Original subtitles

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.