Original subtitles

NARRATOR: In Asia, in Cambodia...

..hidden in the tropical jungle,

is the largest religious structure ever built.

The majestic temple complex of Angkor Wat.

A sprawling structure 1,500 metres long and 1,300 metres wide,

covering 160 hectares, larger than 200 football fields.

It's 200-metre-wide moat surrounds an intricate central sanctuary.

Five gigantic towers, the tallest of which soars 65 metres,

the height of a 25-storey building.

12th-century builders finished this monumental temple

in just four decades, sparking the fascination of archaeologists today.

Today, experts are using science and technology to unlock the secrets

of Angkor Wat by revealing details invisible to the naked eye.

This is a hand-held 3-D scanner.

What's incredible is that we have a perfect virtual model

of the stratigraphy.

By scanning from the satellite scale to the molecular scale

and the aerial scale, archaeologists can now shed light

on the greatest enigmas of this UNESCO World Heritage site.

How did the architects complete this unprecedented project

in just a few decades?

How did the builders supply this site with stone?

What secrets explain how

this thousand-year-old structure remains standing?

The moat of the temple, it's built in the concept.

The sand, when it's humid, it can become strong like the stone.

Why was this monumental temple built in the first place?

And what remains to be discovered

of the legendary city that surrounds it?

We see this mound in many areas,

but we don't know exactly what it is.

History, science, engineering exposed as never before.

The Temple of Angkor Wat finally reveals itself on every scale.

In south-east Asia is Cambodia.

In the north of the country is an area known as Angkor.

It's a site known all over the world,

thanks to the unique remains of the Khmer Empire.

More than 100 temples dating from the 9th to 15th centuries

are hidden amongst 400 square kilometres of tropical forest,

an area four times larger than the city of Paris.

These legendary ruins are visited by 2.5 million people every year

who all want to explore one special temple - Angkor Wat.

It's the largest religious monument on the planet,

dedicated to the god Vishnu and built in just a few decades.

I think Angkor Wat probably constructed

during the life of King Suryavarman II

from approximately 1113 to 1150.

It's about 37 years.

So it's amazing, right?

An extraordinary feat because every part of this superstructure

was built in stone,

a very heavy and labour-intensive material to transport and use.

Examining the stones on a microscopic scale reveals

that this is a sedimentary rock - sandstone.

One cubic metre, it's about one tonne, so it's very heavy.

The material choice raises a mystery.

Examining the site on a satellite scale shows

that the nearest quarries are many kilometres from Angkor.

Some of them came from 30, 40, 50 kilometres away

from the temples themselves.

The blocks of stone are very, very heavy,

and literally millions of these blocks had to be transported

from these distant quarries to the sites of the temples.

We know roughly that about 10 million blocks

have been used on Angkor Wat.

How did Khmer builders transport millions of tonnes of sandstone

in such a short space of time?

To solve this mystery,

archaeologists today rely on the most advanced technology

at their disposal to hunt for clues on a satellite scale.

The Angkor Archaeological Park is overseen

by the APSARA National Authority.

Kim Samnang is the director of its geographical information system...

..a service that collects and analyses mapping data

enriched in recent years by the global view of satellite images.

The zone of Angkor is very big, very huge.

And when we have these kind of satellite areas,

we can learn more how the ancient people

organise or set up their urban cities in that time.

When we zoom in to the Angkor Wat, you can see the structure,

the main temple here, the moat.

And then, another example,

you can see the Angkor Thom as well.

Examining the site on a satellite scale exposes Angkor Wat's location

amongst a vast expanse of ruins.

To the north, the royal city of Angkor Thom is surrounded by

a giant square moat.

A little further west, the satellite images reveal

a huge rectangular area eight kilometres long

and more than two kilometres wide.

We can see clearly from the satellite, the 'baray'.

It's a reservoir that's built by humans in that time,

built by hand, and can store about 56 million cubic metres.

Satellite images also reveal traces of other reservoirs now dried up

but all once connected to Angkor Wat by a series of canals.

One of the more interesting features that we see about Angkor

is its hydraulic network,

its network of canals and reservoirs,

which has been the focus of a lot of debate, a lot of research

and a lot of scholarship in the last few decades.

There are many theories about what these reservoirs

and canals represent.

A sophisticated irrigation system for the population

or for sacred water to symbolically bless the city?

But now another idea has been suggested, and it could help

solve the mystery of how stone was transported to Angkor Wat.

From the stone quarry site, stone quarry site here,

there is one ancient road that connects from here.

You see, the straight line like this close to that area?

The latest scientific research makes it possible to visualise

how the hydraulic network of Angkor was once connected to the quarries.

At the foot of the sandstone source, a network of canals and rivers

stretches some 40 kilometres all the way to the centre of the city,

a route for efficiently transporting millions of tonnes of sandstone.

By connecting the quarries to the central area

of Angkor with canals,

and the blocks would be presumably floated down the canals,

perhaps on rafts made of bamboo.

That was a giant undertaking.

Thanks to their ingenuity,

Khmer builders supplied their site with vast quantities of sandstone.

Examining the material on a microscopic scale

exposes that this sedimentary rock is composed of 40% felspar

and almost 50% quartz.

These are very hard minerals,

which makes the stone difficult to shape.

So how did workers building these sandstone walls

carve the stones quickly?

World Monuments Fund is conducting a restoration project

at Phnom Bakheng.

And this for us is a great opportunity to get to see

more in detail about how the temple was built.

Khmer engineers built the temple of Phnom Bakheng from sandstone

in just 30 years.

Approximately in a day,

a team of two craftsmen will cut and finish two blocks.

Here we're in front of thousands and thousands and thousands of blocks,

so we can imagine how 30 years was really actually a short time

to build this whole structure.

And what we also can imagine is there were a lot of people

working in this working site.

To finish Angkor Wat in just 37 years,

archaeologists calculate that the labour force

could have numbered in the hundreds of thousands.

But building the entire temple in sandstone using traditional methods

would have taken too long,

so Khmer builders must have discovered an engineering technique

that sped up the work.

In Phnom Bakheng, the dilapidated state of the sandstone walls

gives archaeologists a surprising clue about the construction process.

For instance, at this corner,

we can see that a lot of the original wall stone units

were actually collapsed.

And this gives us a chance to see more in detail

the foundations inside the temple mounting.

In the case of Phnom Bakheng,

actually the temple mounting was built around the natural hill.

Here, ancient builders laid sandstone

onto the bedrock of a natural hill to save time.

But at Angkor Wat they couldn't use the same trick.

It was constructed on a flat foundation of sand.

Now 3-D modelling makes it possible

to visualise a remarkable secret hidden inside the structure.

Underneath an outer layer of sandstone blocks,

architects used a red rock called laterite.

Actually behind every sandstone unit,

which we see being part of the raised platforms,

there are laterite blocks.

Examining laterite on a microscopic scale exposes an iron-rich mineral

with a honeycomb structure

whose properties make it particularly easy to work with.

When it is extracted, when it is quarried,

it's actually very much filled with water.

So it's just left drying and then it hardens.

By building from the foundations up using laterite

and then placing the sandstone on top,

Khmer builders sped up the construction of Angkor Wat.

But once the different types of rock had been cut,

they still had to be lifted to build the towers of the central sanctuary,

which are up to 65 metres high.

Today, restoration teams use cranes to quickly move enormous blocks,

weighing between one and three tonnes.

But in the Middle Ages, without modern tools,

how did the Khmer builders solve this problem?

With the help of the latest technology,

experts can now reveal clues that are invisible to the naked eye.

Today, a team from the APSARA National Authority's

photogrammetry laboratory is visiting the site of Banteay Thom,

a small temple built a few decades after Angkor Wat.

Its towers are made of thousands of sandstone blocks.

But how did builders reach the top?

The team plans to take thousands of photographs to create a 3-D model

of the site and record every tiny detail of the ruins.

The operation would be impossible without the use of drone technology.

The drone is really important for the 3-D model for the temple

because we cannot climb up to the top of the temple.

So we need the picture on the top.

By flying over areas usually impossible to reach,

the drone takes unique shots of the temple,

covering a full 360-degree view in minutes.

The team then processes the photos on a computer.

They use powerful software to automatically place the images

in a digital space and create a point-cloud volume.

They can then texture the volume

to re-create the temple in minute detail.

With this 3-D model, we can show everything on the top,

on any direction from anywhere.

We can zoom inside and go to see everything in accuracy

about the decoration, about the structure.

And we can go any place that we cannot go by the human.

By studying the 3-D model of Banteay Thom from all angles,

the team finds strange markings on the sandstone blocks.

Hundreds of holes from the base to the top of the temple.

These are vital clues to solve the mystery

of the construction process.

In Angkor Wat, we can observe in many of these blocks, these holes.

And really refers to the way that the stones were brought into place.

Originally, the holes are where wooden pegs

would have been installed in order to lift the blocks

and bring them to their final location.

3-D imagery exposes a theory

about how Khmer engineers used these holes.

They inserted large wooden sticks directly into these notches

on each side of the stone, allowing them to handle and move the blocks

much more efficiently.

But this doesn't explain how builders lifted them so high up.

I think the way of moving the stone from the bottom to the top,

using the scaffolding is not enough to support the stone

because the stone is too large.

Simple wooden or bamboo scaffolding

could not support the weight of the blocks.

So how did the builders haul tonnes of stone

to heights of 65 metres?

Now archaeologists have found new clues at the site of Rong Chen,

an unfinished mountain temple dating from the 9th century,

allowing them to suggest a theory

that can be revealed using 3-D modelling technology.

Builders constructed earthen ramps

to drag blocks to the top of the construction site.

Then they dismantled the temporary ramps.

Thanks to this ingenious process, the Khmer builders were able to haul

millions of sandstone blocks to the highest points of their site.

But once the blocks were in position, the work was far from over.

The mystery is how, in only four decades,

did the Khmer carve kilometres of bas-reliefs,

thousands of small columns and monumental towers.

To finish in time, architects had to plan ahead

and ensure their work was ultra-efficient.

The process of the construction is the structure built first,

you know, from the base up to the top,

and then the carving went down from the top to the bottom.

It would have been extremely difficult to carve the stone units

when they were not yet in place,

because only once the blocks are perfectly sitting in their place

that it is possible to carve the details to this level.

And it is thanks to this extreme precision in the setting of stones

that we can now have the feeling

that basically it's just one big stone.

Today, examining Angkor Wat from the air,

the monumental temple appears lost in the jungle.

And on the ground, the many structures surrounding it

seem to have been abandoned.

But the temple walls are covered in carvings that illustrate the lives

of people living in a great civilisation.

So where did their homes go,

and how big was the city that once stood here?

EVANS: The temples are made of stone, they're very durable.

They survive relatively well until the present day.

But the cities that grew up around these monuments

were made of wood and other non-durable materials

that have just disappeared almost completely

in this tropical environment.

Which presents a real challenge for us trying to reconstruct

what happened, you know, everyday life around these temples.

Now scientists are lifting the veil

on the true size of the city of Angkor,

thanks to a cutting-edge technology

that operates on an aerial scale - LiDAR.

LiDAR can, like, take out the forest from the ground.

Attached to a helicopter, LiDAR uses light waves.

The rays pass through the forest canopy

and are reflected back to the LiDAR sensors,

allowing researchers to chart the topography

of the ground in unprecedented detail.

In 2012, an international team scanned

an incredible 370 square kilometres around Angkor in just 20 hours.

We were able to map huge areas of the landscape very, very quickly

and with exceptional precision and detail,

achieving in a few hours of flying what may have otherwise taken

years or even decades on the ground.

Results were obviously a revelation for us.

The LiDAR revealed something astounding about the landscape

around Angkor Wat.

Hidden within the temple enclosure,

the team discover an astonishing urban grid

made up of countless man-made hills and depressions.

The Geographic Information System Department keeps

all the LiDAR data collected by the international teams.

See?

This is LiDAR areas.

Different colours represent different elevations.

The darkest are the highest ones.

You can see the grid and you can see the deep shape

upon the small points.

And the dyke.

You see? And a small mound.

And then the traces of the... could be the road here.

That is the way the people organise the cities.

This is the landscape in which people lived,

for the most part, in the central area.

In city blocks, square city blocks,

surrounded by roads or canals.

They built their wooden houses on top of mounds

to stay above the floodwaters during the wet season.

And they also dug ponds into the ground

to keep water during the dry season for household use.

Of course, the wooden neighbourhoods have disappeared,

but the traces of their earthmoving, the traces of these ponds and mounds

remain on the surface.

By using LiDAR, experts discovered the hidden elaborate urban structure

filling the enclosure of Angkor Wat.

All around the sacred temple,

archaeologists have uncovered more than 370 square kilometres

of urban structure.

Evidence of a high population density.

It's very impressive, you know.

Through this information, I can say there are many people

inside the compound, but also outside the compound as well.

The picture that we have today of monuments covered in jungle

are kind of cliched

and the temples themselves represent in a way the kind of skeleton

of the society that lived there

in the middle of Angkor a thousand years ago.

In fact, it would have been a very busy place, full of life,

full of colour, full of noise, full of people moving around.

These builders living at the foot of the Angkor Wat construction site

form part of a gigantic medieval city called Angkor,

the capital of the Khmer kingdom.

SOKRITHY: We probably believe that

more than 800,000 people live in the capital city,

so roughly 1,000,000.

This is maybe the biggest city in the world at 12th century.

At its height in the 12th century,

the capital was the centre of a gigantic Khmer Empire

that can only be visualised on a satellite scale.

From its very beginnings in the 9th century,

the kingdom had an incredible urban organisation.

EVANS: Angkor, in a way, is unique in human history

for the degree of transformation of the landscape

that took place as part of this extraordinary project

of urbanism and engineering.

But how widespread were these structures around the kingdom?

What was the real size of the city?

Now French archaeologist Jean-Baptiste Chevance

is revealing the answer.

In 2017, his field work, illuminated by a vast LiDAR campaign,

exposed the urban remains of one of the very first cities in the empire,

located in a remote area 40 kilometres north-east of Angkor Wat,

the city of Mahendraparvata.

In this remote massif of the Phnom Kulen Plateau,

teams used LiDAR to sweep an enormous area

stretching 900 square kilometres,

revealing ruins previously unknown to researchers.

Today, Jean-Baptiste is trying to reach a temple

identified by this new technology.

He uses GPS to track his position and plot a route.

It's the only way to navigate the jungle

and reach the hidden ruins in order to explore them further.

The temple is 40 kilometres from Angkor Wat,

giving an indication of the scale of the Khmer Empire ruins

hidden in the region.

But the most important discovery revealed by LiDAR

remains invisible from the ground.

As early as the 9th century,

Khmer builders had constructed residential quarters

around the temples, stretching 1.5 kilometres

on each side, connected to each other by large communication routes.

The discovery illustrates

the extraordinary building capacity of the Khmer

from the very beginning of their empire.

Examining the area on a satellite scale

is the only way to expose the true footprint of Angkor urban planning.

For more than 600 years, builders constructed cities

and a vast communication network throughout the empire.

There were very different scales of infrastructural development

going on, from the local scale where you had pathways and roadways

connecting houses and neighbourhoods.

But it was also something much bigger and much larger than that.

For instance, Angkor was connected to various far-flung provinces

across mainland south-east Asia by this vast network of highways.

If we, you know, calculate all the road structures in the capital city,

it's more than 400 kilometres long.

But since the 12th century,

Angkor Wat has survived the test of time.

So how did this UNESCO World Heritage site remain standing

for nearly a millennium?

What are the secrets of its incredible longevity?

According to Mr Im Sokrithy,

Director of the Conservation of Angkor Park Monuments,

the 12th-century builders had learned from their past mistakes.

They built a small pyramid,

but the temples collapsed.

3-D technology makes it possible to visualise the structural problem

posed by heavy Khmer temples.

The monumental weight of the sandstone towers

exerts phenomenal pressure on the structure,

a pressure that tends to push the stones to the sides.

The King of the Angkor Wat

understands that failure of the structure,

and that's why they built the central tower

with some kind of support.

The precision of 3-D scanned images reveals a technique

that builders used which is difficult to see with the naked eye.

Around the central tower, to counter the forces exerted on the structure,

they placed 11 staircases intended to act as buttresses.

But only the central staircase, which was less steep,

could be used to reach the top floor of Angkor Wat.

Many people misunderstanding.

This is the real staircase getting up to the central tower.

But others were planned

used as supporting structure to the central tower.

This secret of construction allowed the towers

to stand the test of time.

Specialists have calculated the number of stone blocks

and estimate that Angkor Wat weighs 10 million tonnes.

A colossal mass built not on bedrock

but on millions of cubic metres of compacted sand.

A thousand years ago, we don't have the technique

to make the foundation.

And the soil, especially in Siem Reap,

is not strong enough

to support this kind of heavy construction.

So what is the secret to the sand's resistance?

Dr Hang is the director of the APSARA National Authority.

A hydraulic engineer by training,

he has devoted his life to restoring and studying the city's waterways.

According to his research,

the secret to the temple's solidity is water.

The moat of the temple, it's very important.

It's built in the concept.

By infiltration, they go to fit the sand layer under the temple,

to keep the sand humid to support our temple.

Once again, the latest technology makes it possible to visualise

the ingenious concept employed by the Khmer.

Thanks to the moat, water seeps through the walls

that support the sand foundations.

But why is wet sand any better than dry

at supporting 10 million tonnes of temple?

We can say the sand when it's humid, it can become strong like the stone.

The Khmer ancestors, they understand that very well.

Examining the sand on a microscopic scale

exposes what makes this material particularly resistant.

Water distributes amongst the grains,

acting as a glue between each particle

to create a new state, a solidified sand aggregate.

The enormous moat of Angkor Wat

contributes to the incredible longevity of the temple.

But the engineering challenge can only be fully appreciated

on a satellite scale.

In order to sufficiently moisten the foundations,

the moat is proportional to the scale of the monument -

200 metres wide, 5 kilometres in circumference,

containing up to 1.5 million cubic metres of water.

The moat is connected to a network of canals and reservoirs

to guarantee the stability of the temple, even in the dry season.

The temple, it's built for the god.

So it is why they want to make sure that the temple, it stands forever.

Restoration work allows experts to decipher the ancient techniques.

Because the blocks need to interlock with each other,

we need to make sure that there is a perfect match

between a stone and the one next to it.

When we look at the Angkor temples, we're often fascinated by the fact

that the joints in between blocks and between sandstone blocks

are extremely, extremely thin

and so tight that we cannot even slip a credit card inside.

How did they make such perfectly fitting joints

to lock the blocks together?

This was actually achieved, this technique

of rubbing, or frottage,

where basically the two blocks would be rubbed together

with each other, against each other,

and an abrasive sand would be placed in between.

And so they would have been rubbed until the fine joints were reached.

3-D imaging technology exposes

the technique used by the Khmer builders.

By means of abrasion,

the irregularities of the sandstone are ground down

until the blocks fit together perfectly.

Then when they erected the walls,

the builders would slide a smaller stone called a key

between the large blocks, which would lock the structure.

Sandstone and laterite were not the only materials used

by Khmer craftsmen.

In 2012, an important discovery brought to light

other ancient techniques.

The discovery sheds light on a little known aspect

of Angkor craftsmanship.

They adorned temples and sculptures with decorations of precious metal.

The central towers were covered with gold leaf.

But the Khmer used metal for more than just decoration.

Builders cut notches in the sandstone to fit metal pieces,

many of which, like the decoration, have been stolen by looters.

What did these objects look like

and what were their roles in the temple's engineering?

This question is now being closely studied by science.

In the CEA Paris-Saclay laboratory,

archaeometallurgist Stephanie Leroy uses the latest technology

to decipher the history of these mysterious objects.

Today she is studying a precious specimen

dating back nearly 1,000 years and found in 2018 on the Angkor site.

3-D imagery helps visualise the role of these metal clamps

in the structure of the temples.

These objects were used to join the sandstone blocks together.

Examining the metal on a microscopic scale

exposes a material that is easy to work with

to create the shape of the clamp - iron.

This very resistant metal was usually used by the Khmer

for weapons and tools,

but here it was used to reinforce the structure of the temple.

In Angkor Wat, builders placed hundreds of thousands of clamps

inside the structure to ensure impressive longevity.

This latest revelation poses a new mystery for archaeologists.

Where did the Khmer find so much iron to build this temple?

By studying clamps from Angkor Wat before they are restored,

Stephanie looks for clues on a microscopic scale.

Thanks to cutting-edge technology, a scanning electron microscope,

the laboratory is now trying to determine

the origin of the Angkor Wat clamps on a molecular scale.

On satellite images, the quarry site is located in Phnom Dek,

literally meaning the Iron Mountain.

The site is some 100 kilometres east of the city

and has been exploited since the 11th century.

But when samples from the temple of Angkor Wat itself were studied

in 2020, the results were unexpected.

The origin of the iron used at Angkor Wat

remains a great enigma of this sacred temple.

The Khmer built this engineering wonder with one goal in mind -

to build an eternal monument.

But this building is still shrouded in mystery.

Why did King Suryavarman II order the construction

of the largest temple ever to rise from the ground,

and what does the fascinating architecture

of this incredible monument symbolise?

In the 19th century, when archaeologists began to study

the symbolism of the largest Khmer temples,

it was impossible for them to easily visualise its gigantic plan.

But today, thanks to the latest satellite technology,

archaeologist Ginevra Boatto can observe the entire 160 hectares

that the building covers.

The use of aerial pictures or satellite images today

is very helpful in the sense that it helps us

get a better understanding, seeing better

these incredibly huge structures like Angkor Wat.

It measures 1,500 metres

along its west-east axis

and 1,300 metres along the north-south axis.

When we look at its moat,

a total perimeter of 5 kilometres, approximately.

But why build a moat and a monument of such magnitude?

The architecture has been designed for the heaven

and they apply it on the space.

When planning Angkor Wat, the architects were inspired

by the image of the heavenly paradise

according to Hindu mythology.

3-D modelling makes it possible

to better decipher the hidden symbolism of Angkor Wat's layout.

This pyramidal structure called Temple Mountain

is topped by five towers,

representing the five peaks of a mythical mountain, Mount Meru,

home of the God Vishnu, to whom the temple is dedicated.

A sanctuary surrounded by not one but three stone enclosures

evokes the mountain ranges surrounding the celestial residence.

Finally, the gigantic moat symbolises the cosmic ocean

separating humans from the gods.

In a kind of a magical way,

a certain power is also given to this place.

This temple becomes a heavenly palace,

it becomes heaven on earth.

The king wanted a paradise on earth but his ambition didn't stop there.

To understand, you have to look at the structure of the temple

on an aerial scale.

In this case, specifically,

when we look at one of these aerial views,

we can appreciate a geometric pattern, which is called mandala...

..which was commonly used in Hinduism and Buddhism,

which shows a square with the four openings at the four cardinal axes

and the circle inscribing it with a central point.

This central point represents the axis mundi,

the central axis of the cosmos.

Therefore, having this kind of geometric pattern

imposed on Angkor Wat means that Angkor Wat is placed

around the axis of the world and is the axis of the world.

Builders used this symbol for the foundation of the temple.

Khmer engineers first drew the sacred mandala

directly on the ground to define the location of the superstructure.

Then they dug out the earth to place the foundations

in the form of a huge amount of compacted sand.

During the excavation of the foundation for Angkor Wat,

a well was also dug at the bottom of the foundation.

Today, 3-D modelling techniques help to expose the mysterious structure

discovered in the 1930s.

From the foot of the central tower,

a 20-metre-long well joins the base of the temple

where the builders dug a cavity.

But why did they form this structure within the foundations?

To better understand the role of this underground cavity,

experts travelled some 100 kilometres north of Angkor

to the site of another Khmer temple.

This is Prasat Khnar,

a little-known site whose construction began

approximately three centuries before Angkor Wat.

Now, for the first time, an international team

is carrying out an excavation campaign

to understand its history and construction.

One long building is of particular interest to researchers.

It's an ashram, or monastery, dating from the 9th century

and partially destroyed.

The floor has long disappeared,

giving archaeologists a unique opportunity to dig a trench

and study the foundations of the temple.

It's a chance to discover what could be hidden

in the depths of Angkor Wat.

The study is facilitated by cutting-edge technology

now being tested by these University of Toronto researchers.

This is a hand-held 3-D scanner that uses two cameras

and a light system to develop an image.

It's called stereoscopic vision.

So, you have two eyes in order to have a sense of depth.

This does exactly the same thing.

The scanner uses stereoscopic vision

similar to the principle of human vision.

Both camera images are sent directly to the internal computer.

Instantly, just like the brain,

a software program calculates a three-dimensional model,

allowing archaeologists to study bas-reliefs,

statues and other ruins in precise detail.

But today, this tool is an opportunity to observe

the foundations of the Khmer monastery

and reveal details in three dimensions.

You can really pick up the relief there, you see, Giles?

And from that basis you can draw the actual stratigraphy.

That's nice.

In just a few minutes, the trench is fully modelled.

If I can take a look at what you have here.

It's very good. OK.

So what's incredible is that we have a perfect virtual model

of the stratigraphy.

So in a sense, if this was ever destroyed or once it's buried,

we can revisit the trench and reanalyse it.

By zooming in on this digital model,

the archaeologists observe a recess which forms a small cavity.

So, in fact, we found a small looter's pit

that didn't go very deep.

From this pit, looters stole precious objects

that had been placed by the builders.

But which is very common in Angkorian temple construction

is offerings that are placed right at the bottom of the foundation

as a way to bless and sanctify

the major cult building of the monastery that was founded here.

It's likely that many years ago, this temple was, in fact, looted

for these very precious objects.

Situated 20 metres below the main tower,

at the very heart of the foundations,

this space, sheltered from looters,

was used to protect precious offerings to the gods.

But some archaeologists believe this cavity had another function.

They think the Khmer deposited the ashes of King Suryavarman II here,

meaning that Angkor Wat is more than a temple, it's a gigantic tomb.

No coffin or ashes that could prove this theory have been unearthed

during excavations, but the idea is commonly accepted by archaeologists.

Angkor Wat, the largest religious building on the planet,

was built at the request of the king to embody the centre of the universe

at the home of the god Vishnu.

This symbol of Khmer civilisation has survived for almost 1,000 years.

But according to the legend, the capital of Angkor

brutally disappeared in the 15th century.

So how did this enormous city,

finally revealed by LiDAR, really decline?

Why did the population leave this gigantic capital?

There was an idea, for instance, one theory

that something catastrophic happened at Angkor

and there was a mass migration of people away from Angkor

into alternative places.

Many theories exist about the disappearance of the capital...

..the occupation of forces from the nearby Ayutthaya kingdom,

violent political or religious disorders...

..or natural disasters that forced hundreds of thousands of inhabitants

to leave the city in a hurry.

In fact, the LiDAR tends to show us that that was not the case,

actually, because the cities that came after Angkor

were very modest in size by comparison.

What we really need to do from that point onwards

is to complement these horizontal surveys we've provided by LiDAR

with other kinds of information right down to the microscopic scale.

In 2019, to check whether or not the city had suddenly been abandoned

by its Khmer population,

an Australian team turned to microscopic research.

By carrying out core sampling and collecting soil samples

from the moats of Angkor Thom,

scientists studied sediments present in the soil,

exposing a biological archive of the city's history.

Pollen analysis made it possible to determine the precise use of land

year after year.

EVANS: What we would then expect to see

if there were periods of abandonment

is gradually species reclaiming that landscape

and eventually pollen from forests reappearing

at the top of that archive.

That was not the case, actually.

What the cores have shown is that there was a very gradual

resumption of this sort of jungle environment at Angkor,

spanning many, many centuries,

and probably a progressive abandonment of particular areas,

even as other areas continued to function and remained occupied.

But just as advanced technology has shed new light

on the history of Angkor, it has also raised new mysteries.

Following the LiDAR campaigns, archaeologists discovered

strange alignments of earthen mounds at various sites,

360 structures whose function are still eluding specialists.

We see these mounds, these alignments,

and mound, a small mound like this,

in many areas, like Phnom Kulen, as well.

But we don't know exactly what it is.

Decade after decade, archaeological research,

aided by the latest technology, has exposed many of Angkor's secrets.

From the gradual decline of the city from the 15th century

to the mountain temple symbolising paradise on earth,

built in just a few decades, using ingenious techniques.

Scientific progress has allowed investigators

to reveal an incredible capital

hidden underneath vast expanses of forest.

But many great mysteries of the Khmer builders

remain to be solved.

BOATTO: Basically, everywhere there is a new temple, a new ruin

that has been discovered in the jungle, among the trees.

So it's a never-ending experience.

And there is so much still left to understand

and to discover about this civilisation.

Captions by Red Bee Media (c) SBS Australia 2022