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Narrator:
The leaning tower of pisa:
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00:00:05,172 --> 00:00:09,274
One of the world's
most iconic monuments.
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00:00:09,276 --> 00:00:14,246
For nearly 1,000 years,
it defies the laws of gravity.
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00:00:18,519 --> 00:00:21,687
Its incredible tilt triggers
a race against time
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00:00:21,689 --> 00:00:24,023
For the world's top engineers.
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00:00:29,964 --> 00:00:34,433
Today investigators
use high-tech tests...
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00:00:34,435 --> 00:00:36,301
That is a lot of load.
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00:00:36,303 --> 00:00:39,004
...To uncover hidden dangers.
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00:00:39,006 --> 00:00:42,241
The soil cannot take
any more stress.
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00:00:42,243 --> 00:00:45,177
Narrator:
Why does the tower tilt?
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00:00:45,179 --> 00:00:47,579
Can it survive
another earthquake,
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00:00:49,817 --> 00:00:53,352
And what could finally
make it fall?
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00:00:53,354 --> 00:00:54,853
Burland:
It would literally explode.
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00:00:54,855 --> 00:00:57,790
The whole tower
would just crash down.
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00:00:57,792 --> 00:00:59,691
Narrator:
To unravel its mysteries,
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00:00:59,693 --> 00:01:03,362
We will deconstruct
this marble giant.
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00:01:03,364 --> 00:01:07,032
We'll explore
its structural core
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00:01:07,034 --> 00:01:12,971
And expose its roots underground
to reveal the hidden secrets
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00:01:12,973 --> 00:01:17,076
Of this medieval
engineering wonder.
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00:01:17,078 --> 00:01:20,079
-- Captions by vitac --
www.Vitac.Com
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Captions paid for by
discovery communications
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The leaning tower of pisa
in Italy:
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One of the most recognizable
buildings on the planet.
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It's constructed 800 years ago
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00:01:35,830 --> 00:01:38,363
As a bell tower
for the city's cathedral.
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00:01:40,901 --> 00:01:43,602
The leaning tower stands
at the heart of the piazza
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00:01:43,604 --> 00:01:48,307
Dei miracoli, or miracle square.
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00:02:09,763 --> 00:02:11,763
This medieval bell tower
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00:02:11,765 --> 00:02:14,800
Is an immense column
of fine white marble.
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00:02:18,239 --> 00:02:23,509
Nearly 300 steps spiral
around its central shaft,
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Ascending eight pillared stories
to a lofty belfry.
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00:02:29,049 --> 00:02:33,018
At more than 180 feet,
this medieval skyscraper
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Is defined not by its height
but by its 5-degree lean.
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00:02:40,861 --> 00:02:44,596
With a staggering overhang
of more than 12 feet,
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Why does this monumental tower
lean so precariously?
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00:02:48,402 --> 00:02:56,341
♪
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♪
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Valerio ascani dedicates
his professional life
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To unraveling
the tower's mysteries
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00:03:10,157 --> 00:03:12,824
Starting with
what explains its tilt.
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00:03:16,630 --> 00:03:20,232
Do the medieval architects
make a basic building error?
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00:03:22,269 --> 00:03:26,038
Valerio believes that the answer
is hidden in plain sight,
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Written into the very stone
of the tower itself.
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A bas-relief by the entrance
to the tower
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Reveals an unexpected image
for a church building.
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00:04:02,977 --> 00:04:07,379
This unusual imagery
doesn't stop there.
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00:04:07,381 --> 00:04:12,584
Most bell towers are square
in shape, not this one.
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The hollow cylinder
of the tower itself
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Also echoes that
of another structure.
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00:04:28,335 --> 00:04:31,436
Pisa is not on the sea.
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It's landlocked,
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00:04:34,642 --> 00:04:37,576
So why a building
that looks like a lighthouse?
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00:04:40,414 --> 00:04:44,650
Could it be a key to solve the
mystery of why the tower leans?
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00:04:48,255 --> 00:04:50,956
The answer may lie in an
astonishing discovery
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Just 800 feet from the tower.
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♪
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Beneath the city,
drowned in thick black mud,
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00:05:04,905 --> 00:05:10,809
Archaeologists unearth
the remains of 30 wooden ships.
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00:05:10,811 --> 00:05:15,614
Within them, fishing nets, ropes
and baskets are frozen in time.
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00:05:18,252 --> 00:05:21,053
Their cargo gives a clue
to their age.
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00:05:23,223 --> 00:05:24,923
Roman pottery and coins
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Reveal these ships are more
than 2,000 years old,
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00:05:32,533 --> 00:05:34,966
But why are there
ancient roman ships
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Buried beneath the city
of pisa?
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Domenico barreca is
the archaeologist in charge
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Of rebuilding
these extraordinary ships.
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00:06:22,783 --> 00:06:26,685
2,000 years ago,
pisa sits right on the coast.
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00:06:26,687 --> 00:06:30,489
A complex system of waterways
and lagoons link it to the sea.
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00:06:33,694 --> 00:06:38,497
Ships use them to sail directly
into the heart of the town
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To deliver cargo of wine,
oil and fruit,
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00:06:44,872 --> 00:06:47,939
But gradually the waterways
fill up with silt,
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00:06:47,941 --> 00:06:50,642
Burying the shipwrecks
in a marshy grave.
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00:06:56,283 --> 00:06:59,751
Over the centuries, the
coastline drifts to the west,
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00:06:59,753 --> 00:07:02,954
Leaving pisa landlocked
miles from the sea.
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00:07:08,629 --> 00:07:11,830
The leaning tower of pisa
resembles a lighthouse
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Built literally on
its hidden seafaring past.
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The tower's foundation
sits on a mix of sand and clay.
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One of the most iconic monuments
on the planet is built on a bog.
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Is this why it leans?
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00:07:36,924 --> 00:07:39,591
Burland: The answer is,
watch some children
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Trying to build a model
brick tower on a soft carpet.
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00:07:44,565 --> 00:07:47,199
John burland is known
for his work on the world's
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Most challenging
engineering projects.
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Here is some firm rubber,
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Which represents
this very soft ground,
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So let's start building a tower,
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00:08:01,148 --> 00:08:04,316
And we try and make it
as straight as possible.
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00:08:07,054 --> 00:08:09,087
Now you can see that,
at a certain height,
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00:08:09,089 --> 00:08:14,192
That's perfectly stable,
so we'll go a little bit higher,
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00:08:14,194 --> 00:08:16,428
Keeping it as straight
as we can,
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00:08:16,430 --> 00:08:20,198
But give it one more
brick like that.
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00:08:20,200 --> 00:08:22,834
If the tower
has a very small disturbance,
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00:08:22,836 --> 00:08:28,340
Like a fly sitting on one side,
the center of gravity moves,
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00:08:28,342 --> 00:08:31,443
And it just runs away
with itself.
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Narrator: The ground is so soft,
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It's incapable of resisting
the momentum of the tower.
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00:08:38,085 --> 00:08:42,220
Burland:
We watch it, and over it goes.
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00:08:42,222 --> 00:08:43,889
[ laughs ]
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Narrator: How much longer can
the tower of pisa
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Continue to defy
the laws of gravity?
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00:08:51,398 --> 00:09:00,372
♪
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♪
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00:09:16,223 --> 00:09:19,324
Narrator:
The leaning tower of pisa,
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00:09:19,326 --> 00:09:23,194
Built to be the most impressive
bell tower in medieval europe,
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00:09:26,099 --> 00:09:29,734
It's been fighting gravity
for over 800 years.
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00:09:32,239 --> 00:09:34,706
What could make it collapse?
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00:09:34,708 --> 00:09:41,279
♪
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More than 32,000 marble blocks
make up the tower.
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Stacked eight stories high,
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They weigh an immense
16,000 tons,
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00:09:56,763 --> 00:09:59,564
But it's not solid marble
all the way through.
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00:10:01,735 --> 00:10:04,102
Behind the smooth marble face
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00:10:04,104 --> 00:10:07,038
Sits a core of rough
limestone and mortar.
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00:10:10,077 --> 00:10:12,043
This solid backbone
keeps the tower
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00:10:12,045 --> 00:10:17,015
Standing for 800 years,
but is there a hidden weakness?
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♪
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Pietro croce
is a structural engineer.
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00:10:30,797 --> 00:10:35,667
He's an expert on the loads that
put buildings under pressure.
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00:10:35,669 --> 00:10:37,435
He wants to find out
whether the tower
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00:10:37,437 --> 00:10:40,839
Can survive the stresses
created by its own tilt.
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00:10:45,412 --> 00:10:48,647
Is there a clue
in the tower's unusual shape?
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00:10:59,926 --> 00:11:03,561
The tower is not straight.
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00:11:03,563 --> 00:11:06,231
It's not easy to see
with the naked eye,
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00:11:06,233 --> 00:11:09,334
But the tower bends
from the base to the top.
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00:11:12,039 --> 00:11:18,476
Pietro investigates the unusual
banana shape from the inside.
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00:11:18,478 --> 00:11:20,178
He wants to find out
how it affects
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00:11:20,180 --> 00:11:22,414
The loads acting on the tower.
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00:11:25,519 --> 00:11:27,752
He starts at the sixth floor.
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00:11:41,334 --> 00:11:45,837
The measurements
don't seem to add up:
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16 feet and 7 inches
on the north side,
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16 feet and 11 inches
on the south.
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00:12:01,154 --> 00:12:04,923
These odd measurements
are not mistakes.
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00:12:04,925 --> 00:12:06,558
They're built-in.
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00:12:09,596 --> 00:12:13,431
In 1172, 5 years
into construction,
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00:12:13,433 --> 00:12:16,868
The tower is already
leaning to the north.
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00:12:16,870 --> 00:12:22,073
Engineers add height to one side
to counteract its lean.
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00:12:22,075 --> 00:12:24,042
Construction continues,
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00:12:24,044 --> 00:12:27,846
But pisa suddenly goes to war
with its neighboring states,
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00:12:27,848 --> 00:12:30,949
And building stops
for 100 years.
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00:12:32,986 --> 00:12:35,186
When construction
finally resumes,
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00:12:35,188 --> 00:12:39,090
Three more stories shift
the center of gravity,
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00:12:39,092 --> 00:12:41,760
And the tower now
leans to the south.
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00:12:43,964 --> 00:12:48,800
Finally, they build the belfry
lopsided to offset the lean.
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00:12:48,802 --> 00:12:51,436
It's the final curve
of the banana-shaped tower.
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00:12:51,438 --> 00:12:57,809
♪
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00:12:57,811 --> 00:13:01,846
The tower's peculiar shape
is the result of a rescue job,
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00:13:01,848 --> 00:13:03,848
But it doesn't stop it leaning.
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00:13:06,453 --> 00:13:10,021
The tower still overhangs
by 12 feet.
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00:13:12,192 --> 00:13:14,025
The more the tower leans,
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00:13:14,027 --> 00:13:16,728
The greater the stress
put on its structure.
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00:13:19,733 --> 00:13:22,867
Pietro investigates whether
the stonework can withstand
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These immense forces.
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00:13:24,638 --> 00:13:29,440
♪
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The tower's stresses
are concentrated
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00:13:31,511 --> 00:13:34,179
Around the first floor
on the south side.
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00:13:36,416 --> 00:13:38,917
It bears the full brunt
of the lean.
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00:13:42,022 --> 00:13:43,888
The most vulnerable point of all
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Is where the tower's walls
intersect with its stairwell.
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00:14:05,512 --> 00:14:08,046
A huge void has been created
in the walls
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00:14:08,048 --> 00:14:09,914
To make room for the stairwell.
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00:14:12,752 --> 00:14:15,286
Here the masonry
is at its thinnest,
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00:14:15,288 --> 00:14:18,756
And yet the stresses
are at their highest.
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00:14:18,758 --> 00:14:21,125
The walls buckle
under the pressure.
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00:14:37,677 --> 00:14:40,078
This is the danger zone.
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One thing we know:
If it fails here,
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00:14:58,765 --> 00:15:01,065
At the wall on the first floor,
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00:15:01,067 --> 00:15:03,434
The consequences
could be catastrophic.
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♪
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The marble giant faces
monumental threats,
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00:15:18,818 --> 00:15:23,254
But some are more visible
than others.
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Is there a hidden threat high up
in the belfry?
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00:15:41,508 --> 00:15:45,410
♪
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Narrator: The leaning tower
of pisa in Italy:
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00:15:49,416 --> 00:15:53,718
It's been waging war
with gravity for over 800 years,
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00:15:56,389 --> 00:15:59,624
But a leaning tower can reach
a critical point when even
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00:15:59,626 --> 00:16:02,026
The smallest disturbance
can send it crashing.
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00:16:02,028 --> 00:16:09,267
♪
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Towering 160 feet
above the town,
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00:16:12,739 --> 00:16:18,209
The belfry holds seven
colossal bronze bells.
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00:16:18,211 --> 00:16:25,016
The largest weighs almost 8,000
pounds, as much as two cars.
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00:16:27,320 --> 00:16:31,823
Stone projections on the arches
support sturdy wooden beams.
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00:16:33,927 --> 00:16:36,594
These huge blocks of wood
suspend the bells
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00:16:36,596 --> 00:16:38,663
With thick iron straps.
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00:16:40,867 --> 00:16:45,370
With a combined weight of more
than 23,000 pounds,
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00:16:45,372 --> 00:16:48,873
Could the bells be a threat
to their own bell tower?
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00:16:48,875 --> 00:16:52,110
[ bells tolls ]
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[ bells tolling ]
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Structural engineer pietro croce
investigates
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00:17:01,154 --> 00:17:04,522
The forces generated by
the bells in the leaning tower.
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00:17:08,294 --> 00:17:10,661
He visits
the marinelli bell foundry
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00:17:10,663 --> 00:17:12,764
In the italian town of agnone.
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00:17:19,739 --> 00:17:23,574
Established in 1040 ad,
the marinelli foundry
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00:17:23,576 --> 00:17:26,778
Is one of the oldest
family businesses in the world.
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00:17:29,716 --> 00:17:34,018
One of its bells hangs today in
pisa's landmark leaning tower.
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00:17:46,299 --> 00:17:48,633
The sheer size and weight
of the bells
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00:17:48,635 --> 00:17:51,636
Makes just hanging them
a risky business.
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00:17:53,706 --> 00:17:57,608
Armando marinelli is the
26th generation of his family
198
00:17:57,610 --> 00:17:59,243
To run the foundry.
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00:18:17,831 --> 00:18:20,665
Bell towers do fall down
200
00:18:20,667 --> 00:18:23,935
And none more dramatically
than the bell tower
201
00:18:23,937 --> 00:18:27,605
Of st. Mark's basilica
in venice, northeast Italy.
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00:18:29,876 --> 00:18:31,409
In the 16th century,
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00:18:31,411 --> 00:18:33,311
The bell tower
in st. Mark's square
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00:18:33,313 --> 00:18:36,414
Stands over 300 feet tall.
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00:18:36,416 --> 00:18:40,518
Five swinging bells ring out
for 400 years,
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00:18:40,520 --> 00:18:45,256
But in 1902, without warning,
a huge crack appears,
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00:18:45,258 --> 00:18:48,526
And within 3 days,
it crashes to the ground.
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00:18:53,700 --> 00:18:56,000
Could the same fate
befall the iconic
209
00:18:56,002 --> 00:18:57,735
Leaning tower of pisa?
210
00:18:57,737 --> 00:19:03,274
♪
211
00:19:03,276 --> 00:19:06,244
Pietro croce wants to test
what kinds of stresses
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00:19:06,246 --> 00:19:08,412
A bell generates
when it swings.
213
00:19:16,289 --> 00:19:19,090
Pietro chooses a bell
of similar size and weight
214
00:19:19,092 --> 00:19:21,125
To bells
in the leaning tower.
215
00:19:24,130 --> 00:19:27,398
He places markers on the bell
216
00:19:27,400 --> 00:19:29,867
And uses a high-speed
digital camera
217
00:19:29,869 --> 00:19:33,304
Filming at 960 frames per second
218
00:19:33,306 --> 00:19:36,274
To track the motion
of the bell as it swings.
219
00:19:39,646 --> 00:19:43,214
This will allow him to calculate
the points of acceleration
220
00:19:43,216 --> 00:19:45,616
And deceleration when it moves.
221
00:19:45,618 --> 00:19:50,154
♪
222
00:19:50,156 --> 00:19:52,089
How much greater
are the forces created
223
00:19:52,091 --> 00:19:54,759
By a swinging bell
than a static one?
224
00:19:54,761 --> 00:19:59,897
♪
225
00:19:59,899 --> 00:20:02,967
Pietro works with university
colleague maria luisa
226
00:20:02,969 --> 00:20:04,835
To make his calculations.
227
00:20:31,598 --> 00:20:34,065
They conclude that the bell
generates forces
228
00:20:34,067 --> 00:20:36,934
Twice its weight
when it swings halfway.
229
00:20:39,038 --> 00:20:41,105
What if it swings all the way?
230
00:20:53,586 --> 00:20:56,921
A bell that exerts 1 ton of
static load on the tower
231
00:20:56,923 --> 00:21:00,825
Could create four times
that amount with each swing.
232
00:21:05,031 --> 00:21:09,367
Swinging bells in a belfry
generate a horizontal force
233
00:21:09,369 --> 00:21:12,336
That acts on the tower
through the wooden axle.
234
00:21:14,440 --> 00:21:16,474
When many bells
swing separately,
235
00:21:16,476 --> 00:21:21,112
The forces cancel out,
but when they swing together,
236
00:21:21,114 --> 00:21:24,749
The bells create a dangerous
surge in one direction.
237
00:21:24,751 --> 00:21:30,454
♪
238
00:21:30,456 --> 00:21:32,156
How much damage are the bells
239
00:21:32,158 --> 00:21:34,425
Doing to the
leaning tower of pisa?
240
00:21:34,427 --> 00:21:39,697
♪
241
00:21:39,699 --> 00:21:43,567
Pietro takes a closer look
inside the belfry itself.
242
00:22:21,374 --> 00:22:26,143
Today no one is taking
any chances.
243
00:22:26,145 --> 00:22:29,347
The bells no longer swing.
244
00:22:29,349 --> 00:22:32,216
Automated clappers create
the chimes that ring out
245
00:22:32,218 --> 00:22:33,851
Over the city of pisa.
246
00:22:36,356 --> 00:22:38,923
Despite the forces
ranged against it,
247
00:22:38,925 --> 00:22:41,525
The leaning tower still stands,
248
00:22:43,930 --> 00:22:47,465
But how does the tower
survive over 800 years
249
00:22:47,467 --> 00:22:49,400
With its precarious tilt?
250
00:22:51,471 --> 00:22:54,372
Could the answer lie
with a curious pair of towers
251
00:22:54,374 --> 00:22:56,540
95 miles away?
252
00:23:11,391 --> 00:23:14,225
♪
253
00:23:14,227 --> 00:23:17,194
Narrator:
The leaning tower of pisa:
254
00:23:17,196 --> 00:23:21,565
It's a marvel
of 12th-century engineering,
255
00:23:21,567 --> 00:23:23,267
A precarious balancing act
256
00:23:23,269 --> 00:23:26,103
That seems to defy
the laws of physics.
257
00:23:29,742 --> 00:23:34,044
What hidden secret lies behind
the tower's miraculous survival?
258
00:23:37,250 --> 00:23:41,018
Does the answer lie with these
two medieval watchtowers
259
00:23:41,020 --> 00:23:44,355
95 miles away
in the city of bologna?
260
00:23:44,357 --> 00:23:50,027
♪
261
00:23:50,029 --> 00:23:54,231
The towers of bologna
are simple brick skeletons
262
00:23:54,233 --> 00:24:00,671
With metal braces
and iron rings.
263
00:24:00,673 --> 00:24:05,309
The taller tower rises 318 feet
into the air
264
00:24:05,311 --> 00:24:09,180
And has a 2-degree lean,
265
00:24:09,182 --> 00:24:11,482
But the shorter tower
dangerously tilts
266
00:24:11,484 --> 00:24:15,119
More than double this amount.
267
00:24:15,121 --> 00:24:18,355
Both towers use the same
materials
268
00:24:18,357 --> 00:24:23,627
And have the same foundations
in the same dense sand and clay.
269
00:24:25,665 --> 00:24:28,332
If their construction
is identical,
270
00:24:28,334 --> 00:24:31,435
What accounts for the difference
in height and lean?
271
00:24:34,941 --> 00:24:38,008
Could the answer to this mystery
also reveal the secret
272
00:24:38,010 --> 00:24:41,679
Of the leaning tower of
pisa's incredible balancing act?
273
00:24:41,681 --> 00:24:49,553
♪
274
00:24:49,555 --> 00:24:52,957
You might ask yourself,
"why is it that a shorter tower
275
00:24:52,959 --> 00:24:55,993
Is leaning more
than a taller tower?"
276
00:24:58,164 --> 00:25:03,100
Narrator: Nick o'riordan is
a geotechnical engineer.
277
00:25:03,102 --> 00:25:07,872
O'riordan:
It is almost always the passage
of time which holds the key
278
00:25:07,874 --> 00:25:12,109
To why things
are the way they are.
279
00:25:12,111 --> 00:25:15,679
Narrator: Nick believes a clue
to the puzzle lies in the soil
280
00:25:15,681 --> 00:25:17,481
The two towers are built on.
281
00:25:17,483 --> 00:25:22,586
♪
282
00:25:22,588 --> 00:25:26,457
In a london laboratory,
he investigates.
283
00:25:26,459 --> 00:25:29,660
This force is basically
modeling the effect
284
00:25:29,662 --> 00:25:32,997
Of loading from the tower.
285
00:25:32,999 --> 00:25:35,432
In the cylinder
is a bore sample of soil
286
00:25:35,434 --> 00:25:37,501
Taken from deep underground.
287
00:25:40,306 --> 00:25:42,339
Fellow engineer
christopher russell
288
00:25:42,341 --> 00:25:45,776
Helps nick set up the test.
289
00:25:45,778 --> 00:25:49,213
It's immediately taken load,
a lot of load.
290
00:25:49,215 --> 00:25:51,849
In fact,
that is a lot of load.
291
00:25:51,851 --> 00:25:54,785
The test mimics what happens
to the soil underground
292
00:25:54,787 --> 00:25:59,089
When a building
is erected quickly.
293
00:25:59,091 --> 00:26:01,992
The force is building up
and building up.
294
00:26:04,263 --> 00:26:08,332
At first, the soil gets stronger
as it becomes more compact.
295
00:26:11,270 --> 00:26:15,873
So, 30 percent
gain in strength.
296
00:26:15,875 --> 00:26:19,009
But the load increases faster
than the soil's ability
297
00:26:19,011 --> 00:26:20,778
To settle and strengthen.
298
00:26:23,049 --> 00:26:26,517
So this is as though
the tower is being
299
00:26:26,519 --> 00:26:29,720
Built very rapidly indeed.
300
00:26:29,722 --> 00:26:33,624
The soil can only withstand
so much pressure.
301
00:26:33,626 --> 00:26:36,627
It's hit its peak,
and it's coming over the top.
302
00:26:40,266 --> 00:26:42,933
That is failing.
303
00:26:42,935 --> 00:26:45,669
That is failed soil.
304
00:26:49,442 --> 00:26:53,177
The experiment reveals
the dangers of hasty building.
305
00:26:57,149 --> 00:27:00,951
They examine their soil sample
to reveal further clues.
306
00:27:04,323 --> 00:27:06,624
O'riordan: The shearing
of the soil causes failure,
307
00:27:06,626 --> 00:27:09,960
And you get a polished surface
like this.
308
00:27:09,962 --> 00:27:13,297
If you can imagine walking
through a very heavy clay field
309
00:27:13,299 --> 00:27:16,300
With wellingtons
and you skid on the surface,
310
00:27:16,302 --> 00:27:19,470
Then you get the same sort
of shiny surface
311
00:27:19,472 --> 00:27:25,142
Where your boot sole
has skidded, shearing the soil.
312
00:27:25,144 --> 00:27:27,344
You can just see
the shiny surface
313
00:27:27,346 --> 00:27:31,382
And see the actual
movement lines.
314
00:27:31,384 --> 00:27:35,819
The soil is pushed
to its breaking point.
315
00:27:35,821 --> 00:27:38,722
There comes a point at which
the soil cannot take
316
00:27:38,724 --> 00:27:40,124
Any more stress,
317
00:27:40,126 --> 00:27:43,661
And it's at that point
that the builder ordinarily
318
00:27:43,663 --> 00:27:45,229
Would start to panic
and say,
319
00:27:45,231 --> 00:27:50,534
"oh, my tower is on the move,
and I better stop building,
320
00:27:50,536 --> 00:27:54,805
And maybe I'd better reduce
the height of that tower."
321
00:27:54,807 --> 00:27:57,408
Narrator:
Nick believes that this is
the fate of the shorter
322
00:27:57,410 --> 00:28:01,111
Of the two bologna towers.
323
00:28:01,113 --> 00:28:03,614
The medieval builders
have to knock the top off
324
00:28:03,616 --> 00:28:06,383
To save the whole structure
from collapse.
325
00:28:11,157 --> 00:28:14,625
12th bologna
is filled with slender towers,
326
00:28:14,627 --> 00:28:16,660
Each a symbol of wealth
and power.
327
00:28:16,662 --> 00:28:22,232
♪
328
00:28:22,234 --> 00:28:26,003
Two rival families,
garisenda and asinelli,
329
00:28:26,005 --> 00:28:28,839
Compete for the tallest tower
right next to each other.
330
00:28:28,841 --> 00:28:33,811
♪
331
00:28:33,813 --> 00:28:38,182
The garisendas' tower initially
outpaces the asinellis',
332
00:28:38,184 --> 00:28:40,084
But when it leans dangerously,
333
00:28:40,086 --> 00:28:42,319
They are forced
to decapitate it.
334
00:28:45,591 --> 00:28:50,527
The slower-rising asinelli tower
never tilts more than 2-degrees
335
00:28:50,529 --> 00:28:54,498
And eventually wins this
medieval tortoise-and-hare race.
336
00:28:57,169 --> 00:28:59,336
The slow build of
the asinelli tower
337
00:28:59,338 --> 00:29:03,841
Allows the soil
to settle and consolidate.
338
00:29:03,843 --> 00:29:05,709
That's how they learned
empirically that
339
00:29:05,711 --> 00:29:07,311
You can't build them
too quickly.
340
00:29:07,313 --> 00:29:10,881
You've got to build them
in stages.
341
00:29:10,883 --> 00:29:13,550
Narrator: The story of the
leaning towers of bologna
342
00:29:13,552 --> 00:29:17,354
Also shines light on the most
famous leaning tower of all.
343
00:29:20,359 --> 00:29:23,894
When the tower of pisa is
being built in the 12th century,
344
00:29:23,896 --> 00:29:27,264
Local wars halt work
for 100 years.
345
00:29:29,668 --> 00:29:33,737
It's this that allows the soil
to settle and strengthen
346
00:29:33,739 --> 00:29:37,474
And ensures
the tower's survival.
347
00:29:37,476 --> 00:29:39,610
If they had built
it straight up,
348
00:29:39,612 --> 00:29:42,913
It would probably have got up
to about the fifth level,
349
00:29:42,915 --> 00:29:46,917
And it would've fallen over,
no question about it.
350
00:29:46,919 --> 00:29:49,753
Narrator: The tower survives,
351
00:29:49,755 --> 00:29:54,057
But 800 years later,
it's in peril.
352
00:29:54,059 --> 00:29:57,427
Every month,
it leans a fraction more,
353
00:29:57,429 --> 00:30:00,664
Inching to a point of no return.
354
00:30:00,666 --> 00:30:05,302
Will a high-profile rescue
mission save it or destroy it?
355
00:30:05,304 --> 00:30:09,873
♪
356
00:30:21,620 --> 00:30:25,455
♪
357
00:30:25,457 --> 00:30:28,992
The leaning tower of pisa:
358
00:30:28,994 --> 00:30:33,664
One of the world's
most iconic monuments.
359
00:30:33,666 --> 00:30:37,401
By the 1990s, the lean
is increasing every year.
360
00:30:39,672 --> 00:30:45,475
It's incremental but relentless
and can only end one way.
361
00:30:45,477 --> 00:30:53,116
♪
362
00:30:53,118 --> 00:30:58,121
In 1995, engineer john burland
joins a high-profile
363
00:30:58,123 --> 00:31:00,891
Italian commission
to save the tower.
364
00:31:03,896 --> 00:31:07,331
He's got a fine line to tread.
365
00:31:07,333 --> 00:31:09,600
There were two unacceptable
outcomes
366
00:31:09,602 --> 00:31:10,834
For our work on the tower.
367
00:31:10,836 --> 00:31:15,439
One we don't talk about,
mainly it falling over,
368
00:31:15,441 --> 00:31:17,841
But the other one,
which was equally unacceptable,
369
00:31:17,843 --> 00:31:19,376
Was a straight tower.
370
00:31:19,378 --> 00:31:22,379
I mean, the pisans didn't want
a tower that was made vertical
371
00:31:22,381 --> 00:31:26,316
Because it's no longer
a leaning tower.
372
00:31:26,318 --> 00:31:28,018
Narrator:
John develops a computer model
373
00:31:28,020 --> 00:31:31,221
To predict every aspect
of the tower's tilt,
374
00:31:33,559 --> 00:31:37,861
But his figures
reveal an enigma.
375
00:31:37,863 --> 00:31:41,164
The model produced everything
on the inclination beautifully,
376
00:31:41,166 --> 00:31:42,566
Almost perfectly.
377
00:31:42,568 --> 00:31:44,701
It's just that,
right at the very end,
378
00:31:44,703 --> 00:31:49,606
The model told us
that the tower was falling over.
379
00:31:49,608 --> 00:31:52,743
Narrator:
According to john's model,
the tower will fall over
380
00:31:52,745 --> 00:31:56,513
If it leans at more
than 4.75 degrees,
381
00:31:59,051 --> 00:32:03,854
But it's already reached
5.5 degrees.
382
00:32:03,856 --> 00:32:07,791
It's a finding with
chilling implications.
383
00:32:07,793 --> 00:32:11,795
Our conclusion was that
the tower must be very,
384
00:32:11,797 --> 00:32:18,368
Very close to falling over.
385
00:32:20,773 --> 00:32:24,708
Narrator:
An urgent solution is needed
to save this historic monument.
386
00:32:27,212 --> 00:32:30,814
The pisa commission
finally agrees on a rescue plan.
387
00:32:30,816 --> 00:32:35,686
♪
388
00:32:35,688 --> 00:32:42,793
They fix 10 steel cables to the
foundations on the north side,
389
00:32:42,795 --> 00:32:47,731
Which extend
150 feet underground,
390
00:32:47,733 --> 00:32:52,836
And attach to massive anchors
in the dense sand,
391
00:32:52,838 --> 00:32:56,173
But the soil is waterlogged.
392
00:32:56,175 --> 00:32:58,942
To stop the excavations
from flooding,
393
00:32:58,944 --> 00:33:03,880
Engineers freeze the ground
with liquid nitrogen,
394
00:33:03,882 --> 00:33:06,416
But the ground suddenly swells,
395
00:33:06,418 --> 00:33:09,086
And the tower tilts
more in 1 day
396
00:33:09,088 --> 00:33:12,522
Than it has in a whole year.
397
00:33:12,524 --> 00:33:14,958
Is the tower about
to be lost for good?
398
00:33:14,960 --> 00:33:21,865
♪
399
00:33:21,867 --> 00:33:25,635
Burland: It was very, very
frightening and dramatic.
400
00:33:25,637 --> 00:33:28,505
That was the time
I wrote in my diary,
401
00:33:28,507 --> 00:33:30,674
"I fear we may have
lost the tower."
402
00:33:33,112 --> 00:33:36,747
Narrator:
In desperation, engineers fix
additional counterweights
403
00:33:36,749 --> 00:33:39,416
To the opposite side
of the tower's lean.
404
00:33:41,754 --> 00:33:45,055
They were stacked one
by one on the north side
405
00:33:45,057 --> 00:33:46,390
Of the tower here,
406
00:33:46,392 --> 00:33:48,458
So it's a bit like sitting out
on a sailing boat.
407
00:33:48,460 --> 00:33:52,662
You sit out and stop
it leaning so much.
408
00:33:52,664 --> 00:33:56,366
Narrator:
The emergency fix stops
the tower from toppling over,
409
00:33:58,370 --> 00:34:02,239
But its lean is worse than ever.
410
00:34:02,241 --> 00:34:05,809
How can they save
the 16,000-ton giant now?
411
00:34:08,647 --> 00:34:11,014
John proposes a radical idea.
412
00:34:11,016 --> 00:34:16,253
♪
413
00:34:16,255 --> 00:34:20,323
In preparation, john's team
secures heavy-duty steel cables
414
00:34:20,325 --> 00:34:22,059
Around the table
415
00:34:22,061 --> 00:34:24,061
And anchors them
into the ground.
416
00:34:27,866 --> 00:34:31,468
Engineers dig a tunnel up to the
foundations on the opposite side
417
00:34:31,470 --> 00:34:37,574
Of the lean and carefully remove
soil with great precision.
418
00:34:40,279 --> 00:34:41,978
Over the course of a year,
419
00:34:41,980 --> 00:34:44,281
The tower settles
into the cavity,
420
00:34:44,283 --> 00:34:47,417
Reducing the overhang
by nearly 2 feet.
421
00:34:49,822 --> 00:34:54,791
In 2001, the cables
and counterweights are removed,
422
00:34:54,793 --> 00:34:56,860
Revealing the leaning tower
of pisa
423
00:34:56,862 --> 00:34:58,795
In all its glory once again.
424
00:34:58,797 --> 00:35:03,934
♪
425
00:35:03,936 --> 00:35:07,404
The tower,
it's just extraordinary.
426
00:35:07,406 --> 00:35:11,675
I mean, it just glows, so it's
a privilege actually to go back
427
00:35:11,677 --> 00:35:13,076
And think you've played a part
428
00:35:13,078 --> 00:35:16,980
In conserving this
incredible monument.
429
00:35:19,651 --> 00:35:25,021
Narrator:
Today, the tower stands
straighter and safer,
430
00:35:25,023 --> 00:35:28,925
But does that mean
it won't ever fall?
431
00:35:28,927 --> 00:35:31,695
Italy is in one of the most
notorious seismic zones
432
00:35:31,697 --> 00:35:34,731
In the world.
433
00:35:34,733 --> 00:35:38,401
Could the tower still be brought
down by a powerful earthquake?
434
00:35:38,403 --> 00:35:44,241
♪
435
00:35:57,456 --> 00:36:00,857
Narrator:
The leaning tower of pisa:
436
00:36:00,859 --> 00:36:03,660
An 800-year-old medieval
masterpiece
437
00:36:03,662 --> 00:36:05,028
Saved from tipping over
438
00:36:05,030 --> 00:36:07,531
By the efforts
of modern engineers.
439
00:36:07,533 --> 00:36:13,470
♪
440
00:36:13,472 --> 00:36:15,639
But the leaning tower
is in one of the world's
441
00:36:15,641 --> 00:36:17,807
Deadly earthquake zones.
442
00:36:19,912 --> 00:36:22,812
Is it still at risk
from a cataclysmic collapse?
443
00:36:22,814 --> 00:36:30,253
♪
444
00:36:30,255 --> 00:36:35,926
In 2016, a major earthquake,
445
00:36:35,928 --> 00:36:38,528
Measuring 6.5
on the richter scale,
446
00:36:38,530 --> 00:36:41,431
Hits the town of amatrice,
south of pisa.
447
00:36:45,938 --> 00:36:48,171
The town is flattened.
448
00:36:48,173 --> 00:36:50,607
Around 300 people die,
449
00:36:50,609 --> 00:36:53,410
And 4,000 more
are left homeless.
450
00:37:17,035 --> 00:37:20,136
One of the surviving buildings
dominates the skyline.
451
00:37:23,909 --> 00:37:27,477
It's the town's 82-foot-tall
clock tower.
452
00:37:29,982 --> 00:37:32,782
Astonishingly,
underneath the scaffolding,
453
00:37:32,784 --> 00:37:35,852
This medieval building
remains intact.
454
00:37:50,569 --> 00:37:58,875
♪
455
00:37:58,877 --> 00:38:02,746
Every structure
has a natural frequency.
456
00:38:02,748 --> 00:38:05,081
This is the rate at which
it will vibrate
457
00:38:05,083 --> 00:38:08,818
When it's jostled or struck.
458
00:38:08,820 --> 00:38:11,121
Tall
objects have a low frequency.
459
00:38:11,123 --> 00:38:13,657
They move from side
to side slowly.
460
00:38:16,028 --> 00:38:18,628
Short structures
have a high frequency.
461
00:38:18,630 --> 00:38:21,031
They move from side
to side quickly.
462
00:38:23,602 --> 00:38:27,170
In an earthquake, a shorter
structure will start vibrating
463
00:38:27,172 --> 00:38:29,339
At its natural high frequency,
464
00:38:31,443 --> 00:38:33,877
But an earthquake's
own high-frequency waves
465
00:38:33,879 --> 00:38:37,047
Can intensify the vibration
of a shorter building.
466
00:38:40,452 --> 00:38:43,286
This can make it shake more
and more violently
467
00:38:43,288 --> 00:38:44,954
Until it falls apart,
468
00:38:49,661 --> 00:38:54,230
But it's a different story
for taller buildings.
469
00:38:54,232 --> 00:38:56,366
The high-frequency waves
of an earthquake
470
00:38:56,368 --> 00:38:59,636
Don't match the low frequency
of a tall building.
471
00:39:02,207 --> 00:39:04,874
This is why it can be
left unscathed.
472
00:39:17,122 --> 00:39:19,756
The tower of pisa's tall
and slender shape
473
00:39:19,758 --> 00:39:22,158
Helps protect it
from seismic activity
474
00:39:22,160 --> 00:39:24,361
Like the amatrice clock tower.
475
00:39:31,136 --> 00:39:35,205
But the pisa tower also leans.
476
00:39:35,207 --> 00:39:37,807
The fact that the tower
is leaning means
477
00:39:37,809 --> 00:39:44,514
That it has much higher stresses
on one side than the other,
478
00:39:44,516 --> 00:39:46,750
So the fact that
when it vibrates
479
00:39:46,752 --> 00:39:49,652
It is generating high stresses
480
00:39:49,654 --> 00:39:53,022
Would mean that
it would be more vulnerable.
481
00:39:53,024 --> 00:39:55,959
This presents a puzzle.
482
00:39:55,961 --> 00:39:59,062
The lean of the tower creates
such a vulnerability
483
00:39:59,064 --> 00:40:02,432
That even moderate earthquakes
in the region should topple it.
484
00:40:05,470 --> 00:40:10,407
Against the odds, the leaning
tower of pisa remains standing.
485
00:40:10,409 --> 00:40:12,041
Why?
486
00:40:14,646 --> 00:40:16,713
[ laughs ]
487
00:40:23,321 --> 00:40:24,821
Narrator:
The leaning tower of pisa
488
00:40:24,823 --> 00:40:27,824
Still holds onto
some of its mysteries.
489
00:40:27,826 --> 00:40:34,631
♪
490
00:40:34,633 --> 00:40:38,568
It stands today as the true
miracle of miracle square,
491
00:40:41,673 --> 00:40:43,740
Capturing the imagination
of visitors
492
00:40:43,742 --> 00:40:45,542
From all over the world.
493
00:40:47,846 --> 00:40:51,881
Burland:
It's thrilling to go back and
just to see everyone enjoying it
494
00:40:51,883 --> 00:40:53,850
And what their reactions are.
495
00:40:57,022 --> 00:41:00,690
Narrator: The iconic leaning
tower not only still stands
496
00:41:00,692 --> 00:41:03,927
But looks set to survive
for centuries to come.
497
00:41:19,478 --> 00:41:23,313
The tower of pisa survives
800 years of gravity,
498
00:41:23,315 --> 00:41:26,149
Structural stress
and earthquakes.
499
00:41:29,287 --> 00:41:33,223
Through a combination
of incredible coincidences
500
00:41:33,225 --> 00:41:39,362
And blind luck,
the leaning tower still stands,
501
00:41:39,364 --> 00:41:42,665
A testament to medieval
and modern engineering.
502
00:41:42,667 --> 00:41:52,242
♪
503
00:41:52,244 --> 00:41:54,577
[ bells tolling ]
504
00:41:54,579 --> 00:42:04,220
♪
505
00:42:04,222 --> 00:42:10,793
♪
44856
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