

doesn't end here.

The bridge is imbued with a special ability:

It can be swung out

and then brought back to a neutral position.

- When you think about wanting to move a bridge,

you're now merging two systems.

You have this structural system which is static, a bridge,

but then you're turning it into a machine

by introducing gears and motors and pinions and racks

and counterbalance and ropes and pulleys.

It's changing the nature of this structure

to be much more complex.

- [Narrator] Even in the '30s,

this bridge was outfitted with an electrical system

that allowed for the easy movement of its platform.

- Moving a bridge of this size is a complex feat on its own,

especially considering the electric systems

that were available in the 1930s.

But it's also been designed and built

with a backup ability to be opened manually if needs be

with the force of just one or two people.

And it's amazing to me that it still works

to this day, some 80 years later,

because the Germans were at the forefront

of technology and engineering.

- [Narrator] But the design of this bridge

seems to raise only more questions as to its true purpose.

When engaged, the platform swings out into the canal,

seemingly cutting off water traffic.

- Movable bridges today

allow for two types of traffic to use them.

We have vehicle traffic on top

that goes over the bridge when the bridge is in place,

and then we move that bridge outta the way

via a swinging motion or a tilting motion or a lift motion

to allow for marine traffic

to go under and through the waterway.

But this bridge had a completely different intent.

- But when this bridge opens,

it's right in the middle of the river still.

It's still blocking the waterway,

which doesn't seem like any kind of design of bridge

that you'd need for building a road or a highway.

So, what is going on here?

- [Narrator] Perhaps a clue for Germany's motivation

could be gleaned from the location itself.

While this territory is now part of Poland,

it was previously occupied by Germany.

- This is territory which is really vulnerable.

It's quite open, relatively flat.

It's a fairly accessible corridor for a foreign army.

- [Narrator] And this area is very close

to the heart of the Third Reich: Berlin.

In fact, one can travel by vehicle from this region

to the German capital in less than three hours.

- So, it hits all of the high marks

for German obsession with it, right, to protect it.

- [Narrator] Could this structure's proximity to Berlin

explain its design?

- The fact that when this bridge opens,

not only does it still block the waterway,

it also creates a gap in the roadway,

and it makes you think, "Maybe that's exactly the point.

"Maybe this is a defensive structure

"to prevent people accessing

"what's on the other side of the bridge."

- [Narrator] Suddenly, a clearer picture

of Germany's intention for this bridge begins to form.

- It was used to control who and when

was able to traverse that waterway.

- [Narrator] But the swinging bridge

is only a piece of a much larger puzzle.

Dams and canals are put into place throughout the region

with the ability to quickly flood areas.

- It is a little medieval, using water to instill chaos

or control the access to this parcel of land.

We don't really use water in that way today.

We use water in other ways.

We harness it for energy turbines to generate energy.

We use water for good. [laughing]

This was a much more sinister use of an energy source.

- To build a concrete wall to prevent trucks and tanks

from approaching your position is very expensive.

We have to move a lot of material to construct a wall.

It's not simple, it's not easy.

Whereas if I build a couple of dikes,

I build a couple of dams and have some controls to pump

or to allow water to flow naturally,

I can achieve essentially exactly the same thing

with far less manpower,

with far less movement of material and goods.

- Any time an army has to cross a river,

it becomes more tricky.

So, if you up the ante by building

some complicated fortifications,

you make it easier to defend.

- If we imagine those invading forces

trying to seize the area and trying to reach Berlin,

for sure, they would be stopped by this hydrology system,

which makes the area around impossible to break through.

- [Narrator] While a few waterworks

to allow for a highway might be one thing,

this is on a scale so immense

it has the rumblings of a fortification

rather than roadwork.

- This is an immense infrastructure project.

I mean, it really is on a grand scale.

But it was done in a way that wasn't grossly obvious

because you were building small pieces at a time

that built up to a much larger thing,

rather than having tens of thousands of people

building one large obvious structure all at once.

- [Narrator] What could the Nazis' intentions be

for this region and the massive hydro works

that they've created?

- Hitler's looking around and seeing threats everywhere.

The French have already started building the Maginot Line,

the world's most technically advanced static line of defense

from the late-'20s onwards.

So, he sees this threat from the west,

and Hitler's answer to that is his own Maginot Line,

the Festungsfront on the eastern border.

- [Narrator] Beyond the hydrological defenses,

an even more massive construction project

begins in the region.

Steel and concrete is quickly smuggled into the area.

- Could this have been interpreted as a highway at the time?

This was very obviously military-related.

You look at it, it just looks [pausing] evil.

- I can't imagine how you can keep a secret.

The continuous delivery of trucks

full of aggregate, full of concrete,

full of just all of the materials

that you need for your construction.

- [Narrator] Polish spies and intelligence

manage to infiltrate the area

and suspect that this is something

far greater than highway construction.

- Polish intelligence have absolute knowledge

that from the other side of the Polish border,

something about 30 kilometers from Poland,

Germans are building huge fortification line.

- [Robert] You get a sense of real dread

because you've faced the German army before.

You know that these are fortifications

the likes of which you've not really seen before

at any point in the war.

And this is not a state that's giving up easily.

- The road to Berlin is wide open.

Festungsfront is the gate that slams it shut.

- [Narrator] But this is just the tip of the iceberg.

Unbeknownst to Soviet and Polish intelligence,

Germany is building something underground

that is more secret and colossal

than they had ever imagined.

Across a vast area in eastern Germany,

a massive construction project is secretly underway.

Germany believes whatever invading force comes

will be vastly different

from what it had previously seen in World War I.

But this is just the tip of the iceberg.

Germany sets to building hundreds of concrete structures

across the Festung region.

Jutting from the earth, over three feet tall,

are some cement dragon's teeth.

- They stretch as far as the eye can see.

- It's obviously purposeful and out of place,

and it gives me a sinister feeling.

- [Narrator] These aren't merely concrete structures.

They're reinforced with steel

to ensure their destruction would be nearly impossible.

- In general, as engineers,

when we see a reinforced concrete structure, we start going,

"Oh my god, how am I actually going to move this?"

- What on you see on the surface

are these dragon teeth jutting out of the ground.

But what you don't necessarily see

is that there are huge great slabs of concrete underground

that connect all these teeth together.

They combine them as one great long carpet

of dragon's teeth, if you'd like.

- [Narrator] What could the purpose

of these jagged structures be?

- World War I has shown that the next war

will probably be mobile.

Tanks almost played a crucial role,

and the Germans were caught on the back foot.

Hitler's determined not to make that mistake this time.

- They recognized that tanks were going to get bigger

and better and faster and stronger.

You need to build a blockade to prevent an attack

of this mechanized new military machine

that you need some incredible embattlement,

some system to prevent those tanks

from reaching your facilities.

- There's no way any motorized vehicle

was gonna make its way successfully across this.

- [Narrator] Why would Germany favor such a strange design?

- Another option to stop vehicles

might be a really large wall.

You could build a vertical wall.

That would also need a really large footing

underneath the ground to prevent that wall from overturning.

Whereas with the dragon teeth,

we have individual superstructure,

structure that's above the ground,

tied together with a more efficient underground footing,

so you'd likely use a lot less material.

You don't have to build all the vertical rebar,

you don't have to form up the vertical wall,

and you have probably a smaller footing

to deal with that overturning.

- The construction of the dragon's teeth at Festung

is so strong that after the war,

Polish army would like to use it

and they were trying to remove it.

But they used so many resources to do it,

but totally unsuccessful.

That's the proof how strong it is

and how complicated the construction is.

- [Narrator] While the dragon's teeth

are powerful enough as they are,

they still possess yet another devastating function.

- As if the front itself wasn't heavily defended enough,

the Germans even think up this mini canal system

connected to the dragon's teeth

which could be flooded at a moment's notice.

- [Narrator] The dragon's teeth wouldn't stand alone

to fend off an attacking force,

but act in tandem with other elaborate defensive measures.

- The dragon's teeth are the foundation,

kinda this concrete permanent structure

of this defense line, but there would also be trenches,

there would be barbed wire,

there'd be other mechanisms in there

to deter people from going in, soldiers,

and other smaller vehicles passing through.

It was part of a system that was built up

and layered upon to build an entire defense line.

- It's absolutely massive.

It's designed as effectively a dividing military line

between themselves and the eastern territories.

- [Narrator] Beyond this complex series of waterways,

trenches, minefields, and anti-tank barriers

lie some of the most sophisticated

defensive structures ever built.

Hitler had been a corporal in World War I

and saw firsthand the downfall of primitive trench warfare.

- Germany really learned a lot from World War I.

A huge amount of the military casualties in World War I

were as a result of the conditions in the trenches.

- And the biggest problems were, one, rain,

'cause the men were out in the open

and any trench or bunker would flood;

two, rats, the infestation from rats; and three, disease.

If it was a rainy season,

you were ankle or knee-deep in muck,

and diseases were spread.

- [Narrator] In 1929, France has started to develop

the next generation of static warfare: the Maginot Line.

Comprised of interconnected bunkers,

it was to be the next generation of static warfare.

- So, the Nazis are in an arms race with the French.

The French have the Maginot Line,

and now the Nazis have Festungsfront.

- What these fortifications do is to try to close up

that really vulnerable corridor into Berlin.

That's a knife that is thrust at the heart of Germany

in no uncertain fashion.

- [Narrator] Germany's answer to what's been seen

on the Maginot Line is the Panzerwerk,

a heavily-defended bunker.

- Those bunkers are like nothing we've seen

anywhere else before.

- [Narrator] Germany develops

its own advanced bunker system, the Panzerwerk.

- This system would enable the Germans

to maintain almost indefinitely a defense.

It was really ideal for making sure

nobody could ever get past this point

and was an innovative idea to address all the issues

that occurred in the trenches in World War I.

It really was an amazing undertaking

to protect themselves from future attacks.

It's gonna change the battle entirely.

- [Narrator] The bunkers were built

with massive concrete walls

that were between three and 10 feet wide.

- If you spot someone pouring

nine- or 10-feet-thick concrete walls,

that's something to pay attention to.

- Whatever is down there,

if you're pouring that much concrete,

you want it to be protected.

- Reinforced concrete is a great way to construct a building

that starts to become virtually indestructible.

Short of launching massive ton-sized munitions,

these structures were gonna be essentially impenetrable.

- The bunkers are also built into the hill.

They're using this natural landscape to help build form,

to create these bunkers, to hide them as well,

and to allow these structures to be buried

to protect them from any impacts

from bombs or other artillery.

- [Narrator] What could the Nazis be hiding

beneath these bunkers

that made them go to such great lengths to defend them?

Germany sees France's Maginot Line to the west

and sets to creating its own massive defensive line

called the Festungsfront to protect the road to Berlin

from any eastern-invading forces.

- What on Earth is going on down inside these bunkers

that would make the Nazis put so much effort,

so much expense into fortifying them?

- [Narrator] Fixed atop each bunker

were steel domes known as cupolas.

These armored domes were constructed

with thick steel walls at least three feet wide.

- Some of these cupolas weighed

the equivalent of two T-34 Russian tanks.

That is incredibly heavy!

- Those cupolas was extremely strong and powerful,

but also extremely expensive.

- It's not necessarily overkill, though,

if you are designing it for a specific purpose.

You must be putting them there

so that it protects you from bombs.

- You can see one of the cupolas has gouges out of it.

It looks like clay almost,

and those are remnants of shelling.

And what happens is the shell with this huge energy

hits this steel and the steel deforms

and dissipates that energy through deformation of the steel.

- [Narrator] These weren't simply domes.

Utilizing unique mechanics, each cupola could be rotated,

becoming a deadly turret.

- There had to be some sort of track at the bottom

and some sort of wheels to, one,

bear the weight of those cupolas,

as well as allow someone to manually rotate these cupolas

via a gearing system to translate

that energy, that human energy,

into moving this massive chunk of steel around.

It's likely a pretty efficient system.

- [Narrator] Concealed inside of the cupola

were state-of-the-art machine guns

capable of frightening firepower.

- To top it all off, the Nazis bring the MG 34 to bear.

This was considered the most advanced

general purpose machine gun of its time.

In '36, they're already ready

and they're arming that line to the teeth with them.

I've seen them myself,

and the idea of trying to charge up against this machine gun

capable of firing 900 rounds a minute,

it brings chills to my body.

- [Narrator] World War I brought a frightening new weapon

to the battleground: mustard gas.

- The prolific use of mustard gas in World War I

prompted the Nazis to build this complex

so that it was protected against any gas attacks.

- You had to think about, "How do we deliver enough oxygen

"for the occupants," and manage also these explosions

that were happening that could suck that air pressure out,

that could change the toxicity of the air,

that could cause people to die, to suffocate.

- But it also had state-of-the-art

air filtration and ventilation systems,

meaning that if they needed to,

they could stay underground for months at a time.

- [Narrator] Tucked away in one of the rooms,

experts find the remnants of a mysterious machine.

- When you look at this machinery

and the room that it's all contained within,

it's quite eerie, to be honest.

You get a feeling deep down that this is all geared

toward something quite brutal, something quite sinister.

- [Narrator] Pipes lead from this ominous machine

to the surface level of the bunker

and then stretch all the way down to massive reservoirs.

- When you look at the mechanics

and the infrastructure within this room,

it seems to be that it's all about moving

large amounts of fuel and moving it very fast,

which to me says one thing: flamethrower.

- Positioned at the top of each of these bunker complexes

is a two-and-a-half-ton FN flamethrower

capable of firing out sheets of flame

that would've incinerated anyone for up to 60 meters.

- Fire is a very effective deterrent.

A soldier's far more willing to charge

against a position with a gun

than they are with a flamethrower.

It's visually obvious to the attacking soldiers.

It's a psychological barrier for the attacking force

to try and take that position.

- The rush of flames, the visual, it's just fear-inspiring.

It would be crushing.

- [Narrator] Each FN flamethrower came equipped

with a staggering reservoir of fuel,

which housed an impressive 2,500 liters of fuel.

- For comparison, today's SUVs,

an average SUV has 50 to 60 liters of fuel in it.

This flamethrower had access to 2,500 liters of fuel.

That's 50 times a single SUV,

and it would burn 27 liters of those in one minute.

That would burn half a tank of your SUV fuel in one minute.

It's unreal.

- I just can't imagine the intensity

or the size of flame that that's gonna produce.

Absolutely terrifying!

- [Narrator] Still deeper in this bunker complex

awaits another secret.

- As if all of these defenses weren't enough,

the Germans have the MG 19, a grenade launcher

capable of firing 120 rounds a minute.

If you're an advancing infantryman,

not only would the ditches slow you down,

not only would the barbwire stop you,

but these grenades will cut you to pieces in seconds.

- [Narrator] The MG 19 grenade launcher

contains one of the earliest targeting systems,

accounting for trajectory, wind speed, and velocity.

This weapon could deliver a shell

on advancing forces with deadly accuracy.

- In the 1930s, the Germans started developing

systems that would automatically calculate

all of those parameters for you

so that you could more accurately guide the shell

or direct the shell to where it needed to land.

- [Narrator] Still, there was yet another devastating weapon

housed in these concrete bunkers:

the MG 08 heavy machine gun stationed at the bunker door.

This weapon was capable of delivering a deadly barrage

that could devastate enemy troops

with a range of over 6,000 feet.

The design of the bunker was so complex

it even had special modifications

which protected the operator of the MG 08.

- In the bunker wall, they would've had openings

for machine guns, and the openings are stepped.

So, at the interior, the opening is quite small,

and it steps out as you get to the exterior of the wall,

and this would allow a machine gun

to protrude out into the opening

and allow it to maneuver from left to right.

And it also protected the user by stepping in that opening

to keep that opening as small as possible,

but allow the range of motion of the gun

to be as wide as possible at the same time.

- [Narrator] But the deadly design

of this bunker doesn't end here.

If invading forces manage to make it into the bunker,

a surprise awaits them.

A strange metal sheet would swing up,

revealing a pit below.

- It's a trap door.

It's like something out of a medieval castle.

I guess it proves that sometimes the old methods

are the most effective.

- Even if you get to these positions

and you start taking these positions,

you're certainly still not gonna be able

to get into the larger complex

and attack the soldiers therein.

- The Nazis, they wanted to build themselves

and protect themselves

further and deeper into their bunkers,

and how can they do that?

How can they stop people from entering further?

So, various characteristics like narrow hallways

and inset machine guns here and there,

and traps, essentially, to prevent any sort of assault

from getting deeper into the bunker.

And they would have stores and safe spaces

further in their bunker for them to survive, basically.

They wanted to lock themselves in.

- The Festung is the most advanced

and modern fortification system maybe even in 20th century.

- When you actually look at these structures,

they're terrifying.

They have enough armament

and capabilities to defend themselves.

They have enough people and supplies

almost to defend themselves in perpetuity.

"Even if the attackers come,

"I'm comfortable sitting in here, defending myself.

"Let them wear themself out

"and fall at the feet of my door in futility."

That's kind of what these structures were.

- You put this together, the heavy artillery,

the dragon's teeth, the canals,

the ditches, the barbed wire,

the machine guns, the grenade launches,

and then, to top it all off,

this complex series of flamethrowers;

I mean, if any infantrymen had been able to get close,

they would've been absolutely terrified

and it would've been sheer murder.

- [Narrator] This is just the tip of the iceberg.

Beneath these bunkers lie even more secrets.

Breaking the terms of their defeat in World War I,

the Nazis are building a top-secret massive complex

of defensive systems to guard the gateway to Berlin.

- This is the advent of the Autobahn,

the advent of these mega-projects

that are really on a scale

that is well beyond what man had done previously,

that would be something that any attackers

would not be anticipating in that era.

I mean, this is totally new to the world.

- [Narrator] Inside of the bunker,

a secret lurks in the bowels of this defensive line.

Buried up to 180 feet

are a series of interconnected tunnels so massive

it questions how they were constructed in the first place.

- Imagine if you had made it through

all that defensive concrete.

Well, it turns out that's just the beginning.

What's beneath your feet

is a whole network of underground tunnels.

It's an underground city down there.

- It's extensive.

There's a lot of planning and engineering

and building and resources and thought that goes into this.

It's difficult to wrap your head around

how they were intending to basically live here, if need be.

- If you imagine that you can go downstairs

and walk through these tunnels for three or five days

and you will still not reach another part of that system,

you can realize how huge it is.

- [Narrator] These massive tunnels were bored into the earth

by the company Siemens & Halske,

who had built the Berlin subway system.

The tunnel system impressively connects

to the hundreds of bunkers above ground

and stretches for miles underground.

- Imagine tunneling in the 1930s under duress

and having this ambitious scope and length of tunnels.

It changes that whole dynamic.

It makes a lot riskier.

- Tunneling in any environment is challenging.

And the technology that is available now

has advanced way more, and it's still very challenging.

So, to think about tunneling in the 1930s

and to think about the length of these tunnels

and how deep they had to go

with the technology that was available then,

it must've been an insane amount of work.

- [Narrator] Even more astounding,

these tunnels were bored in sand.

- Tunneling in sand is an unstable endeavor.

It's loose, it's not compact, it doesn't self-support.

Not only did they have to excavate the sand,

deal with that excavated material, get it out of the way,

but they also had to pump concrete down

to re-support these tunnels

so that they wouldn't cave in on themselves.

- [Narrator] France's Maginot Line

only contains six miles of tunneling,

which connected to select bunkers on the surface.

But here, the Festung tunnels stretch for over 20 miles

and connect to every topside bunker.

To top it all off, these tunnels were outfitted

with an impressive rail system that was entirely electric.

- When you understand there were underground train networks

which connected at least a few dozen

elaborate tunnel systems-

- If we imagine the modern subway

with all those tunnels and subway stations,

it's quite comparable, in fact.

In Warsaw, we had less subway lines than in the Festung.

- [Narrator] This rail line could transport troops

across miles of tunnels to defend any location,

should an attack occur.

- This is the next level of military defenses.

Talking about bunkers with up to 10-foot-thick walls

with shafts running down under the ground to 180 feet

to reach rail tracks that allow you to travel

across 22 miles of interconnected systems.

- If you can't get the food

and the resupplies to the troops,

you're not really gonna be effective.

And building an underground electrified railway

was a great way for the Germans

to ensure those troops were well-supplied, comfortable,

and made sure they had everything they needed

to sit there for literally months of attack

and not have any real supply issues.

- In the underground tunnels, there were storages,

bunkers, a lot of ammunition, lot of food supplies.

So, for three months, they can act in total autonomy.

- Everything you can possibly imagine is there,

and you can come away from that

with an obvious sense of dread.

- [Narrator] These tunnel systems

included their own water reservoir, communication systems

linking the full network of structures,

hospitals, and its own dedicated ventilation system.

Over 20,000 troops could be housed

in these underground tunnels, where they had enough supplies

to last for up to three months.

- These structures, while huge on the inside,

really aren't that giant on the outside.

So, the attacking force is looking at it,

going, "Well, that's really not that bad."

I mean, this is totally new to the world.

Nobody had built any system of this size,

of this interconnected bunkers

that could house tens of thousands of soldiers.

There was a lot of thought put into this design

in order to maximize its ability to defend.

- [Narrator] Germany has spared no expense

in building these fortifications,

and they've come at a great cost.

- Unlike in later complex engineering projects

of the Second World War,

the German Nazi regime have no slave labor to build this.

You have to understand, this is a project

costing hundreds of millions of Reichsmarks.

This is one of the biggest things

they'll do in 1930s Germany.

And it's all under the nose of the Allies.

- [Narrator] The Festungsfront defense line

is extremely technologically advanced.

It requires a specialized military crew

to effectively utilize the wide range of devastating weapons

employed at the bunkers and to operate

the complex systems of rails and the tunnels below.

- In 1938, there was many troops specialized

in fighting in such facilities,

very specialized troops knowing the system,

knowing how to use it effectively.

- [Narrator] This defensive line would be insurmountable

for any invading forces, but things were changing.

In 1938, Hitler inspects the system,

and suddenly construction comes to a complete halt.

- But why?

What possible reason could there be

to just abandon such a huge investment?

- What we see today is just a piece of this whole system.

They didn't end up finishing this project,

but they still achieved so much in six years.

- [Narrator] If it were to be completed,

this site would contain

nearly 13 million cubic feet of concrete.

- That's just unimaginable.

- If we look at Yankee Stadium, for example,

it would not be nearly in that range;

maybe around 20,000 cubic meters.

Here, we're talking 360,000 cubic meters of concrete

that woulda been used had the project gone to completion.

That's a lot of concrete.

- [Narrator] Why would the Nazis put so much effort

into this massive system,

only to halt it before it was finished?

In under six years, the Nazis have managed to create

one of the most technologically advanced

and powerful defensive systems the world has seen.

Then, all of a sudden, before it's completed,

Hitler orders it stopped.

Then, in May of 1940, the Nazis invade

the Netherlands, Belgium, and France

and introduce a new terror to the world: the Blitzkrieg.

[explosions booming]

- Blitzkrieg is all about the rapid advancement of troops

using a combined and coordinated approach,

from the air and on land,

using tanks, infantry, and mechanized vehicles.

- You throw everything at one concentrated point,

and the enemy cannot stop that.

- No one had seen anything like this before.

And it was very different to the static warfare

that had played out during the First World War.

- [Narrator] The shift from static trench-based warfare

to the rapid advancement of troops

leaves defensive lines like Festungsfront in the dust.

- But it's actually the Nazis themselves

that prove that this First World War mentality

of static defense isn't worth the paper it's written on.

In 1940, they carve through the Maginot Line

like a knife through butter.

- World War II was itself a revolutionary curve,

so defensive networks like that

became not exactly redundant,

but certainly not as impressive

as they were designed in the 1930s.

- [Narrator] In spite of its technological feats

and devastating armaments,

Festungsfront's importance all but disappears.

- There's a ton of resources poured into this

that never really came to fruition.

It was almost a waste, in a way.

There was a lotta planning and foresight

in anticipation of what might happen here,

and it never came to.

- Constructing this site took miles and miles of tunneling.

Tons of concrete and steel needed to be erected.

A train line was built.

How on Earth did they achieve all of that?

- It would be difficult for any government today

to build one of these projects,

not only the science and the engineering, but the cost.

In today's dollars, we're talking billions of dollars

for a lotta these projects.

- The fortification system has its place,

but it is very much rooted, I think,

in the lessons of World War I

rather than in thinking about how the next war

might be informed by these game-changing technologies

that we start to see at the end of World War I.

- [Narrator] Germany continues to evolve

its military strategy.

And on June 22nd, 1941,

the Nazis would execute Operation Barbarosa

and begin their charge into Soviet territory.

- When you consider Blitzkrieg

and the speed at which the Nazis

sweep through the occupied territories,

it's kind of obvious that the Festung

should now become all but abandoned.

Germany's ever-expanding borders

are now hundreds of miles away,

and Hitler is feeling invincible.

- Germany's priorities shift

from building these defensive structures

to an offensive posture,

both on the east and on the Western Fronts.

- [Narrator] Festungsfront is now considered

to be in safe territory

and it's unceremoniously stripped of its units and supplies

and left nearly derelict.

On July 15th,

less than a month after invading Soviet territory,

the Nazis capture Smolensk.

- Germany is now practically knocking on Moscow's door.

- [Narrator] But the Blitzkrieg technique

reveals its limitations.

- Blitzkrieg is vulnerable,

and it's really vulnerable to its assets.

It's vulnerable in the sense that that emphasis on mobility,

on speed, means that if anything goes wrong

to slow down that speed,

those forces are left pretty ill-equipped.

That can turn into a real liability.

- [Narrator] The tides are changing,

and Germany will realize that string of successes

in the east will come at a heavy cost.

[explosion booming]

With their Blitzkrieg strategy,

Germany has driven their forces deep into Soviet territory,

but they're brought to a crushing halt

when they reach Stalingrad.

- But the tides are changing.

Suddenly, that German advance is stopped.

And slowly at first, but ever faster,

the Nazis are being pushed back.

- They're losing hundreds of thousands of soldiers

across the various fighting fronts between 1944 and 1945.

At Stalingrad alone,

the entire Sixth Army is completely wiped out.

- [Narrator] Germany quickly retreats

across its former occupied territories.

Further devastating Hitler's plans,

the Allies have landed along the Western Front.

The walls are closing in on Nazi Germany.

- Hitler realizes he's vulnerable in his Eastern Front.

So, at the last minute, he rushes to renovate,

to refit, to expand this east wall.

- Even though it's unfinished,

could the Festung defenses now prove to be the catalyst

to turn the tide back in the favor of the Germans?

- [Narrator] The Nazi regime diverts

as many resources as it can to Festung

and prepares for battle, but the Soviet army

will prove to be a truly formidable entity.

- And it is an army bent on revenge.

It's an army that wants to exact a terrible price

for the suffering of its own people,

and so it unleashes fury, hell on Earth.

- I cannot imagine how frightening it would be

to be living in the eastern territories

and have the Red Army come through.

It would've been the most terrifying experience

of your life.

- [Narrator] As the Soviet army closes in,

Festung prepares to see if it can fulfill its purpose

to protect Berlin.

- Even though it was unfinished,

the Festung would've been their ultimate test.

- [Narrator] But what kind of forces

have been roused up by Germany

to fortify this massive defense line?

- In 1945, Goebbels and Hitler put together

what's know as the Volkssturm,

which is essentially boys as young as 16

and men as old as 60

who are given obsolete weapons or no weapons

and told to go out and defend the German Reichs.

- German forces, they had to improvise.

And they have no knowledge about the system,

did not know how huge it is.

There was normally equipment inside

that you could use to defend such a fortress.

- In the world of engineering,

no matter what system or machine or structure you build,

if it's operated by humans,

it's only ever gonna be as effective as the humans using it.

And if you can't supply it with the skills that's needed,

then it's always gonna be operated at suboptimal level.

- [Narrator] Despite Germany's last-minute rush

to staff the Festung defense line,

it's far too little and far too late.

- The Red Army overran it in three days,

which is really nothing when you consider

the size and scale of these fortifications.

- [Narrator] But because so much of this defense line

is buried, it remains a secret initially.

- Most of the Soviet troops don't even know

that they make their way through

this huge fortification line.

- [Narrator] Original plans for the Festung defense line

saw construction continuing up until 1951.

Its already-impressive defenses

would've been made even more formidable.

- At the end of this story,

you're just left wondering what could've been.

Had the Festung been completed

and had it been manned with fully-trained operatives,

might the outcome of the war have been different?

- [Narrator] Ultimately, the systems contained

within the massive Festung defense line

are so sophisticated that the Russians

and later the Polish couldn't operate them.

So, instead, they stripped the site

of the most valuable metal and technology

and then abandoned the site.

A section of the massive Festung tunnels

were even filled with waste, acting as a trash disposal site

for the Polish army in the region.

- Every element of the Festung was high-tech on this period.

We have to realize that these dragon teeth

is connected with those hydrologic lines,

with those bunkers connected to each other.

And as we see, it wasn't also high-tech for the '30s,

of 20th century, but even now it's really impress.

- [Narrator] Today, these fortifications still stand

as a testament to their over-engineered construction.

- They look like they would have in 1944.

It's almost as if you took a time machine.

- These bunkers have walls that are nine or 10 feet thick.

It's just an unreasonable use of concrete in a wall.

It's just a significantly heavier purpose

than just resisting environment loads.

- It's an immense amount of work.

The scale of it, to me, is impressive.

- It's like a rabbit warren, it really is.

It's kinda this underground endless array of technologies

and building capacities, and it's scary.

- [Narrator] The Nazis built

one of the most advanced defensive lines on Earth,

one that outlived its own era.

Ironically, when it mattered most, it was rendered useless.

But the ghosts of this complex hint at what could've been

had events played out differently.

[suspenseful orchestral music]