All language subtitles for Ancient.Superstructures.S01E04.720p.HDTV.x264-CBFM

In the Middle East

between the Red Sea
and the Dead Sea is Petra.

A city nestled in the middle
of the Jordanian desert,

in a vast rocky massif.

Built over 2000 years ago

by a nomadic Arab population
called the Nabataeans,

many people consider it
the eighth wonder of the world.

It is an incredible place.

The colours of Petra
are just amazing.

You feel you are not on this planet.

Petra is known for its huge facades,
carved out of the rock,

which are often used
as scenery for films.

But taking to the skies,

we discover an archaeological site
stretching 260 square kilometres

with over 850 remains.

An immense city which still has
many secrets to uncover.

It's like a puzzle, a huge puzzle.

We only have a glimpse
of this glorious city.

This city, forgotten for centuries,

was rediscovered by a Swiss explorer
200 years ago.

Today, archaeologists are still
trying to complete its story

and unravel the mysteries
that elude them.

So we need some good pictures
of this area.

With the help of the most modern
scientific tools,

they can probe
the work of ancient builders.

These technologies are giving us

real insight
into the building methods.

By examining Petra
from different perspectives,

from the infinitely big
to the infinitely small,

experts can detect details
that are invisible to the human eye.

It's very exciting. We're
almost on the telephone to the past.

Thanks to digital tools,

the luxurious structures from Petra's
golden age are brought back to life.

This mythical city, which was
at its peak in the 1st century B.C.

can finally reveal
its true splendour.

Petra demonstrates opulence.

These cascading fountains and pools.
It's almost as if it's showing off.

A Las Vegas of its time.

We will unlock the secrets
of this sprawling desert city

by examining
its spectacular structures,

starting with its most
famous monument, Al-Khazneh.

It is so dramatic, because it's
sculpted out of the rock itself.

What techniques did the Nabataeans,
a population of nomadic traders,

use to build such tall structures
without modern tools?

How did their masons transform

this raw stone
into an architectural diamond?

And how did they find water
in the middle of the desert

and create one of Antiquity's
most advanced hydraulic systems?

It had water and that is
Petra's secret.

The main ingenious engineering
of the Nabataeans

was their hydraulic engineering.

History, science, construction -

examined like never before.

Petra reveals its secrets
on every scale.

To understand Petra, you first
have to understand the secrets

of this fascinating city's
construction.

It was built in the 1st century B.C.

by a population of nomads,
the Nabataeans.

At the time, and still today,
each visitor would enter

by following this astonishing
and winding natural passage,

known as the Siq.

1200 metres long and, in places,
less than 3 metres wide,

it leads to this breath-taking
building, Al-Khazneh.

The outside was meant to be seen.

This was...to woo.

For archaeological experts,
this monument is a true enigma.

Now, thanks to new technology,
they can unlock its secrets.

It's just mind boggling,
the skill that went into it.

The lines are so clean,
the reliefs are so beautiful.

And there's just no mistakes.

If you look at
the detailed carvings,

not only of the figures,
but the decorations.

Perfect!

This incredible lavishly decorated
temple is about 39 metres high,

the equivalent
of a 12-storey building.

But how did the Nabataens
carve a structure

which is as monumental
as it is perfect, from solid rock?

The interesting thing
when we see the Khazneh

is first obviously its scale.

It's always been a mystery as to how

they were able to construct these
sculptures so high above the ground.

In this arid desert, wood resources
are practically non-existent.

It is impossible to find sufficient
planks to build scaffolding.

So, how did the Nabataeans raise
themselves 40 metres from the ground

in order to sculpt Al-Khazneh?

To solve this mystery,
archaeologists today,

combine traditional
observation techniques

with sophisticated
technology

revealing details that the human eye
can't easily perceive.

Qais Tweissi is the site's
technical curator

Thanks to this machine
called a "total station",

he can take precise measurements
of the building.

Then, he processes
the data on a computer

to build an extremely
accurate 3D projection.

Which gives us a very good accuracy,
which is about 0.01 mm,

which is very good
for documentation.

The 3D model lets experts examine
the smallest details of the facade.

But do any clues remain

that could reveal the Nabataeans'
construction techniques?

Details like the remains
of fastenings for scaffolding?

The first feature
which become apparent

are notches on either side
of the monument.

Changing the scale,
thanks to digital zoom,

reveals the shape
of these strange notches.

They are deeper at the base
than at the top.

They appear to be shaped
for a human foot.

But did Nabataean builders use them
to access the construction site?

One of the theory for those foot
holes is for climbing the monument

looking for special finds like gold,

and also for broking
the figures of the monument.

So these notches were hollowed out
by raiders and not by the builders.

Thanks to a careful examination of
the facade and the digital model,

archaeologists have also ruled out
other false leads.

For example, these small holes

are not the remains of fastenings
for scaffolding,

they're bullet holes
or marks from tools

made long after
the construction of Al-Khazneh.

To solve the enigma
of its construction,

experts have had to find
other clues.

Thibaud Fournet is a French
architect and engineer at the CNRS.

He has been studying one particular
cliff for several months,

using a variety of techniques.

This cliff is the site of a unique,
partially carved building

known as
the "unfinished monument".

The site allows Thibaud to analyse
the ancient builders' work.

This 4-column monument was intended
to run down the side of the cliff.

Only the upper parts
have been carved.

Which means that the stone-cutters
started from the top.

But how could they have worked
on such a steep cliff?

You can see this reddish part.
Yes.

I need on this part,
good pictures from quite close.

Today, Thibaud wants to observe
architectural features

at the top of the facade.

Just like the Nabateans,

he has also had to find a way
to access the cliff.

Thanks to this drone,

the team can collect clues
that were unattainable until now,

in order to process them in 3D.

These previously unseen images,

alongside traditional observations
made 20 years ago,

allow archaeologists
to piece together

the secrets of the Nabataeans'
construction techniques.

To sculpt this kind of structure,

ancient stone-carvers
started by smoothing the rock,

creating a flat wall which marked
out the front of their building.

On this blank canvas of rock,

the Nabateans hollowed out
small terraces

which allowed them to sculpt
the rock face in complete safety.

After completing the upper parts,

workers needed extreme attention to
detail as they gradually descended,

finishing with the foundations.

The fact that they had to do
the whole facade in one go

with no chance to restart
or to repair a mistake

is something very courageous and
a risk that nobody could bear today.

In Petra, you need preciseness
and engineering,

that means if a mistake would go,

that means the whole facade
is destroyed.

Researchers haven't found any plans
or written records about Al-Khazneh,

but they assume that the Nabataeans
planned their construction

to the smallest detail.

At Al-Khazneh,
the ancient builders erased

all traces of their
construction as they worked.

But taking a step back,

we can see evidence
that this structure was also built

using the inverted
construction technique.

To the right of the monument,
these stairs,

discovered at the same time
as the site in the 19th century,

allowed workers to access
the construction site.

But a mystery remains.

How can this monumental facade stand,

when it was impossible
for the Nabataeans

to test the strength
of its foundations

before they started building?

The images taken by drone
reveal an immense massif

which overlooks Al-Khazneh.

Hundreds of tons of rocks
weigh down on the monument.

How could the Nabataeans be sure that
these structures wouldn't crumble

underneath this immense weight?

One of the principles
of construction

is that you have to
transfer the load

from its highest point
down to the ground,

and when you look at the sculpture,
each of the elements,

the weight is transferred down
to the ground through the column.

At first glance, the support element
seems obvious.

A monumental lintel
held up by six immense columns,

cut directly into the rock face.

But appearances can be deceiving.

On 19th century engravings,

we discover that one of
the load-bearing columns was broken.

It wasn't repaired until the 1960s.
Yet the building didn't collapse.

The 3D model of the monument, built
using data from the total station,

reveals the solution to
what holds up the structure.

From the side, we realise that
the lintel is built into the cliff.

So are all of the columns,
except for the two central ones.

So it's the rock wall itself which
is supporting most of the weight,

and not the facade which is almost
like an optical illusion.

The interior of Al-Khazneh
confirms this hypothesis.

Usually it is closed to the public.

But Qais Tweissi has special
permission to explore inside.

Surprisingly, the interior of
Al-Khazneh is much less spectacular

than the outside suggests.

This looks from outside very huge,

and from the interior,
small chamber.

One master chamber,
ten metres high and undecorated

leading to three
much smaller rooms.

This transition
from gigantic to miniature,

between exterior
and interior,

shows that the Nabataeans
knew about force dynamics,

and were careful to evaluate
the risk of collapse.

You need to triangulate any loading,

so that it can go over the structure
and back down to the earth,

and if you hollow it out to too
great an extent, it would collapse

and it would have brought
the facade down with it.

These modest rooms pose question
about the very nature of Al-Khazneh.

Even today, experts still
can't agree on its exact function.

One thing is for sure, this
structure was designed to impress.

It is the first thing you see
when entering the site.

Thanks to the support of the cliff,
the Nabataeans ensured

that this spectacular monument would
remain standing 2000 years later.

For centuries, Petra has survived
many dangers -

erosion, earthquakes,
enemy attacks.

But the city also has to withstand
another major challenge.

A hazard that's unexpected
in the desert.

From this blue sky,
it's hard to believe

how it mists up during
the winter season.

THUNDER RUMBLES

Between October and January, the
city can be hit by torrential rain.

And the rain here
always starts suddenly.

The other side of this is that
there is the danger of flash floods.

In the event of a storm,

rainwater flows between gaps in
the rock and funnels toward the Siq.

This natural gorge can transform
into a furious torrent

threatening the entire city,
starting with Al-Khazneh.

Even in my 12 years here,
there were two times where you saw

the water roll into centre city
and wreak havoc,

that major boulders
moved along.

And you realised that people needed
to be evacuated out of the city.

In November 2018, 4000 tourists
had to be urgently evacuated

to avoid being trapped
by devastating mudslides.

But what did the Nabataens do
when faced with this danger?

To answer this question,

archaeologists first looked
for clues at the Siq -

the narrow gorge
that allows access to the city.

The Siq was the weakest point

of entire infrastructure
of the city of Petra,

so they had to protect
this paved road from destruction.

During the first excavations
in the 19th century,

archaeologists discovered an amazing
passage carved through the rock.

A tunnel, three metres wide and
five metres high, leading to a canal.

We're now in the middle of

one of the most impressive
hydrological structures in Petra.

This tunnel was partly cut
in the rock 2000 years ago.

This tunnel was associated
with another diversion dam

that was constructed not far
away from it

and both structures
were constructed

in order to manage
the water floods

and prevent them
from entering the city.

The dam was built
near the city's entrance,

diverting water towards
the tunnel and away from the Siq.

An ingenious solution that prevented
the destruction of Petra's jewel.

Other dams, discovered later,
completed this protection system.

Ahmad Hasanat is an expert
in digital archaeology.

By taking thousands of pictures
of the protection system's remains,

he has been able to make 3D digital
models of 36 individual sites.

We are standing in one of these dams

which is a clear example
about how the Nabateans

used to protect
their cities from flash floods.

Today, most of the dams have been
restored and returned to service.

Nabataean system was re-erected

by the Jordanian department
of Antiquities

for the protection of people,
tourists going into Petra.

It's the first time ever
in history of archaeology

that I did archaeology
not only for curiosity

but to get to know
how the system works

and whether it's possible
to restore it,

that it works again, and
that's what I did, it works again.

The system still functions and is
proving to be an ingenious feature.

Because the Nabataeans
faced another major problem -

how to supply the city with water
all year round

when it only rains in winter?

It has 200 millimetres
of water a year,

whereas in Western Europe
we have 900.

They needed to harness that.

There is no life without water.

How did the city's 30,000 inhabitants
obtain fresh water?

And how did they cultivate the land
to feed themselves?

The water at Petra has a reliable
source up in the mountains.

But it is some distance
from the city.

Satellite imagery reveals that
the closest waterway, the Ain Moussa,

is situated 9km away.

So how did ancient engineers
provide drinking water

to the city's inhabitants, without a
nearby water source or regular rain?

The first answer can be found
at the dams -

built to protect the city
from flash floods.

So Nabataeans built this,

first of all to protect the city
from the flash floods,

and then to use the water,
to collect the water,

because they were
very careful and wise

to collect every drop of water.

Every drop of rain that
trickled down the rock face

was channelled and directed into
water tanks where it could be stored.

There were as many as
200 tanks in the city.

Many are still visible
to the naked eye.

Some from high up, others
like this one, from the ground.

But there was a problem.

The water that collected
in these tanks wasn't drinkable.

It was not only water,
it was dung from the birds,

it was branches from bushes,
it was dry leaves, god knows what.

And then the water had to be
collected and to still down,

and then all the material
fell down to the bottom.

In 1999, Ueli Bellwald carried out
tests on the rainwater.

The results allowed him to understand

how the Nabataeans
obtained drinking water.

They stored water in a first pool

and waited for the waste
to sink to the bottom

then the water
flowed into a second pool

followed by a third,
controlled by a system of valves.

From the first to the next
to the next,

and at the end of this filtering
process, the water was really clean.

This filtration system
made the water cleaner for drinking.

But how did it reach the city?

The answer can be found here
in the Siq.

On either side of this narrow rocky
gorge are shallow grooves

that Ahmad Hasanat thinks
were carved by the Nabataeans.

One diverted rainwater.

The other channelled in spring water
from the neighbouring mountains.

Actually there is a complicated
system of pipelines

to provide the water
for every single house.

So every single house in Petra
gets the water like our days.

Thanks to his meticulous
digital work,

Ahmad was able
to compile details

of all of the discoveries
made by previous archaeologists.

It allowed him to build
a complete map

of the complex Nabataeans'
hydraulic system.

29km of pipes and five aqueducts

that channelled an astonishing
46 million litres of water

into the city each day.

A change of scale
allows us to discover

other elements of this
stunning hydraulic system.

Some of the rock-cut channels

retain the faint traces
of terracotta pipes.

If we look up close here,

we can see that the edge
of these pottery pipes,

and also we can see how these pipes
connected together.

The Nabataeans developed
a system of pipes

that was highly advanced
for the time.

Terracotta pipes were
embedded into the rock,

fixed in place with mortar

and the joints sealed
with a waterproof coating.

They made first a clay
like the paste for bread

and then they wrapped this clay
paste around a wooden cylinder

and fixed the two ends together

and then they were firing
these enormous pieces.

These terracotta pipelines preserved
the quality of drinking water.

But the Nabataeans faced
another major problem.

How could these pipes
withstand enormous pressure

created by the steep slope
between the source,

at an altitude of 1300 metres,

and the centre of the city
500 metres below?

Inside the pipelines,
with each percent of gradient,

the pressure inside the pipeline
increases by one bar.

We know from tests that these
terracotta pipes could bear

an internal pressure of 9 bars.

With a slope between 4 and 5 percent,
terracotta pipes won't explode

but they still remain
80% filled with water,

allowing for a regular flow.

This is exactly the angle of
the channels discovered in Petra.

Through experience, the Nabataeans
had managed to determine

the perfect angle for their pipeline
to withstand the pressure -

another feat for the era -

while their system
of aqueducts and valves

let them adjust the flow rate
in their hydraulic network.

Now, the city could be supplied
with sufficient water all year round.

In Petra,

the population
which grew to 30,000 people,

could rely on being able to drink

eight litres of water a day.

They could rely on about 40,000
cubic metres of water per day,

which makes it really incredible.

It means that they had water not
only for the normal domestic uses

but also for public uses.

The Nabataeans figured out
how to transport

an incredible volume of water
into the middle of the desert.

But what could they use
this abundance for?

In the heart of the city,

the study of ruins next to the Great
Temple reveals something astonishing.

At first glance,
these piled slabs and pebbles

don't suggest the presence of water.

The first archaeologists even
thought this was a marketplace.

But, in 2013, a study of soils
on a microscopic scale

made it possible to make
an incredible discovery.

And through the archaeologist
analysis for sand samples,

they have proved that those samples
have seeds of palm tree.

Which give us the idea that

this garden was full of palm tree
as the main tree for the garden.

Amongst the grains of sand,

scientists found remains of
fossilised seeds and plants.

Which meant that the Nabataeans
had mastered irrigation

to create lush gardens.

And that's not all.

By analysing the ground
with georadar,

researchers have detected
the presence of an enormous pool.

Instead of a market,

there used to be an incredible
aquatic complex here

which today's digital imagery
makes it possible to recreate.

Above the garden, there is
this unique swimming pool.

And this swimming pool

is about 44 by 25 metres
and about 2 metres of depth.

It's actually like the size of
Olympic swimming pool nowadays.

An Olympic-sized swimming pool,
one century before Christ.

It seems excessive in a desert
where water is so precious.

But the abundance of water
was key to the Nabataeans

establishing their power
in the region.

Water was used to serve
the city's splendour.

The mastery of the water

and then creating these
cascading fountains and pools,

it's basically showing off.

The Nabataeans were originally
nomads, caravan drivers.

They knew that in the desert,

water is the most important
source of wealth.

And they took full advantage of it.

In terms of Nabatean economy,
it relied heavily on trade.

The main trade
that brought the most money

was the trade of incense and myrrh.

In ancient times,

incense and myrrh were luxurious,
rare and expensive goods.

Petra became so rich due do its
location at a strategic crossroads.

Today, archaeologists
and historians believe

that the abundance of water
and luxury in the city

were key to the success
of the Nabataean people.

Petra demonstrates opulence

and people would be drawn
from great distances

to come and marvel at this society

and also to spend money there.

It was a financial Mecca,
a Las Vegas of its time.

The city stood out
as a place of opulence.

It was the Dubai
or Las Vegas of its time.

In Petra, everything was made
to dazzle visitors.

Al-Khazneh, as well as
other monuments.

Today scientists are studying

the remains of 850 buildings
from every angle.

They want to understand how the city
developed over the centuries.

These constructions have suffered
the ravages of time

much more than those which
were carved into the cliffs.

However, thanks to new technology,

experts can reveal
their full splendour.

In the heights of the city,
at the summit of Jabal Khubtat,

a French archaeological mission
has taken an interest in these ruins,

which were first discovered
at the start of last century.

So this is the place?
Yes, that's the place,

I think we should stand here.

Archaeologists have long believed
that this was a place of worship.

To verify this theory,

Thibaud Fournet is focussed
on recreating the entire building.

He will use drone photography
to achieve his goal.

I need the main pictures
on this main monument

from this angle to this one,

on different flights, from
the top and the front of the cliff.

By employing a drone to photograph
these remains

from many different angles,

photogrammetry
can be used to visualise

the dimensions and volumes
of the ruined buildings

and reveal their architecture.

3-dimensional re-creations,
in conjunction with excavation work

has revealed a building
measuring 225 square metres,

and composed of seven rooms.

And the research
shows something surprising.

This wasn't a place of worship

but an incredible spa
hanging from the cliff.

Thanks to these digital models,

Thibaud could even reconstruct
the exact path taken by bathers

who visited this luxurious spa.

According to Thibault, this layout is
typical of the thermal architecture

from Latin civilisation.

The Nabataeans must have built
these baths under Roman influence.

Historians believe
that these invaders

took over Petra without a fight.

It became a Roman province
in 106 A.D.

Today, scientists are trying to
understand how the Romans influenced,

or even changed
the city's architecture.

When flying over the site, traces
of Roman presence are evident.

Like this impressive
ancient amphitheatre -

33 rows of seats,

and a 50-metre orchestra area
holding 5000 spectators.

But other sites
can only be uncovered

through the development of complex
investigative techniques.

Shall we go up and set up there?
Yes.

For several years, French experts
have been investigating

one of Petra's most important
monuments - the Qasr al-Bint temple

literally meaning "the castle
of the pharaoh's daughter".

This building was constructed
by the Nabataeans around 30 A.D.

The monument has been
severely damaged over time.

Francois Renel leads a team
of ten archaeologists

supported by
ten Jordanian workers.

They are meticulously
excavating the ruins of the temple.

The team is hunting for traces
of the Roman building works.

The Romans added their own materials

to the top of the limestone and
sandstone of the Nabataean buildings.

Materials like white marble
which they considered more precious.

Experts believe they changed
this Nabataean place of worship

into a temple dedicated
to the glory of their Gods.

The team have carried out a detailed
topographical survey of the site

allowing them to recreate
the dimensions and volume

of the Qasr al-Bint.

Then modelling software could bring
these ruins back to life,

recreating, in three dimensions,

the structure as it looked
2000 years ago.

Thanks to the Roman Empire, Petra
experienced glory days once again.

But without the architectural genius
of the Nabataeans,

these constructions would have
disappeared long ago,

wiped out by earthquakes.

To understand how,
it's necessary to take to the sky.

Satellite imagery reveals

the very particular geographical
situation of this site.

Petra was built
over a seismic fault.

The city is on the boundary
of two sliding plates -

the African plate
and the Arabian plate.

In this area, two continental
plates are moving.

One is shifting below the other

and the other one is shifting over.

As the plates slide over each other,

they can cause particularly
violent earthquakes.

We think about Petra
and we should always remember

that there were many earthquakes
that influenced the city's history.

The biggest one was in May 363 and
devastated the centre city of Petra.

Archaeologists have studied
the Qasr al-Bint

in an attempt to understand
how it could survive centuries

in this highly seismic zone.

Within the partially destroyed walls,

they found a clue -
the remains of wood.

Some experts think that this is
the key to withstanding tremors.

Their engineers, they understood

that they needed to deal
with the tectonic problems.

The Nabataean engineers had invented

one of the first anti-seismic
measures in history.

They had this idea

that this piece of wood
in the middle of the building

should control the cracks,

the vertical and the horizontal
cracks within the building.

Experts think that the framework
of wooden beams,

made of a more flexible material
than the stone around it

absorbs the energy of earthquakes

combatting the effects of stress
in the walls,

the raising of foundations
and even twisting.

Somehow, it's controlling
the balance of the building

which makes a smooth movement
in the blocks of the building

and this technique
is actually approved,

this is a successful technique.

The Nabataeans had risen to
the challenge posed by earthquakes

and they even took some benefit
from the phenomenon.

Because it was seismic activity

that created this city's
exceptional scenery.

In Greek, Petra means "stone"

and it is this extraordinary
raw material

that gave its name to the city.

But what is so particular about it?

And how did the Nabataeans
manage to work with it?

It's not possible
to understand Petra

without understanding its geology.

Petra is carved in a natural rock

so it is important to
understand the raw material.

Seismic activity allowed
the raw building materials

used by the Nabataeans
to rise to the surface.

This is a sedimentary rock
called sandstone

with very specific properties.

Petra is surrounded by mountains
that are made of igneous rocks.

But Petra itself is sandstone

and sandstone is formed
by layers effectively under the sea.

And that means this sandstone
is not like granite,

a stone which is strong and compact,

but it's only like cement,

the grains are still the same
as they ever were

but they are glued together by lime.

When you observe sandstone
under a microscope,

you realise that it is composed of

millions of compressed
grains of sand.

This characteristic explains
how the stone is so easily cut.

To work it, the Nabataeans
could resort

to relatively basic techniques.

Some of their tools even left marks
that are studied by scientists today.

It shows one way

the Nabataeans enhanced
the beauty of their landscape.

When flying over Petra,

the variety of colour
in the monuments is impressive.

The colours and the design
in the bedrock itself,

it's very, very impressive.

An extraordinary range which
stretches from yellow to purple,

as well as pinks, oranges and reds.

This is the only place

where sandstone becomes art.

The colours of Petra
are just amazing.

You cannot stop looking or gazing
at the facades of Petra.

The builders understood
that to embellish the city,

they could utilise the incredible
range of colours

that the sandstone naturally offered.

All different colours of sandstone
of Petra are visible.

Each single layer represents
the different chemical compositions

depending on the typology of
iron oxide which is present inside.

When sandstone forms, it preserves a
record of the different types of sand

deposited over time.

And one element
is particularly spectacular.

There's so much embedded quartzite
in the sandstone,

it refracts the light

and so you get these different
coloured grains within the sand,

which through this reflection,

makes all these different colours
and different bands,

which brings the sculpture to life.

It would be dead
without that geological composition.

It's the composition of the
sandstone, in part thanks to quartz,

that gives the city its splendour.

The Nabataeans used the colours
of this rock to great effect,

but they also suffered
from this stone's weaknesses.

Because the strength
of sandstone varies greatly.

By choosing to carve
their structures here,

the builders were taking a big risk.

It's likely that the instability
of the rock

accounts for
the number of construction sites

found abandoned here.

Even today, the nature of the stone
poses a threat to Petra.

A team of scientists from UNESCO,

led by Georgia Cesario
and Giuseppe Delmonaco,

have the delicate task
of securing this site

and safeguarding
this jewel of the desert.

All the monuments are affected
by similar factors

like wind and water erosion.

It's really fundamental for
the people who take care of the site

to understand this concept in
order to act and to prioritise.

The most fragile area is the Siq -

the passage through which half
a million tourists pass each year.

In May 2019,
just in the place where I am,

a small stone fell
from the top of this cliff,

which is one of the highest
points in the site itself.

And fortunately
it happened in the night

and so we didn't have any
injuries for the visitors.

But in May 2015, a big block
fell from this cliff.

It happened around 11 o'clock
when people were passing through.

Today, new technology
like GPS sensors

give advance warnings
of this type of accident.

Eleven dangerous spots
have been identified at the Siq

and Georgia's team, aided by
rock climbing specialists,

have installed nets in the areas

where the rock is most likely
to crumble.

Then the team installed a highly
sophisticated surveillance system.

On the top of the block,

we put a thermometer and a tiltmeter
to measure the inclination in time.

This device monitors the rock
24 hours a day

sending alerts to the UNESCO team

who can intervene in the event
of danger and evacuate the Siq.

Through fieldwork
and thanks to digital models,

scientists today are trying to bring

the ancient city of Petra
back to life.

For centuries
the city was abandoned,

and even the Nabataeans'
architectural genius

wasn't enough to ensure
they would be remembered.

This ancient people disappeared
in the 7th century,

prompting the fall
of this superb city.

Why did the citizens of this
prosperous and busy city

abandon Petra and flee?

There was an economic decline.

The cause of Petra's lost status
can be traced 500 kilometres away

to the city of Palmyra's
rising economic importance.

But another disaster
played a role, too.

The big turning point,
when damage started again

was definitely
the earthquake of 363 AD.

All the water system, the cisterns

and the aqueducts
were damaged heavily,

and were not, no longer
reconstructed in the future.

The ending of Petra,
it will finally come

when they no longer maintained
those water access points.

There is not enough rainfall there

to sustain agriculture
on a normal basis.

Even the ingenious anti-seismic
measures of the Nabataeans

weren't enough to save the city
from destruction.

But some scientists have suggested
another theory -

that in 400 A.D. the city
fell victim to a different tragedy.

By analysing the ground
of the town's main road,

they discovered the presence
of unusual white sand,

totally different to the local
red or orange sand.

There was a very irregular event

which deposited white sand
in amongst the buildings of Petra.

And that could only have come

from an area some distance
away from Petra.

So it was a monumental flood
which happened

and it could well have
destroyed the entire population

certainly in the centre of the city.

But today this theory is disputed

because signs of life
and even subsequent building work

have been discovered in Petra.

So life went on

but it did not go on

as rich or as strong as before.

These Byzantine churches,
with their famous mosaics,

are the final, glorious chapter
of a city

which is thought to have finally sunk
into oblivion in the 7th century A.D.

The lost city of Petra
was only rediscovered

in the beginning of the 19th century
by a Swiss explorer.

Since then,
this marvel of architecture

has been observed from every angle
and on every scale

by hundreds of scientific missions,

revealing new parts
of the city's history.

We only have glimpses
of the Nabateans,

we only have glimpses of their
glorious city, capital city.

Piece by piece, the secrets
of this city's construction

have been revealed though
meticulous work on all scales.

But mysteries still endure.

So for now,
this pink city of the desert

will remain the perfect
playground for archaeologists,

architects and hydraulic engineers,

all passionate about the incredible
heritage left by the Nabataeans.