Original subtitles

NARRATOR: These are the remains of the medieval city

of Angkor in Cambodia.

Former capital of one of the world's greatest civilisations,

and once, the biggest city on Earth.

In many respects, Angkor is totally unique.

The things that were achieved here were unparalleled

throughout all of human history.

NARRATOR: Grand temples like Angkor Wat...

[BIRDS CHIRPING]

...massive engineering projects...

and huge reservoirs.

This was once a vast city, teeming with life.

One has to really stop and be in awe of what has taken place here.

NARRATOR: Explorers and archaeologists

have been coming here for over 150 years...

to find out about the people who built Angkor...

and to try to discover why they abandoned the city.

[HORN HONKING]

Now, archaeologists are using a sophisticated mapping technology

called LIDAR to help solve the mystery of what happened here.

By revealing a lost world beneath the trees,

they allow us to imagine how the great city of Angkor once looked.

DAMIAN: LIDAR is an incredibly valuable tool

because what it allows us to do is to breathe life

back into this landscape.

NARRATOR: By unlocking the secrets

of how this medieval metropolis flourished...

-[THUNDER RUMBLING] -...they're also shedding new light

on the dramatic events leading to its fall.

BRENDAN: That's what we describe as a one-two punch,

and I think that was really the part where they realised

things started to go horribly wrong.

NARRATOR: This new technology has revolutionised archaeology.

And it helps to explain why the world's greatest

medieval metropolis was abandoned to the jungle.

[CRICKETS CHIRPING]

NARRATOR: Eight-hundred years ago,

a vast city flourished here in the Cambodian jungle.

Angkor was the capital of the Khmer Empire.

By the end of the 12th century,

the Khmer people had dominated Southeast Asia

for hundreds of years.

The jewel in Angkor's crown?

Angkor Wat.

The biggest religious complex on Earth.

But the story of Angkor and its people

didn't end with the completion of this great temple.

[CRICKETS CHIRPING]

Forty years later, and one kilometre to the north,

construction began here, at a new site called Angkor Thom.

Its walls and moat are over 12 kilometres long.

They enclose an area three times larger than medieval London.

Angkor Thom would become the new seat of imperial power,

a symbol of Angkor's golden age.

Archaeologists have been studying this great royal enclosure

for over a century.

But the world of the people who lived here and beyond its walls

largely remains a mystery.

Dr Damian Evans is now trying to reveal the city's secrets.

Eight-hundred years ago, we would have been standing

in the middle of a vast city, teeming with life.

Unfortunately, almost all of that city

was made of non-durable materials like wood and thatch

and has completely rotted away.

The stuff that's remaining,

the huge temples, this wall that we're standing on,

is a very small and unrepresentative part of the whole city of Angkor.

And so, this is the fundamental challenge

that we're now trying to address,

to try and reintroduce people into this landscape

and understand it as a living city, as a lived-in space,

rather than just a collection of empty and abandoned monuments.

NARRATOR: The new technology is called LIDAR .

It's now being used to reveal the lost world beyond the temples.

LIDAR works by firing laser beams through the foliage

to measure the elevation of the land surface, beneath.

Billions of data points are captured,

creating a ghostly outline of the medieval city.

This LIDAR map gives archaeologists a revolutionary new way

of investigating the history of Angkor.

Some of LiDAR's biggest revelations

lie beneath the jungle beyond the great moat of Angkor Thom.

With the tree cover removed...

LIDAR reveals the outline of a grid of city streets

stretching into the distance.

It allows us to build a graphic reconstruction

revealing the scale of Angkor in its golden age.

A formally planned metropolis, with tens of thousands of houses.

Over three-quarters of a million people lived and worked

in this bustling city all around the stone temples.

DAMIAN: The LIDAR data really transforms our vision of Angkor

as a-- as a lived-in space.

What it shows us is that this downtown area spread

far into the landscape beyond, uh, and also was accompanied

by this huge network of infrastructure, of roadways,

of canals, um, of neighbourhoods that tied these far-flung areas

of Angkor into the city centre, where we are now.

NARRATOR: By the end of the 12th century,

Angkor was one of the most sophisticated cities in the world.

The LIDAR survey

reveals the complexity of its vast water management network.

At the heart of the system were massive reservoirs

to store water from the annual monsoon.

In dry years,

this network was a lifeline for hundreds of thousands of people.

In wet years, it helped control the flow of floodwater

through the city.

By the time Angkor Thom was built,

the Khmer were masters of their environment.

And their power and ambition

was made clear in a new temple at its heart.

The Bayon.

Construction began on the Bayon towards the end of the 12th century.

It was commissioned by the same monarch

who built Angkor Thom's imposing walls.

Jayavarman VII, one of the greatest Khmer kings.

Professor Roland Fletcher is using the LIDAR data in his study

of the rise and fall of Angkor.

Jayavarman VII plays a pivotal role in the story.

This immensity of Jayavarman VII's temple illustrates his significance.

He, in a sense, epitomises everything that the Khmer world has been doing.

NARRATOR: Khmer kings had been building stone temples

for hundreds of years.

But Jayavarman VII now took Khmer temple building to a new level.

ROLAND: The significance of Jayavarman VII

is that he builds as many major temples

as have been built in the preceding history of Angkor.

So, this is an absolutely tremendous building programme.

NARRATOR: The Bayon was this great king's statement

of power and authority.

ROLAND: The conventional view, and I think it's a reasonable one,

is that these faces are the faces of Jayavarman VII.

They are the profound representation of what he is doing.

The faces look out in every direction across the city

and across the empire.

[CRICKETS CHIRPING]

NARRATOR: Today, the stone faces stare across

a vast expanse of jungle.

The LIDAR survey reveals the original view of the city.

Little now remains of the bustling metropolis around the Bayon.

But on the walls of the temple itself,

the lives of the people who lived here during the reign

of Jayavarman VII can still be seen.

Dr Julia Esteve lives here in Cambodia.

She's spent 12 years studying life in Angkor at its peak.

[INDISTINCT CHATTER IN KHMER]

It's really lovely to be here at night

and to be all alone in the temple.

I can take the time to look at everything, look at the bas-relief.

I can even touch it, even though I'm not supposed to. [CHUCKLES]

And, uh, yeah, it's really quite magic, I have to say.

NARRATOR: The carvings run for over half-a-kilometre around the temple.

[CAMERA SHUTTER CLICKING]

There are over 300 separate scenes

with thousands of meticulously sculpted figures.

Few representations of ordinary Khmer life survive in other temples.

The bas-reliefs of the Bayon are very special because they give us

a window on the daily life of the Khmer people

at the end of the 12th century.

[INDISTINCT CHATTER IN KHMER]

NARRATOR: From farmers to fishmongers,

these carvings reveal the pattern of everyday life

in the golden age of Angkor.

The Khmer enjoyed games and gambling.

Cock fighting seems especially popular.

JULIA: The carving we see here is particularly interesting

for comparisons with daily life nowadays.

Uh... in fact, we see preparation for a banquet

and we see a lot of, uh... food being cooked,

for example, a pig here, held by two men

is about to be put in boiled water in a cauldron,

maybe to skin it, or just to boil it.

Over there, we have also a lot of people holding little cups,

we can assume of rice wine, and it seems to be a time of peace.

And it fits well with the idea we have of Jayavarman VII's reign.

NARRATOR: But the carvings also reveal this to be a land

of dynastic rivalries and conflict.

Large parts of the Bayon are covered with images of war.

They record a bloody battle between two Khmer armies.

ROLAND: Jayavarman VII comes to power in a very unpleasant civil war.

He clearly is opposed by a significant portion

of the Khmer elite.

And, this is a violent enough and unpleasant enough phenomenon

that he portrays the defeat of a Khmer army

on the walls of the Bayon.

Essentially, this is a method of putting in stone,

"I'm not going to forget,

my descendants are not going to forget."

This was a vicious war.

NARRATOR: Having won the crown, this great warrior-king

now unleashed a religious revolution.

ROLAND: Jayavarman VII is not only a great military leader,

he also introduces a major religious change

in the form of making Mahayana Buddhism

the primary religion of the state.

[CRICKETS CHIRPING]

[MONKS CHANTING]

NARRATOR: Today, Buddhism is the state religion of Cambodia.

It is practised by more than 95 percent of the population.

[MONK CHANTING]

But before Jayavarman VII claimed the throne,

Angkor's kings had been almost exclusively Hindu.

Their legacy seen in monuments like Angkor Wat.

[BIRDS CHIRPING]

Jayavarman VII was now using religious reformation

as a tool to consolidate his power.

ROLAND: The key thing that Jayavarman VII is doing

is he's removing the preceding great families

who controlled that enormous Hindu religious system.

And they vanished from the record.

And a new story starts with Jayavarman VII.

NARRATOR: In 1181, Jayavarman VII began the biggest building programme

in Angkor's history.

During his reign, he would pour the empire's resources

into the construction of major stone temples and shrines

throughout the city.

One of the biggest lies just beyond the walls of Angkor Thom...

Preah Khan.

Preah Khan means "sacred sword" in Khmer.

It was built in 1191, on the site of one of Jayavarman VII's

greatest battlefield victories.

Like many Khmer temples,

Preah Khan was a centre of administrative

and financial power,

as well as a monastery and a place of learning.

Tax levied here on Angkor's rice farmers went directly to the king.

As the city prospered,

Jayavarman VII's temples became fabulously wealthy.

A 12th-century inscription suggests that 60 tons of gold

once lined the walls of this central shrine.

It's thought that these holes were used to support the panels of gold.

Its value today would be about two billion pounds.

[TREES CREAKING]

Much of the temple has been destroyed by the jungle.

Preventing the trees from causing further damage

is a major task for architectural conservator Glenn Boornazian.

[TREES CREAKING]

What we're seeing here is a seed that fell one day.

It started to grow and no-one moved it.

And then in the end, we end up with an object,

or, you know, almost a being, like this.

It almost looks like an alien that has come down

and has grabbed onto all aspects of the masonry.

Quite frankly, this will destroy this section of the building.

[TREES CREAKING]

We've got, probably, millions of stones here.

And when we think about what the labour and the craft

and the time that went into the construction of just one stone,

it then helps us understand the...

the amazing effort that took place at that time

to create an incredible site like this.

[METAL CHAIN GRATING]

NARRATOR: Glenn's conservation team

has spent over 20 years working to preserve Preah Khan.

If this is the top of the stone, it has to be a channel... like that.

And then the cable drops in there.

NARRATOR: Today, they're at work on one of the four gateways

to the main temple, the East Gopura.

GLENN: We are moving probably one of the largest stones

that make up the central tower here on the East Gopura.

It's about 2.3, 2.4 metres long

and probably well over a ton in weight.

So, the amount of energy that it takes us

to move it is... is extreme.

What it makes me think is, OK, we're doing this here

in 2014, and we have some really, uh...you know,

I wouldn't call it state-of-the-art equipment,

but certainly, equipment that makes it easy to move this sized material.

And then, if again, if I sort of close my eyes and wonder

how Jayavarman VII and his team in 1190-- in the 1190s was

also moving these stones, it's quite a wonder.

I really can't comprehend that.

NARRATOR: The efforts of Jayavarman VII's workers

are recorded in the Bayon carvings.

They reveal that only the most basic tools were available.

Labourers haul rocks with ropes.

Others use wooden hoists to lower finished blocks into position.

One of the more exciting and wonderful things

that happens here, when you're working

on an ancient temple and you start to move a stone,

I think one of the things that goes through your mind is...

"When was that stone last moved and who actually moved it?"

And if you think about that,

you realise that the last time that stone was moved

was in Jayavarman VII's time.

And it does give you goose bumps.

NARRATOR: The labour required to move a single block gives an idea

of the speed and efficiency of Jayavarman VII's workers.

This effort was multiplied at vast temple sites throughout the city.

The LIDAR map shows the position of Jayavarman VII's temples.

In Angkor, houses of stone were reserved for the gods.

Everyone else lived in homes made from wood or thatch,

including the king himself.

Only the ghostly footprint of these lost buildings remains.

But one vivid first-hand account of life around the temples

still survives.

[DOG BARKS]

[BIRDS CHIRPING]

[DOG CONTINUES BARKING]

MAN: [READING] "At the lowest level come the homes of the common people.

They only use thatch for their rooves and dare not put up a single tile.

[INDISTINCT CHATTER]

Although the sizes of their homes vary

according to how wealthy they are,

in the end, they do not dare emulate the styles of the great houses."

NARRATOR: These are the words of Zhou Daguan, a Chinese envoy

who came to live in the city for nearly a year from 1296.

His journal is a detailed and intimate record

of life in Angkor.

MAN: "In this country, you can go without clothes.

Food and women are easy to come by.

Housing is easy to deal with.

And it is easy to make do with a few essentials."

NARRATOR: With its reservoirs, fertile paddies and bustling streets,

this was a land of plenty.

But to sustain his temple-building programme,

Jayavarman VII needed stone in ever-greater quantities.

The LIDAR survey revealed the outline of some of the Khmer quarries.

Damian is heading out to explore.

Travelling with him is Simon Warrack,

an expert in medieval stonemasonry.

[COWS MOOING]

It's actually really nice to drive out here.

It's beautiful countryside and very scenic.

The thing is, you never know what's going to come at you

out of those trees.

You just have to, uh... keep your wits about you

and expect anything at any time from any direction.

NARRATOR: The quarries lie around forty kilometres

north of Angkor's main temples.

Transporting vast quantities of stone

would have been a major challenge.

The Bayon Temple is around 600,000 blocks,

but the one-- the one thing that you have to bear in mind,

on average, when you're cutting stone there's at least 30 percent wastage.

So, you're bringing down large blocks,

thirty percent of which gets chipped off and ends up, uh... in the floor

for the archaeologists, later on.

It's-- it's massive. It's absolutely massive.

[DAMIAN HONKS HORN]

NARRATOR: Getting to the medieval Khmer quarries today

is a challenge in itself.

DAMIAN: We're four kilometres away.

-SIMON: Still? -DAMIAN: Yeah.

NARRATOR: The road runs out. And Damian and Simon have to walk.

DAMIAN: Yeah. If there's any path that goes right,

we need to swing right.

NARRATOR: They have to pick their path carefully.

From the 1960s to 1990s,Cambodia was torn by conflict and war.

Land mines remain an ever-present danger.

DAMIAN: Mind you, this is all fine. I mean, it's been cultivated,

so, land mines are not too much of a worry.

NARRATOR: But it's not long before the track runs out.

What we're going to have to do is to go bush bashing

at this point, basically,

uh, which is not normally the best idea

in an area that's well known for having a lot of land mines.

Fortunately, there's a gentleman here who apparently knows a way.

Even if there's no path, we can kind of walk through cultivated areas,

which, uh, should be safe.

Um... and he reckons he can take us to those particular quarries

that we're interested in.

NARRATOR: The local farmer leads them across the dry paddy fields

to a safe path through a village.

From here, Damian and Simon can carry on without assistance.

Soon, they see signs of quarrying.

-SIMON: This is big-- -DAMIAN: All the way around here!

SIMON: This is really big.

DAMIAN: One big, huge ridge. It's amazing.

Yeah, it's beautiful, isn't it?

You can really see the chisel marks there and the stepping of the stones.

It's incredibly silent out here, isn't it, in the middle of nowhere?

Just, uh... imagine 800 years ago, there would have been

thousands upon thousands of people chipping away at sandstone

with iron chisels in this area.

I mean, even the sound, [CHUCKLES] must have been incredible.

SIMON: I would imagine that they were probably working in teams.

Do you think they would get paid per block

or do you think they were just told to go...?

My personal opinion is that people would have been rounded up

and pretty much forced to do this kind of work.

It has to have been an incredibly difficult, difficult job.

Um... and, and really unsafe out here, as well.

[LAUGHING] I doubt it was safety first in the 12th century!

NARRATOR: The labourers would have lived

on a simple diet of rice and fermented fish paste.

They removed thousands of blocks from this site.

Archaeologists once thought

there used to be many small quarries in the region.

But LIDAR has now changed this view.

DAMIAN: When you have exposed bits like this, outcrops,

it's very easy to see evidence of quarrying.

The problem is that the quarries weren't always on bits

that stuck out of the ground like this.

Quite often, they were in pits dug into the ground.

And those have filled in centuries ago.

What the LIDAR can do, uh, is it can show us the depressions

that are basically the remains of those in-filled pits.

And using that new information, we can see that

we're looking at a single, vast quarry field, in fact.

NARRATOR: The LIDAR survey reveals many areas

where previously-unknown quarrying took place.

This is the source of many of the estimated five million blocks

in Angkor's temples.

The new map also reveals how so much stone was transported.

It shows canals stretching back to the city.

The blocks were floated to Jayavarman VII's building sites

on rafts.

With a steady flow of stone from the quarries,

Angkor continued to expand and flourish.

[INDISTINCT CHATTER IN KHMER]

[SPEAKING IN KHMER]

NARRATOR: When Chinese traveller Zhou Daguan arrived in 1296,

he was impressed by the vibrant metropolis.

MAN: "There is a market every day

from around six in the morning until midday.

Small market transactions are paid for with some rice

or other grain and Chinese goods.

The ones next up in size are paid for with cloth.

Large transactions are done with gold and silver."

NARRATOR: Zhou Daguan's journal reveals his interest

in Angkor's markets.

It's possible he was sent to gather commercial information

about one of the most successful economies in Asia.

He records a wealth of produce

and an abundance of fresh fish.

But the foundation for the city's wealth was agriculture.

Its fields kept lush

by the sophisticated management of water from the annual monsoon.

MAN: "In general, crops can be harvested three or four times a year.

The reason being that all four seasons

are like our fifth and sixth months,

with days that know no frost or snow.

For six months, the land has rain.

For six months, no rain at all."

NARRATOR: The staple crop was rice.

The expanding city was built around the paddy fields.

[ENGINE REVVING]

[HORN HONKING]

[DOG BARKS]

By the end of the 13th century, Angkor was a sprawling metropolis.

The LIDAR survey, led by Dr Damian Evans,

has covered only a fraction of the city.

Almost 250 square kilometres of Angkor

have been mapped with LIDAR so far.

This is where the major state temples are located.

But the urban sprawl continued much further

into the surrounding landscape.

[ENGINE REVVING]

It's a long ride from the centre of Angkor

to the city's medieval outskirts.

With nearly 20 kilometres on the clock,

Damian is now well beyond the area covered by the LIDAR survey.

A first glance reveals few clues

that these outlying areas would once have been part of the city.

But some historic landscape features survive.

Because we've gone off the edge of the LIDAR map,

what I'm looking at here is mapping data that we acquired

several years ago, from aerial photographs alone.

We can clearly see that there's an enormous square enclosure here.

NARRATOR: The enclosure of Banteay Srei

lies twenty kilometres from the centre of the city.

It's evidence of Angkor's extraordinary expansion.

DAMIAN: One of the interesting things about Angkor

is that in terms of its size and scale,

it's comparable to these megacities that have developed

over the course of the 20th century.

NARRATOR: Banteay Srei is one of many historic sites

found in areas away from the city centre.

They spread far beyond the area of LIDAR coverage

in the heart of the city.

These outlying sites show that Angkor's great urban sprawl

once covered a thousand square kilometres.

It would be another 700 years

before London stole its crown as the largest city on Earth.

Archaeologists are unsure

what the enclosure of Banteay Srei was used for.

But information from the LIDAR survey elsewhere in the city

helps create an image of how its moat might once have looked.

DAMIAN: During the time that this place was built and inhabited,

you wouldn't have had, really, any of this vegetation around

and the banks of this particular moat, here

would've been populated with wooden houses.

So, you would've seen communities on stilted houses

arrayed along the banks of this particular moat.

In fact, Zhou Daguan, when he visited here at the end of the 13th century,

described a system of residence

where people lived along the banks of ponds.

And, of course, we can see the remnants

of those features, here today.

MAN: "The place is unbearably hot,

and no-one can go without bathing several times a day.

Even at night, you have to bathe once or twice.

They may never have had bathrooms,

but every family is sure to have a pond,

or at least a pond to share among two or three families."

NARRATOR: The LIDAR survey reveals over 4,500 ponds

across the centre of the city.

By mapping them, archaeologists can identify dense clusters

of population in long-forgotten neighbourhoods beyond the temples.

DAMIAN: So, we've moved, in just a few short years,

from a picture of Angkor as just a collection of uh...

of cold, grey, stone temples

to a much more nuanced and much more sophisticated picture of Angkor.

As a lived-in space, a vibrant space full of humans and activity.

NARRATOR: Jayavarman VII

used the vast resources of this flourishing city

to construct his many temples and shrines.

But the resources required to maintain them, were even greater.

Evidence for this

can be found in the Cambodian Ministry of Culture's warehouse.

Monumental standing stone slabs known as stele.

Carved with inscriptions, recording how the temples were managed.

This one is from Preah Khan.

ROLAND: The stele that you see here is essentially a record

of the assets of the temple.

It lists the number of villages that are indented to the temple,

the workforce, the events that are occurring,

supplies that have to be delivered.

NARRATOR: This text, written in Sanskrit poetry,

reveals the huge numbers of people required to keep Preah Khan running.

ROLAND: In the Ta Prohm temple stele, you have a really remarkable record.

You are told that 12,640 people worked for this temple.

Gives you some idea of the scale.

There are 615 dancers, which is a very large dance troupe.

You have over 2,000 administrators, you have somewhere between 1,000

and 2,000 teachers and their students.

So, you have a very elaborate administration,

of which you're only seeing a fraction mentioned.

NARRATOR: The LIDAR map has revealed where thousands of temple staff

once lived in the area around Ta Prohm temple.

Feeding them all required the labour of 66,000 rice farmers

in the surrounding fields.

So, if you total up the number of people who support

and work for the Preah Khan temple and the Ta Prohm,

it's over 150,000 people, and that's two medium sized temples.

When you start adding in the staff and the support for places

like Angkor Wat, the numbers begin to seriously skyrocket.

[BIRDS CHIRPING]

NARRATOR: Jayavarman VII's building spree

transformed the dynamics of city life.

By the time the Bayon was completed, over half a million people

were committed to maintaining the temples.

ROLAND: The problem with this is that the majority

of the population of Greater Angkor

is servicing and supplying the temples.

It's sucking resources in all the time

and what the growth of the temple system does is it boxes them in.

[BIRDS CHIRPING]

NARRATOR: Jayavarman VII died in 1218.

Angkor's golden age was over.

During his reign, his labourers had filled his city with temples.

But only one new stone temple was commissioned here

in the years that followed.

The tiny Mangalartha temple

was the last ever to be constructed in the city.

Within decades of its completion in 1295,

Angkor began its final spiral of decline.

But there's more to the fall of Angkor than an over-ambitious king

burdening his people with too many temples.

Archaeologists now believe that the mystery of the city's decline

can be explained by studying the infrastructure

which allowed it to flourish.

[CRICKETS CHIRPING]

Angkor's success was built on its vast water network.

The great reservoir, known as The West Baray,

can hold up to 49 billion litres of water

within its ten-metre-high earth banks.

This reservoir was connected to the wider water network

by an intricate system of canals and embankments.

To the east of Angkor Thom, other large reservoirs also helped

manage the flow of water across the city.

For centuries, Angkor's water network

gave its citizens food security and flood protection.

[INDISTINCT CHATTER IN KHMER]

But by the mid-13th century,

the system was beginning to show signs of its age.

Scientist, Dan Penny, has been investigating

Angkor's mysterious decline for over a decade.

By analysing medieval pollen samples, he's identified a dramatic change

that occurred here soon after Jayavarman VII's death.

DAN: We know that from the time this reservoir was built

in the mid-11th century,

to the time immediately after Jayavarman VII,

it held deep, clear standing water.

And we know that because we find

pollen grains in the sediment in the reservoir.

Pollen grains like this, this is Nelumbo nucifera, the sacred lotus.

And pollen from plants like this and a range of others indicate

the water in this reservoir was quite high and was permanent.

However, after the time of Jayavarman VII, we have a switch

in the kind of plants which were growing here, from these,

to pollen grains like these, which derive from fern spores and grasses.

Which tell us that we've shifted from an open water reservoir

to effectively a swamp or even to dry land.

NARRATOR: These pollen samples

reveal the rapid drying-up of Angkor's reservoirs.

This was a wealthy city.

But centuries of adaptations to the increasingly complex

water network were taking their toll.

DAN: It's ironic, in a way, that even when Angkor was reaching its zenith

its major pieces of water management infrastructure were failing

and were falling into disrepair.

NARRATOR: The decline of this vital system would leave Angkor vulnerable

to what came next.

In the 14th century,

Angkor's ageing water network received a devastating blow.

Evidence for what happened can be found over 700 kilometres away

in present-day Vietnam.

[BIRDS CHIRPING]

[WOODPECKER DRUMMING]

[DRUMMING CONTINUES]

The Lang Biang highlands rise over 2,000 metres.

[INDISTINCT CHATTER]

They are covered in ancient primary forest.

Scientists working here...

are now finding a new explanation for Angkor's decline.

BRENDAN: We're up kind of high here. We're high elevation.

It's mist forest and uh... but you start doing this,

you'll warm right up.

[TAPPING]

NARRATOR: Dr Brendan Buckley and his colleagues

are taking core samples

from a rare species of pine unique to Vietnam's highlands.

Pinus krempfii grow slowly in the chilly mountain air

and can live 1,000 years.

We've found Krempfii that are more than two metres in diameter.

So, this one is 1.5 metres.

There are some that are a lot bigger than this.

[DRILLING]

NARRATOR: This tree is big enough, and so old enough,

to have been growing when Angkor flourished.

Taking core samples doesn't harm the tree.

That's probably about as far as I want to go in this core for now.

I'm going to pull the core out. We use this spoon

and it just slides in under the dowel of wood that I've cut

and when I turn this back, it breaks the end of it off.

So, now I can just pull the core out.

And that's-- that's a beautiful core.

Actually, this is a really...

That's a really beautiful core, you see that?

So, you can see all the rings through time.

NARRATOR: These rings reveal the annual climate

throughout the tree's life.

A wet year results in a wide ring.

A narrow ring reveals a drought.

We've captured the whole record of this tree's life,

its story told year by year by the annual growth rings.

It goes back about 800 years.

Back to the period of time when the Angkor civilisation

reached its end.

NARRATOR: By sampling trees all across Southeast Asia,

Brendan has revealed a dramatic sequence of events

back in the 14th century.

[LOGS THUDDING]

[INDISTINCT CHATTER]

BRENDAN: Good day of coring, gentlemen.

Thank you for the work. Cheers.

ALL: Cheers.

-BRENDAN: Yo. -MAN: Yo.

[ALL LAUGHING]

NARRATOR: The core samples, collected today,

will be added to Brendan's database of over 1,000 from the region.

BRENDAN: But before we get too drunk,

we should also take a look at those cores.

NARRATOR: Each one will be dried and mounted,

like these samples from his lab.

BRENDAN: That tree has got to be a millennial,

that's got to be 1,000 years old.

There's probably 100 rings right there.

NARRATOR: They show that the highpoint of Khmer civilisation

coincided with particularly favourable climate conditions.

BRENDAN: The Khmer built their civilisation on the kindest period

of climate that we had in the last 1,000 years.

They built their whole system

based on the way the climate was at that time.

NARRATOR: But this period of stable climate was coming to an end.

Coming out of that really nice period of climate,

you really start to see this decline in the rainfall,

and that shows up very clearly in the tree ring record.

NARRATOR: The rings in this period suddenly become much narrower...

and remain narrow for over three decades.

BRENDAN: So, when we go back and we see these big suppressions

in the growth rings, we know that we have droughts that took place.

And for them to last for decades like that,

they have to be really significant failures of the monsoon.

NARRATOR: The failure of the monsoon would have placed a severe strain

on the city's crumbling water network...

-[THUNDER RUMBLING] -...but worse was to come.

In the late 14th century, the tree rings become unusually wide.

After decades of drought came a deluge.

So, the Khmer period of decline really was a matter of a few decades

that it went from extreme dry to extreme wet and then back again.

That's sort of what we describe as a one-two punch.

[THUNDER RUMBLES]

So that-- The wet period was something that was equally as bad,

if not more so, than the droughts were.

So, not only do they get hit by drought,

they get hit by massive amounts of water.

NARRATOR: Angkor's aging water network

now faced its greatest challenge.

[BIRDS CHIRPING]

The Siem Reap river flows through the heart of Angkor.

Dr Dan Penny believes that the changing climate here

in the 14th century destroyed the city's water network.

DAN: Rivers in this kind of environment,

very flat plain like this,

will tend to meander when they're left to their own devices.

So, when we see a straight stretch of water like this one

we know for certain that it's artificial.

NARRATOR: This isn't a natural river but a medieval Khmer canal.

On the LIDAR map, the canal can be seen to follow

a straight course for over five kilometres.

It was built during the time of drought

to channel precious water directly into the city centre.

But as the climate went from extreme dry to extreme wet,

the construction of this canal proved to be a tragic mistake.

DAN: So, this system was designed to carry a certain level of water.

But if you put a very much larger volume of water through

a straight channel like this,

the potential for catastrophe is very high.

NARRATOR: The straighter a river, the faster it flows.

And the deeper it will cut down into the riverbed.

These high banks reveal that this happened here

when the climate suddenly became much wetter.

In places, floodwater here cut down nearly ten metres

below the original land surface.

The devastating effect of these floods on Angkor's infrastructure

can be seen here.

Spean Thma is one of the city's few surviving bridges.

It now sits high above the old canal.

If you'd stood where we are standing now

perhaps in the 14th century, you would be standing in water

and this would have been a flowing canal.

The water now is almost ten metres below the bridge,

and in fact, has destroyed its eastern side, leaving the bridge

hanging up the side of the valley.

NARRATOR: The LIDAR map shows the power of the floodwater.

On meeting the stone bridge, it took the path of least resistance,

swerving to carve down through the soft soil of the riverbank,

before re-joining the canal.

But this wasn't the only damage.

LIDAR reveals that the swollen river also breached embankments...

and destroyed people's homes.

Right across the city,

crucial irrigation channels were left high and dry

above the new level of the river.

And sediment, eroded from the riverbed,

was now washed downstream past Angkor Wat,

and swamped the city's southern canals.

Angkor's intricate water network would never recover.

DAN: The destruction of the water management system

was the specific trigger for Angkor's demise as a viable settlement.

In fact, in many ways, it was the scale of the city,

and particularly its water network,

which was vast and complex and deeply interconnected,

that allowed this place to become so vulnerable,

to the point at which, this episode of climate variability occurred

and effectively, it uh... completely destroyed the system.

NARRATOR: With its water network in tatters,

the city's decline accelerated.

But the Khmer civilisation itself didn't die.

In the mid-15th century, the Khmer kings abandoned Angkor

and moved the imperial administration towards the coast.

They built a new city, Phnom Penh, the present-day capital of Cambodia.

Angkor was slowly devoured by the jungle.

But it never completely disappeared like the fabled Atlantis.

Over the following centuries, most of the people simply moved away.

By the time French explorers made Angkor's temples famous in the 1860s,

little of the city could be seen.

And the legend of a mysterious lost civilisation began to grow.

But many of the temples had continued to function for hundreds of years,

including the greatest of them all.

Angkor Wat has been in constant use since the day it was built.

Today, it's visited by millions of tourists.

Now, with the help of LIDAR,

we can see the lost city all around it once again.

One of the greatest achievements in human history.

The medieval metropolis of Angkor.