Original subtitles

Narrator: Manhattan, new york city.

Once it is home to two of America's

Most iconic buildings --

The world trade center Twin towers.

They were giants Of their time.

When constructed, they are An engineering triumph --

The tallest buildings on earth.

The skyscraper has always been An engineering marvel,

But it was the twin towers That really set the precedent.

Narrator: Remembered across the globe For their tragic final moments,

The story of their origins Is rarely told.

Today, investigators use Pioneering technology

To examine the twin towers' Surviving footprint

And secret history...

We can see details you can't see From the human eye.

Narrator: ...And put the materials they Are built with to the test.

They really broke records In structural design innovation.

Narrator: how do engineers push The boundaries of building tall

To erect these towering giants In the heart of new york city?

To solve this mystery, we'll Digitally reconstruct the towers

And reveal the hidden secrets Behind their iconic design

That once defined New york's skyline.

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Captions paid for by Discovery communications

Narrator: New york city...

Today over 6,000 high rises Dominate the horizon,

But back in the 1960s, Only a handful of buildings

Stand higher than 40 stories.

Darton: something radical needed To be done to bring new york,

Really, into the 21st century, And in our culture,

Bold gestures Are often made architecturally.

Narrator: So new york's leaders invest Hundreds of millions of dollars

In a spectacular Building complex.

The architect, minoru yamasaki,

Went through literally Hundreds of plans,

And eventually he succeeded

By coming up with the idea For the twin towers.

Narrator: the world trade center Design features

Five lowrise buildings Surrounding a central plaza.

Rising from the center,

The most ambitious skyscrapers Of all time,

Each soaring more than 1,300 feet

Above lower manhattan.

Over four times taller Than the statue of liberty.

Together they will deliver 10 million square feet

Of real estate.

Every day, 50,000 workers will Flock to this city in the sky,

But no one has ever tried Erecting a building this tall,

So how do they do it?

Eric darton is an expert On the history of new york city.

He investigates the challenges The engineers face.

The idea was to acquire land And to build on a 16-acre site,

Which is not a lot,

Enough office space To accommodate 50,000 people.

That's why the towers had to be As tall as they were.

Narrator: The architects look to what Is then

The world's tallest skyscraper For inspiration --

The iconic empire State building.

No skyscraper had come near To surpassing it in 40 years.

The empire state building Is the high point

Of skyscraper design --

A steel grid reinforced With concrete

And encased in strong limestone.

It weighs 365,000 tons,

But using this building model For the twin towers

Would limit the internal space.

Darton: The empire state building Has 2 million square feet

Of floor space, office space.

The program for the world Trade center

Was 8 million square feet Of office space.

You can't just scale up The empire state building.

There'd be so many columns,

You couldn't put anybody In the building.

Narrator: So experts have to figure out How to build skyscrapers

That each have double

The floor space Of the empire state building.

Professor adrian brugger Investigates

The empire state building And twin towers structures.

Brugger: Here I have a 3d print

Of one of the floors Of the empire state building,

And you can see that we have This very thick core

In the middle here.

Narrator: The model shows how the weight Of the empire state building

Is spread Across a thick central core

And hundreds of columns,

But in order to beat The height of

The empire state building and Accommodate

The 50,000 office workers,

Japanese american architect

Minoru yamasaki And american engineer

Leslie robertson Will have to come up

With a revolutionary solution.

Minoru yamasaki had the idea Of this stress skin

Architecture from nature,

Which is generally A very good place

To get your motivation from.

And he thought of bamboo.

Narrator: bamboo is hollow And its entire weight

Is supported by its outer skin.

It's a very strong plant.

In japan it's also A symbol of prosperity.

The idea is the perfect solution For the twin towers.

Instead of spreading the load Across a grid,

Yamasaki creates a super strong Hollow structure.

All the weight is supported Around the edge.

It's a breakthrough In skyscraper design.

Each tower is made up of 236 steel columns.

They stand just 26 inches apart Around the perimeter.

More than 43,000 windows Complete the outer skin.

Inside, steel plates stretch Across the void,

Topped with nearly A million tons of concrete

To create 110 floors.

The north tower is crowned With a final feature --

A 360-foot communications mast.

Now they have a new task -- To make this outer skin

Strong enough to support The weight of the tower.

Adrian has rare access

To the surviving structures Of the twin towers,

Carefully conserved In the basement of

The 9/11 memorial museum.

He examines The surviving columns

From the outer skin Of the building.

So here you can see one of those Mega columns

That is landing the forces

From 110 stories Into the ground.

And if we look up at the column, We'll see that one of these

Large columns goes through This beautiful trident structure

That then picks up Three smaller columns.

Narrator: The hollow building structure is A breakthrough in engineering,

But this revolutionary concept Goes even further,

To the skeleton itself.

This column is actually hollow On the inside,

And that's purposely done

So that we can Increase its stiffness.

So the material is all Along the perimeter.

Narrator: Not only is the building hollow, But the columns are hollow, too.

Finally the designers use An extra trick

To reduce the overall weight Of the twin towers.

The metal is 4 inches thick At the base,

But tapers to just 1/4-inch At the highest point.

Brugger: They get lighter and lighter As you go up to the top.

And that is essentially to make The building

As efficient As humanly possible.

So this is how Structural engineers

Are able to build this high.

Narrator: Structurally the design Is perfect.

It breaks through The constraints

Of contemporary Skyscraper building.

With this method, the height Seems almost limitless.

It will allow the twin towers To rise taller

Than any other building On the planet,

Reaching An astonishing 1,368 feet,

But engineers face An additional challenge...

Darton: How do you bring people in?

The vast majority of people Going into

The world trade center Came in underground.

Narrator: Office workers arrive at The towers via the subway

And then go straight To the elevators,

But in the mornings, There are 50,000 people

Coming in at the same time.

How can they all travel Through the tower?

In the empire state building,

Elevators occupy almost 3/10s Of the buildings floor area.

If engineers follow a similar Design,

The twin towers would Lose vital floor space.

The taller a building is, The more elevators it needs

To service it,

But the shafts take up Valuable floor space.

The subway system in new york Provides inspiration

Where local trains Stop at every station

And express ones fast track To key hubs.

So engineers add two sky lobbies To the twin towers.

These are special floors Where people can switch

From a large capacity Express elevator to a local one.

So multiple elevators can share The same shaft,

Increasing the usable Floor space.

Engineering masterminds designed These iconic towers

To break the record for the Tallest building in the world,

Ready to steal the crown From the empire state building.

But when construction begins,

Engineers face an unexpected Challenge beneath the ground.

Could it become a serious danger For the building's foundations?

Narrator: The twin towers are once

The most recognizable Skyscrapers in the world

And the tallest buildings Of their time.

The engineers who build them Face huge challenges

From the very start Of their construction.

That's because much Of lower manhattan

Is built on reclaimed land.

When engineers start to dig Their foundations,

They make a surprising Discovery.

Hidden below the pavement, Engineers

Encounter water-logged mud And rubble,

Including a 400-year-old ship Preserved in mud.

As they dig deeper, a modern Obstacle emerges --

An active railway tunnel

That carries 100,000 passengers per day.

And even further down, 70 feet below street level,

They finally reach bedrock.

This exceptionally hard material Is the perfect base to build on,

But in order to secure The twin towers' foundations

Into the bedrock,

The engineers first Have to reach it.

Structural engineer rick ellman Investigates how it's possible

To build a skyscraper On reclaimed land.

These tall towers that are all Around us,

If they were not Constructed properly,

They would lean And very likely topple over.

Narrator: The most famous example of the Dangers of unsteady foundations

Is the tower of pisa in italy.

It's build on marshy land,

Similar to the reclaimed land Of manhattan.

Over time, the soft ground Causes the tower

To tilt dangerously.

But unlike pisa, manhattan has An incredibly tough bedrock

Just 70-feet underground.

The twin towers, foundations Can be firmly anchored

Into this solid rock, But there's a huge problem --

Reaching it.

Ellman: Excavating in reclaimed land Can be a real challenge.

Because of the weak soils And the high ground water,

You're unable To just simply excavate.

Narrator: If they excavate, water and mud Just slide back into the hole,

So engineers come up With an ingenious solution --

They create a wall To surround the foundations.

The wall acts as a protective Barrier.

First, they dig long trenches Around the edge of the site

And fill them with A liquid slurry

To stop them collapsing inwards.

Then they drop reinforced steel Cages into the trenches and pump

In concrete To displace the slurry.

Once these concrete walls Set solid,

Engineers can excavate Everything inside them,

Creating a giant Underground bathtub.

Instead of holding water in, The bathtub keeps it out

So the foundations Can now be built.

At the 9/11 memorial site, Rick ellman has rare access

To the remains of the bathtub.

Ellman: Here we are, 70-feet below The ground surface

In the area that was once Occupied

By the original World trade center foundations.

The space was created using the Slurry wall construction method.

Narrator: The wall holds back the Surrounding mud and ground water

To protect The towers' foundations.

The wall itself is probably 3 to 5-feet thick,

And this system, amazingly,

Is holdings thousands Of pounds of force.

Narrator: The twin towers engineers Are some of the first

To use this pioneering technique In new york.

In 1968, the bathtub is complete

And the main Building work can begin.

But engineers face Additional challenges.

The twin towers are the world's First skyscrapers

Built without stone or brick.

Their flexible metal structure Causes them

To sway in high winds.

How can engineers Stop new yorkers

From getting sea sick inside?

Narrator: April 1973...

In the heart of manhattan, Two skyscrapers are set

To emerge on the horizon -- The twin towers.

Their hollow metal structure Is a revolutionary design.

They are the first skyscrapers Ever built

Without stone or brick.

The engineers use A new technique

To construct these steel giants.

They click the pieces together Like lego.

Engineer sawteen see

Is the managing partner Of the company

That built The world trade center.

All the design of The twin towers

Allowed for faster and safer Erection and construction,

So it went up really fast.

Narrator: Building the pieces off site Allows engineers

To complete them To the highest standard,

And it means the structure can Be assembled quickly on site.

See: in a sense it was like An assembly plant.

Narrator: The twin towers' external skin Goes up first.

59 steel columns per side, Each just 14 inches wide.

They bear most of the towers' Weight.

In the center of each tower Sits a core of 47 steel columns

To give the tower Extra rigidity.

Thin steel plates 60-feet Wide bridge the gap,

Connecting the inner And outer columns,

Anchoring the entire Structure together,

And creating the vast Open floor plans.

It's a futuristic lightweight Structure,

But will this flexible Design be a match

For new york's high winds?

At the top of the buildings, Wind speeds

Can reach 100 miles per hour.

To make it very tall, The building

Had to be very stiff To resist the wind loads.

Narrator: If the building sways too much, It could sheer the steal,

Or the elevators could even Get stuck in the shafts.

So the twin towers become the First skyscrapers in the world

To be evaluated For wind dynamics.

A new field of research is born.

Scale models on the twin towers

And the complex, The surrounding areas

Were built and tested In colorado.

Narrator: In the 1960s, the twin towers' Engineers

Create the fist ever wind tunnel

And put their models To the test.

They shoot 150 mile per hour Winds across it.

The tests show they will hold Strong structurally --

A success, But there's a problem...

Even though the models Can withstand the wind loads,

They sway 1 inch From side to side,

Equivalent to 40 feet On the full size buildings.

Workers at the top will sway With the buildings.

Gazing out of the window At the second tower

Swaying will make the sense Of movement worse.

On the 110th story, A gust of wind

Will trigger instant nausea.

It would be impossible To work there.

So the wind tunnel test provided Wind loads,

But we didn't know whether those Accelerations were acceptable

To people who are In the building, the occupants.

Would they feel sea sick?

We don't know.

There were no guidelines around.

Narrator: How much sway humans can endure

Is a critical question For skyscraper engineers.

To investigate, structural Engineer antony darby

Launches the "V" simulator,

A state of the art virtual Reality skyscraper

At the university Of bath, england.

Heavy duty hydraulics Suspend a custom made office,

Simulating the sway motion of A skyscraper in gusts of wind.

Inside, a volunteer wears 3d glasses

To experience A virtual reality view

Out of the skyscraper windows.

Antony's team records the Volunteer's physical reactions

As the skyscraper's Sway intensifies.

We can measure their response, Their heart rate,

Their eye blink rate,

Things that measure stress Or tiredness.

We can also assess their work Performance,

How that degrades Over time as well.

Narrator: Antony's team collects data From hundreds of volunteers

To determine The breaking point of humans.

He measures the movement In g-force.

The typical g-force felt on A roller-coaster is 4g

Compared to just 0.01g for walking.

What we've shown here Is that below

About 0.005g, People don't feel the motion.

Above 0.005g, you can perceive The motion,

But it might not affect You that much.

But as we steadily get higher up Around 0.04g,

Then it does become intolerable.

Narrator: The simulator tests reveal that 0.04g is the upper limit.

Anything above this force Is intolerable.

Today, this research is used For high-tech skyscraper design

And rigorous testing.

But in the 1960s, tests Like these don't exist.

The twin towers team

Has to invent The very first test themselves.

The twin towers engineers build Their own test rig

And hang it inside a ventilation Shaft in the lincoln tunnel.

Volunteers sit inside The fake office

While two engineers push it From side to side

And ask them to report When they feel sick.

Even these crude tests Are enough to confirm

What the twin Towers engineers feared --

People aren't able to work In these conditions.

In order to reduce the 40-foot Sway, the architects

Create special shock absorbers To reduce

The movement between the wall And the floor.

They install 10,000 of these Dampers across the two towers,

Absorbing huge amounts Of energy,

Reducing the sway To just 3 feet.

They are a success.

They are the first skyscraper Dampers in the world.

See: everything in the original Twin towers

Was innovative And ground breaking --

In the design, In the materials,

In the construction, And erection.

Narrator: The twin towers are a modern Masterpiece

Created with exquisite attention To detail.

By creating these technological Solutions, the engineers

Keep the 50,000 people Inside safe

From these foreseeable dangers.

But the threat that lies ahead Is unimaginable

To its creators --

The deadly impact of the Terrorist attack on 9/11

Surpasses any force engineers Could ever predict.

♪♪

Narrator: in 1973, The twin towers

Become the tallest Buildings on the planet.

These engineering marvels Rise 1,368 feet into the sky,

But no one could foresee The challenges

They would face Over their lifetime.

In 1975, an electric short Circuit failure

Set six stories Of the north tower on fire.

Firefighters manage to control The blaze.

Fortunately the building Was unoccupied

Or there would have been, Potentially,

Some serious consequences.

Narrator: 20 years later, a bigger impact Strikes --

A terrorist plants A 1,200-pound bomb

In the north tower garage.

It explodes With devastating force,

Yet the tower stands strong.

But on september 11, 2001, A tragedy bigger

Than anything imaginable Strikes the towers.

The world watches in disbelief.

Over 17,000 people are working Inside the twin towers

When suddenly a Terrorist-hijacked boeing 767

With 92 people on board crashes Into the north tower.

The impact creates A burning hole

Between the 94th and 99th floors Of the 110-story skyscraper.

The 10,000 dampers absorb The energy from the impact,

And the hollow skeleton design Supports the weight

Of the building Despite the damage.

17 minutes after The first plane hits,

A second Hijacked boeing 767

With 65 passengers and crew On board...

...Crashes into the 77th to 85th Floor of the south tower.

There was a tremendous outburst Of smoke, debris.

Debris from both impacts, Particularly the second one,

Rained down on the whole area.

Narrator: Over 1,000 people perish In these sudden crashes.

While the towers are still Standing,

Over 15,000 people Manage to evacuate.

But the tragedy isn't over.

The flammable jet fuel From the aircraft

Makes the crash much more deadly Than anyone could have foreseen.

When engineers designed the twin Towers, they protect

Against a crash from the Largest aircraft at the time --

A boeing 707.

But the hijacked aircraft Are much large boeing 767s

Carrying 20,000 gallons Of kerosene.

This jet fuel pours down through The 12 miles of elevator shafts

Onto other floors, Spreading like wildfire.

Inside the towers, there are Thousands of tons of office

Furnishings,

Many of them highly flammable

So the fire spreads Unusually quickly.

The firefighters soon realize That there was no controlling

This blaze, And from then on,

It was concentrating On not saving the building

But on rescuing People inside.

Narrator: at exactly 9:59 a.M., Almost two hours

After the first collision,

The south tower collapses...

...In just 10 seconds.

Half an hour later, The north tower goes down, too.

More than 2,000 people Are still inside the buildings.

Darton: It was impossible, I think, For anyone --

Engineer, tourist, new yorker,

To imagine that these towers Could possibly come down.

Narrator: After standing strong for nearly Two hours,

What ultimately causes The towers to collapse?

The twin towers' All-steel structure

Is exceptionally strong,

But its innovative design Hides potential weaknesses.

At the base of each tower,

The steel columns Are 4 inches thick

To bear the weight Of the building above.

But near the top, these columns Are only 1/4-inch thick.

To protect the steel from fire, Engineers coat it

In a fireproofing spray...

...But this protective layer Is thin and easily dislodged.

Could this be the downfall Of these iconic buildings?

♪♪

Structural engineer, Adrian brugger investigates

If the fire could be The cause of the collapse.

Brugger: the ensuing fire raged Inside the building.

The impact caused massive swaths Of windows to be torn out

And this extra damage Created winds

That were actually Fanning the flames.

Narrator: Although the fireproof coating Can withstand the flames,

The impact of the aircraft Leaves it compromised.

The aircraft entered The building

Between 400 And 550 miles per hour

And the impact Lasted only 0.7 seconds.

And of course, During that impact,

We actually had all of That fireproofing stripped.

Narrator: Adrian's team investigates How fire affects steel

That has been stripped Of its fireproofing.

His team uses floor beams

Identical to those From the twin towers.

They load them with 1.4 tons Of weight

To simulate the weight Of each floor of the tower.

They prepare to heat up The steel beam.

We will apply heat By simulating a fire.

Narrator: The temperature of the fire Climbs

To 1,800 degrees fahrenheit --

Equivalent to that Inside the towers.

Now we're heating up the beam With flame throwers,

And you can see The temperature is going up

On our infrared camera.

We're reaching about 500 degrees Celsius,

So that's 800 degrees Fahrenheit.

Narrator: At 800 degrees fahrenheit, The trusses hold strong,

But then they start To show signs of weakness.

Now we see a cherry red spot On the beam that's indicative

Of about a temperature Of 750 celsius,

Which is equivalent To 1,300 fahrenheit.

The beam is actually Deflecting significantly.

Narrator: The steel beam gives way after Just 11 minutes

At 1,300 degrees.

Adrian's investigation shows That without fireproofing,

The floor beams Cannot withstand the high heat

And lose their strength.

The extreme heat weakens The steel floor plates

Causing them to sag,

Pulling the walls In on each other.

The heat also weakens the steel Bolts

That hold the floors in place Causing each floor

To collapse down Onto the one below --

A process known as pancaking.

This catastrophe goes down In history

As america's darkest day.

The wreckage burns for 100 days.

♪♪

Rescue operations salvage The structural remains

Of the twin towers.

Senior vice president Of the 9/11 memorial museum,

Anthony gardner,

Leads the mission to preserve The twin towers structures.

1.8 million tons of debris fill

Ground zero In the aftermath of the attacks.

Essentially it was 7-story Smoldering pile of material

That stretched up From street level

And then also down seven stories To the bedrock level of the site

Where I'm standing today.

Narrator: The scale of destruction Is overwhelming.

But despite the force,

Some structures Survive the collapse.

These elements are conserved On the 9/11

Memorial site to this day --

A display of respect and a place To remember

Those that lost their lives.

One of the columns Of the twin towers

Was found intact In its original position.

So this is the last column that Stood, despite the destruction.

Rescue and recovery workers Adorned it

With all these inscriptions

And tributes To their fallen colleagues.

Narrator: As the clean up operation Evolves, the twin towers

Deep foundations Come into view.

Buried by rubble, can the Damaged bathtub wall

Continue to hold back The hudson river?

What will it take to keep this Area of lower manhattan safe?

♪♪

Narrator: September 11, 2001...

After months of wreckage Clean up,

Just a handful of structures From the towers remain --

A reminder of the fight The skyscrapers

Put up against the attacks.

Can any part of these buildings

Possibly stay Standing to this day?

Above ground, Very little survives --

A single staircase.

A steel column from the core Of the south tower.

And steel tridents that

Once formed the lower facade Of the towers.

But below ground, something Far bigger survives --

The vast concrete bathtub

Built to protect The site's foundations.

But deep cracks span the wall

And many anchoring bolts Begin to leak ground water.

Without repair, the wall Could fail,

Jeopardizing nearby buildings.

What will it take to keep this Area of lower manhattan safe?

Head of conservation, Lisa conte,

Leads the preservation mission At the 9/11 memorial.

Her four-person team Works all year round

To keep this Historic structure standing.

Here we are looking at a portion Of the original bathtub.

Just beyond it is The hudson river.

Narrator: Following the events of 9/11, The damage to the bathtub wall

Threatens new york city With a potential catastrophe.

If the bathtub wall fails,

The area will rapidly flood With groundwater

And water from the nearby Hudson river.

This water would then inundate The subway tunnel

That runs Through the bathtub...

...Threatening widespread Flooding of

The underground rail network.

With the bathtub breached,

The surrounding ground Would weaken, risking subsidence

Or even collapse Of surrounding buildings.

To detect any weak spots In the wall,

Lisa's team turns To cutting-edge technology.

Every year scanning specialist Scott lee

Laser scans the entire wall To millimeter accuracy.

His lidar scanning works like Echo location,

Bouncing beams off the surface

And recording the distance With incredible accuracy.

By doing the lidar or laser Scanning, we create

A very accurate millimetric Model of the entire slurry wall.

Narrator: The team also takes super high Resolution photos of the wall

To record the surface detail.

A platform raises the team Into the air

To capture the wall From every angle.

Every detail matters.

Scott then Combines the lidar scan

And the thousands of photos

Into a super high resolution 3d model of the wall.

We project those really high Resolution photos

Onto that solid surface model

To create a very detailed Photo realistic model.

We can see Every nook and cranny.

Often we can see details you Can't see from the human eye.

Narrator: Armed with this 3d model, Lisa's team can hone in

On the weak parts of the wall That need attention.

The brown that you see here Is corrosion staining related

To the interaction of the water And the steel elements.

Also present, But less easy to see, are salts.

And those are the areas Of white here

And coming down the face here.

Narrator: The 3d scan pinpoints areas

Where new water Starts to seep through.

It's really important that we're Working to preserve the wall

Because everything needs To be cared for

In order to preserve Its integrity over time.

Narrator: The wall shows great strength And resilience.

With the help of lisa's team, It will remain standing for many

More decades to keep The surrounding city safe.

This colossal underground Structure

Is a reminder of The innovative engineering

That made these radical Skyscrapers a reality.

How have the twin towers' Innovations

And their tragic end

Changed the design Of skyscrapers forever?

Narrator: the twin towers, Revolutionary skyscrapers

That have changed The world forever.

What can we learn from The towers

And the events That lead to their collapse?

The impact of two Hijacked airplanes

Creates massive gashes In the towers

And an unprecedented fire That causes them to collapse.

2,753 people die,

But the building's put up A remarkable fight

In the face of the attack.

On the north face Of the north tower

2/3 of the columns Were taken out,

But yet, the towers stood For more than an hour,

Which allowed occupants

Who were below The force of impact to escape.

Darton: The death toll was horrific,

But had they not been as well Built or engineered

In the wonderful way That they were,

They probably would have Collapsed much earlier

And the death toll would Have been exponentially higher.

Narrator: the twin towers are Revolutionary structures,

Revered by engineers All over the world.

They are the first to break Through the ceiling

Of skyscraper height,

Passing this legacy Of innovative

Engineering onto Their successors.

The skyscraper has always been An engineering marvel,

But it was the twin towers That really set the precedent

With their revolutionary Technology,

With all the different Extraordinary inventions

That had to be created To even make it happen

That set the precedent For the design and engineering

Of the even taller buildings Around the world today.

Narrator: Two of the most recognizable Skyscrapers of all time.

Two of the first buildings To use a bathtub design.

They paved the way For skyscrapers

On reclaimed land All over the world.

The hollow tube design Is now adopted

By thousands of buildings Worldwide.

With subway inspired express Elevators paving the way

For people to travel Through cities in the sky.

And their lightweight flexible Structure triggers

A whole new field of research Into skyscraper wind dynamics,

Setting the stage for cutting Edge research for years to come.

13 years after the 9/11 tragedy,

A new building stands at The world trade center site --

The new one world trade center.

Rising 1,772 feet tall, it is New york's tallest skyscraper.

It is designed with features To remember the people

And the original twin towers With protection

As a top priority.

See: like world trade center one And the other one's here

Are designed With every safety feature

That the engineers Could come up with.

Since 9/11, more public Buildings

Like the world trade center And other office buildings

Have adopted Bomb blast criteria.

♪♪

Narrator: today at ground level, Massive voids

Show the footprints Of the towers.

♪♪

So this is the north tower Memorial pool,

And the memorial pools mark The location

Of where the towers stood At the street level of the site.

And here, etched in The bronze parapets

Are the names Of the nearly 3,000 people,

Including my brother, Who perished on 9/11.

Narrator: The twin towers and those Who lost their lives in 9/11

Are never to be forgotten.

♪♪

The twin towers rose On new york's skyline

As a powerful message Of american innovation,

Breaking boundaries And reaching higher

Than anyone Had ever gone before.

Their fall was the darkest day In america's history,

But their legacy lives On through other skyscrapers.

A legacy never to be forgotten.

♪♪

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