Unearthed (2016–…): Season 6, Episode 7 - Leaning Tower of Pisa: The New Mystery - full transcript

The Leaning Tower of Pisa is one of the world's most iconic buildings, and it's also the site of an 800-year-old mystery; using cutting-edge tech, experts investigate the archaeological evidence to reveal when it might finally fall over.

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.

Why does the tower tilt?

Can it survive
another earthquake,

And what could finally
make it fall?

It would literally explode.

The whole tower
would just crash down.

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.

EP - 7 - Leaning Tower of Pisa: The New Mystery

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.

We watch it, and over it goes.

[ laughs ]

Narrator: How much longer can
the tower of pisa

Continue to defy
the laws of gravity?

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

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?

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

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

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.

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?

The leaning tower of pisa:

An 800-year-old medieval

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.

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.

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.


[ laughs ]

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.