Original subtitles

Narrator: The leaning tower of pisa:

One of the world's Most iconic monuments.

For nearly 1,000 years, It defies the laws of gravity.

Its incredible tilt triggers A race against time

For the world's top engineers.

Today investigators Use high-tech tests...

That is a lot of load.

...To uncover hidden dangers.

The soil cannot take Any more stress.

Narrator: Why does the tower tilt?

Can it survive Another earthquake,

And what could finally Make it fall?

Burland: It would literally explode.

The whole tower Would just crash down.

Narrator: To unravel its mysteries,

We will deconstruct This marble giant.

We'll explore Its structural core

And expose its roots underground To reveal the hidden secrets

Of this medieval Engineering wonder.

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Captions paid for by Discovery communications

The leaning tower of pisa In italy:

One of the most recognizable Buildings on the planet.

It's constructed 800 years ago

As a bell tower For the city's cathedral.

The leaning tower stands At the heart of the piazza

Dei miracoli, or miracle square.

This medieval bell tower

Is an immense column Of fine white marble.

Nearly 300 steps spiral Around its central shaft,

Ascending eight pillared stories To a lofty belfry.

At more than 180 feet, This medieval skyscraper

Is defined not by its height But by its 5-degree lean.

With a staggering overhang Of more than 12 feet,

Why does this monumental tower Lean so precariously?

♪♪

♪♪

Valerio ascani dedicates His professional life

To unraveling The tower's mysteries

Starting with What explains its tilt.

Do the medieval architects Make a basic building error?

Valerio believes that the answer Is hidden in plain sight,

Written into the very stone Of the tower itself.

A bas-relief by the entrance To the tower

Reveals an unexpected image For a church building.

This unusual imagery Doesn't stop there.

Most bell towers are square In shape, not this one.

The hollow cylinder Of the tower itself

Also echoes that Of another structure.

Pisa is not on the sea.

It's landlocked,

So why a building That looks like a lighthouse?

Could it be a key to solve the Mystery of why the tower leans?

The answer may lie in an Astonishing discovery

Just 800 feet from the tower.

♪♪

Beneath the city, Drowned in thick black mud,

Archaeologists unearth The remains of 30 wooden ships.

Within them, fishing nets, ropes And baskets are frozen in time.

Their cargo gives a clue To their age.

Roman pottery and coins

Reveal these ships are more Than 2,000 years old,

But why are there Ancient roman ships

Buried beneath the city Of pisa?

Domenico barreca is The archaeologist in charge

Of rebuilding These extraordinary ships.

2,000 years ago, Pisa sits right on the coast.

A complex system of waterways And lagoons link it to the sea.

Ships use them to sail directly Into the heart of the town

To deliver cargo of wine, Oil and fruit,

But gradually the waterways Fill up with silt,

Burying the shipwrecks In a marshy grave.

Over the centuries, the Coastline drifts to the west,

Leaving pisa landlocked Miles from the sea.

The leaning tower of pisa Resembles a lighthouse

Built literally on Its hidden seafaring past.

The tower's foundation Sits on a mix of sand and clay.

One of the most iconic monuments On the planet is built on a bog.

Is this why it leans?

Burland: the answer is, Watch some children

Trying to build a model Brick tower on a soft carpet.

John burland is known For his work on the world's

Most challenging Engineering projects.

Here is some firm rubber,

Which represents This very soft ground,

So let's start building a tower,

And we try and make it As straight as possible.

Now you can see that, At a certain height,

That's perfectly stable, So we'll go a little bit higher,

Keeping it as straight As we can,

But give it one more Brick like that.

If the tower Has a very small disturbance,

Like a fly sitting on one side, The center of gravity moves,

And it just runs away With itself.

Narrator: the ground is so soft,

It's incapable of resisting The momentum of the tower.

Burland: We watch it, and over it goes.

[ laughs ]

Narrator: how much longer can The tower of pisa

Continue to defy The laws of gravity?

♪♪

♪♪

Narrator: The leaning tower of pisa,

Built to be the most impressive Bell tower in medieval europe,

It's been fighting gravity For over 800 years.

What could make it collapse?

♪♪

More than 32,000 marble blocks Make up the tower.

Stacked eight stories high,

They weigh an immense 16,000 tons,

But it's not solid marble All the way through.

Behind the smooth marble face

Sits a core of rough Limestone and mortar.

This solid backbone Keeps the tower

Standing for 800 years, But is there a hidden weakness?

♪♪

Pietro croce Is a structural engineer.

He's an expert on the loads that Put buildings under pressure.

He wants to find out Whether the tower

Can survive the stresses Created by its own tilt.

Is there a clue In the tower's unusual shape?

The tower is not straight.

It's not easy to see With the naked eye,

But the tower bends From the base to the top.

Pietro investigates the unusual Banana shape from the inside.

He wants to find out How it affects

The loads acting on the tower.

He starts at the sixth floor.

The measurements Don't seem to add up:

16 feet and 7 inches On the north side,

16 feet and 11 inches On the south.

These odd measurements Are not mistakes.

They're built-in.

In 1172, 5 years Into construction,

The tower is already Leaning to the north.

Engineers add height to one side To counteract its lean.

Construction continues,

But pisa suddenly goes to war With its neighboring states,

And building stops For 100 years.

When construction Finally resumes,

Three more stories shift The center of gravity,

And the tower now Leans to the south.

Finally, they build the belfry Lopsided to offset the lean.

It's the final curve Of the banana-shaped tower.

♪♪

The tower's peculiar shape Is the result of a rescue job,

But it doesn't stop it leaning.

The tower still overhangs By 12 feet.

The more the tower leans,

The greater the stress Put on its structure.

Pietro investigates whether The stonework can withstand

These immense forces.

♪♪

The tower's stresses Are concentrated

Around the first floor On the south side.

It bears the full brunt Of the lean.

The most vulnerable point of all

Is where the tower's walls Intersect with its stairwell.

A huge void has been created In the walls

To make room for the stairwell.

Here the masonry Is at its thinnest,

And yet the stresses Are at their highest.

The walls buckle Under the pressure.

This is the danger zone.

One thing we know: If it fails here,

At the wall on the first floor,

The consequences Could be catastrophic.

♪♪

The marble giant faces Monumental threats,

But some are more visible Than others.

Is there a hidden threat high up In the belfry?

♪♪

Narrator: the leaning tower Of pisa in italy:

It's been waging war With gravity for over 800 years,

But a leaning tower can reach A critical point when even

The smallest disturbance Can send it crashing.

♪♪

Towering 160 feet Above the town,

The belfry holds seven Colossal bronze bells.

The largest weighs almost 8,000 Pounds, as much as two cars.

Stone projections on the arches Support sturdy wooden beams.

These huge blocks of wood Suspend the bells

With thick iron straps.

With a combined weight of more Than 23,000 pounds,

Could the bells be a threat To their own bell tower?

[ bells tolls ]

[ bells tolling ]

Structural engineer pietro croce Investigates

The forces generated by The bells in the leaning tower.

He visits The marinelli bell foundry

In the italian town of agnone.

Established in 1040 ad, The marinelli foundry

Is one of the oldest Family businesses in the world.

One of its bells hangs today in Pisa's landmark leaning tower.

The sheer size and weight Of the bells

Makes just hanging them A risky business.

Armando marinelli is the 26th generation of his family

To run the foundry.

Bell towers do fall down

And none more dramatically Than the bell tower

Of st. Mark's basilica In venice, northeast italy.

In the 16th century,

The bell tower In st. Mark's square

Stands over 300 feet tall.

Five swinging bells ring out For 400 years,

But in 1902, without warning, A huge crack appears,

And within 3 days, It crashes to the ground.

Could the same fate Befall the iconic

Leaning tower of pisa?

♪♪

Pietro croce wants to test What kinds of stresses

A bell generates When it swings.

Pietro chooses a bell Of similar size and weight

To bells In the leaning tower.

He places markers on the bell

And uses a high-speed Digital camera

Filming at 960 frames per second

To track the motion Of the bell as it swings.

This will allow him to calculate The points of acceleration

And deceleration when it moves.

♪♪

How much greater Are the forces created

By a swinging bell Than a static one?

♪♪

Pietro works with university Colleague maria luisa

To make his calculations.

They conclude that the bell Generates forces

Twice its weight When it swings halfway.

What if it swings all the way?

A bell that exerts 1 ton of Static load on the tower

Could create four times That amount with each swing.

Swinging bells in a belfry Generate a horizontal force

That acts on the tower Through the wooden axle.

When many bells Swing separately,

The forces cancel out, But when they swing together,

The bells create a dangerous Surge in one direction.

♪♪

How much damage are the bells

Doing to the Leaning tower of pisa?

♪♪

Pietro takes a closer look Inside the belfry itself.

Today no one is taking Any chances.

The bells no longer swing.

Automated clappers create The chimes that ring out

Over the city of pisa.

Despite the forces Ranged against it,

The leaning tower still stands,

But how does the tower Survive over 800 years

With its precarious tilt?

Could the answer lie With a curious pair of towers

95 miles away?

♪♪

Narrator: The leaning tower of pisa:

It's a marvel Of 12th-century engineering,

A precarious balancing act

That seems to defy The laws of physics.

What hidden secret lies behind The tower's miraculous survival?

Does the answer lie with these Two medieval watchtowers

95 miles away In the city of bologna?

♪♪

The towers of bologna Are simple brick skeletons

With metal braces And iron rings.

The taller tower rises 318 feet Into the air

And has a 2-degree lean,

But the shorter tower Dangerously tilts

More than double this amount.

Both towers use the same Materials

And have the same foundations In the same dense sand and clay.

If their construction Is identical,

What accounts for the difference In height and lean?

Could the answer to this mystery Also reveal the secret

Of the leaning tower of Pisa's incredible balancing act?

♪♪

You might ask yourself, "Why is it that a shorter tower

Is leaning more Than a taller tower?"

Narrator: nick o'riordan is A geotechnical engineer.

O'riordan: It is almost always the passage Of time which holds the key

To why things Are the way they are.

Narrator: nick believes a clue To the puzzle lies in the soil

The two towers are built on.

♪♪

In a london laboratory, He investigates.

This force is basically Modeling the effect

Of loading from the tower.

In the cylinder Is a bore sample of soil

Taken from deep underground.

Fellow engineer Christopher russell

Helps nick set up the test.

It's immediately taken load, A lot of load.

In fact, That is a lot of load.

The test mimics what happens To the soil underground

When a building Is erected quickly.

The force is building up And building up.

At first, the soil gets stronger As it becomes more compact.

So, 30 percent Gain in strength.

But the load increases faster Than the soil's ability

To settle and strengthen.

So this is as though The tower is being

Built very rapidly indeed.

The soil can only withstand So much pressure.

It's hit its peak, And it's coming over the top.

That is failing.

That is failed soil.

The experiment reveals The dangers of hasty building.

They examine their soil sample To reveal further clues.

O'riordan: the shearing Of the soil causes failure,

And you get a polished surface Like this.

If you can imagine walking Through a very heavy clay field

With wellingtons And you skid on the surface,

Then you get the same sort Of shiny surface

Where your boot sole Has skidded, shearing the soil.

You can just see The shiny surface

And see the actual Movement lines.

The soil is pushed To its breaking point.

There comes a point at which The soil cannot take

Any more stress,

And it's at that point That the builder ordinarily

Would start to panic And say,

"Oh, my tower is on the move, And I better stop building,

And maybe I'd better reduce The height of that tower."

Narrator: Nick believes that this is The fate of the shorter

Of the two bologna towers.

The medieval builders Have to knock the top off

To save the whole structure From collapse.

12th bologna Is filled with slender towers,

Each a symbol of wealth And power.

♪♪

Two rival families, Garisenda and asinelli,

Compete for the tallest tower Right next to each other.

♪♪

The garisendas' tower initially Outpaces the asinellis',

But when it leans dangerously,

They are forced To decapitate it.

The slower-rising asinelli tower Never tilts more than 2-degrees

And eventually wins this Medieval tortoise-and-hare race.

The slow build of The asinelli tower

Allows the soil To settle and consolidate.

That's how they learned Empirically that

You can't build them Too quickly.

You've got to build them In stages.

Narrator: the story of the Leaning towers of bologna

Also shines light on the most Famous leaning tower of all.

When the tower of pisa is Being built in the 12th century,

Local wars halt work For 100 years.

It's this that allows the soil To settle and strengthen

And ensures The tower's survival.

If they had built It straight up,

It would probably have got up To about the fifth level,

And it would've fallen over, No question about it.

Narrator: the tower survives,

But 800 years later, It's in peril.

Every month, It leans a fraction more,

Inching to a point of no return.

Will a high-profile rescue Mission save it or destroy it?

♪♪

♪♪

The leaning tower of pisa:

One of the world's Most iconic monuments.

By the 1990s, the lean Is increasing every year.

It's incremental but relentless And can only end one way.

♪♪

In 1995, engineer john burland Joins a high-profile

Italian commission To save the tower.

He's got a fine line to tread.

There were two unacceptable Outcomes

For our work on the tower.

One we don't talk about, Mainly it falling over,

But the other one, Which was equally unacceptable,

Was a straight tower.

I mean, the pisans didn't want A tower that was made vertical

Because it's no longer A leaning tower.

Narrator: John develops a computer model

To predict every aspect Of the tower's tilt,

But his figures Reveal an enigma.

The model produced everything On the inclination beautifully,

Almost perfectly.

It's just that, Right at the very end,

The model told us That the tower was falling over.

Narrator: According to john's model, The tower will fall over

If it leans at more Than 4.75 degrees,

But it's already reached 5.5 degrees.

It's a finding with Chilling implications.

Our conclusion was that The tower must be very,

Very close to falling over.

Narrator: An urgent solution is needed To save this historic monument.

The pisa commission Finally agrees on a rescue plan.

♪♪

They fix 10 steel cables to the Foundations on the north side,

Which extend 150 feet underground,

And attach to massive anchors In the dense sand,

But the soil is waterlogged.

To stop the excavations From flooding,

Engineers freeze the ground With liquid nitrogen,

But the ground suddenly swells,

And the tower tilts More in 1 day

Than it has in a whole year.

Is the tower about To be lost for good?

♪♪

Burland: it was very, very Frightening and dramatic.

That was the time I wrote in my diary,

"I fear we may have Lost the tower."

Narrator: In desperation, engineers fix Additional counterweights

To the opposite side Of the tower's lean.

They were stacked one By one on the north side

Of the tower here,

So it's a bit like sitting out On a sailing boat.

You sit out and stop It leaning so much.

Narrator: The emergency fix stops The tower from toppling over,

But its lean is worse than ever.

How can they save The 16,000-ton giant now?

John proposes a radical idea.

♪♪

In preparation, john's team Secures heavy-duty steel cables

Around the table

And anchors them Into the ground.

Engineers dig a tunnel up to the Foundations on the opposite side

Of the lean and carefully remove Soil with great precision.

Over the course of a year,

The tower settles Into the cavity,

Reducing the overhang By nearly 2 feet.

In 2001, the cables And counterweights are removed,

Revealing the leaning tower Of pisa

In all its glory once again.

♪♪

The tower, It's just extraordinary.

I mean, it just glows, so it's A privilege actually to go back

And think you've played a part

In conserving this Incredible monument.

Narrator: Today, the tower stands Straighter and safer,

But does that mean It won't ever fall?

Italy is in one of the most Notorious seismic zones

In the world.

Could the tower still be brought Down by a powerful earthquake?

♪♪

Narrator: The leaning tower of pisa:

An 800-year-old medieval Masterpiece

Saved from tipping over

By the efforts Of modern engineers.

♪♪

But the leaning tower Is in one of the world's

Deadly earthquake zones.

Is it still at risk From a cataclysmic collapse?

♪♪

In 2016, a major earthquake,

Measuring 6.5 On the richter scale,

Hits the town of amatrice, South of pisa.

The town is flattened.

Around 300 people die,

And 4,000 more Are left homeless.

One of the surviving buildings Dominates the skyline.

It's the town's 82-foot-tall Clock tower.

Astonishingly, Underneath the scaffolding,

This medieval building Remains intact.

♪♪

Every structure Has a natural frequency.

This is the rate at which It will vibrate

When it's jostled or struck.

Tall Objects have a low frequency.

They move from side To side slowly.

Short structures Have a high frequency.

They move from side To side quickly.

In an earthquake, a shorter Structure will start vibrating

At its natural high frequency,

But an earthquake's Own high-frequency waves

Can intensify the vibration Of a shorter building.

This can make it shake more And more violently

Until it falls apart,

But it's a different story For taller buildings.

The high-frequency waves Of an earthquake

Don't match the low frequency Of a tall building.

This is why it can be Left unscathed.

The tower of pisa's tall And slender shape

Helps protect it From seismic activity

Like the amatrice clock tower.

But the pisa tower also leans.

The fact that the tower Is leaning means

That it has much higher stresses On one side than the other,

So the fact that When it vibrates

It is generating high stresses

Would mean that It would be more vulnerable.

This presents a puzzle.

The lean of the tower creates Such a vulnerability

That even moderate earthquakes In the region should topple it.

Against the odds, the leaning Tower of pisa remains standing.

Why?

[ laughs ]

Narrator: The leaning tower of pisa

Still holds onto Some of its mysteries.

♪♪

It stands today as the true Miracle of miracle square,

Capturing the imagination Of visitors

From all over the world.

Burland: It's thrilling to go back and Just to see everyone enjoying it

And what their reactions are.

Narrator: the iconic leaning Tower not only still stands

But looks set to survive For centuries to come.

The tower of pisa survives 800 years of gravity,

Structural stress And earthquakes.

Through a combination Of incredible coincidences

And blind luck, The leaning tower still stands,

A testament to medieval And modern engineering.

♪♪

[ bells tolling ]

♪♪

♪♪