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An iconic temple,

the largest religious monument
in the world,

Angkor Wat
is an enduring enigma.

The engineering feat of
Angkor Wat is unbelievable.

In the heart of one
of the greatest ancient cities

ever built: Angkor.

One of the largest,

if not the largest, city
in the world at the time.

Home to over 1,000 temples.

Each new king would try
to outbuild the reign before

and build a bigger
and more impressive temple.

But then, the temples
and the city were engulfed

by the jungle.

It mysteriously was abandoned.

Well, so the story goes.

There are few clues...

Almost every wooden remain--

and the vast majority
of structures at Angkor

were built in wood--

deteriorate very, very quickly.

...and many mysteries.

The lack of bodies.

One million people, not a bone,

not a cremated remain.

It's fascinating
and frustrating.

There's another story there,
sitting underneath.

Now archaeologists are
using the latest technologies

to reveal
Angkor's hidden secrets...

Sarah took the lidar data.

She used some machine-learning
algorithms.

...to see through the jungle...

The lidar data is absolutely
mind-blowing.

It was all
an engineered landscape.

...and look into the past...

Really, like the pages
of a history book.

...to find new evidence
of a great civilization.

They didn't just build
Angkor Wat,

which is a spectacular temple, right?

Like, they were transforming
the entire landscape.

"Angkor: Hidden Jungle Empire,"

right now, on "NOVA."

In the jungles of Cambodia,

a spectacular ruin rises
from the trees.

This is Angkor Wat.

A 900-year-old temple,

filled with intricate carvings

and mysterious figures.

It is the largest religious
monument in the world

and a masterpiece
of ancient engineering.

The engineering feat
of Angkor Wat is unbelievable.

It's amazing.

Angkor Wat is the centerpiece

of the ancient city of Angkor,

a marvel of vast infrastructure,

built with a network
of human-made canals

and enormous reservoirs.

Angkor was the heart
of a wealthy and dynamic empire

that thrived for 600 years.

Then, around 1300, suddenly,
the building stopped.

This vast, ornate city
was largely abandoned.

Why?

Archaeologists like Piphal Heng

are trying to answer
that question

and to understand
the people of Angkor.

Monumental architecture
like Angkor Wat

has been its signature.

When you talk about Angkor,
it's monuments.

Part of my archaeological
endeavor

is to understand
Angkorians people's life.

What did they do?

What was the relationship
between the people and the city,

the relationship between
the people and the temple?

And how did that change
through time?

The ancient city of Angkor
was one of the biggest

pre-industrial cities
in the world.

Located in Cambodia,
in Southeast Asia,

its legendary temple,
Angkor Wat,

is its most iconic structure.

But Angkor Wat is the largest
of more than a thousand temples

spread across
more than 150 square miles.

A lot of these temples
have really

been left to the jungle.

So trees are overgrowing them.

The vegetation is everywhere.

And when you walk into them,

you sometimes feel
like you're the first person

that's stepped foot in them
for over a thousand years.

The temples themselves
have a sense

of mystery to them.

You don't know exactly
what happened here

or why they were abandoned.

This land has been home
to the Khmer people

for thousands of years,

living in small kingdoms often
in conflict with each other,

until the year 802,

when Jayavarman II
defeated his rivals

and declared himself a god-king
in the Hindu tradition,

and founded the Khmer Empire.

Nearly 100 years later,

the capital of the empire
was moved to a new site

on a fertile plain--
the city of Ankgor.

Over the next 500 years,

the empire became the dominant
power in Southeast Asia,

ruling over all of what
is now Cambodia

and much of Vietnam,
Thailand, and Laos.

The kings,
who were considered holy,

ruled both political
and spiritual life.

Each successive ruler strove
to demonstrate his greatness

through major
construction projects.

Each new king and each new reign

would try to outbuild
the reign before

and build a bigger
and more impressive temple.

As Angkor grew,
so did the temples,

in number and size,

until the 1300s,

when temple construction
abruptly stopped.

Evidence suggests
that by the mid-1400s,

Angkor stood empty.

It mysteriously was abandoned.

Well, so the story goes.

And so this place has attracted
the fascination

of people from around the world

for many, many hundreds
of years.

Partly because the city itself
is so prodigiously massive,

and partly because all of this

was apparently left
to the jungle

by the Khmer.

The abandonment of Angkor
is an enduring mystery.

Who were the people
who lived here?

And why did they leave?

The answers are important,

not just for the archaeologists
who are excavating here.

Many Cambodians identify
as Khmer

and are deeply invested
in this ancient place.

Angkor is central
to their heritage,

connecting them
to their ancestors.

Angkor holds

a profound place in Khmer life.

The very word "Angkor" is deeply
rooted in our national identity.

Angkor, we can say, for
the Khmer people, it's our soul.

Every one of the Khmer
people say that.

"We want to see Angkor
before we die."

But the full truth about Angkor
and its people has been elusive,

because so much of the city

has long lain hidden
beneath the jungle canopy.

We only know of Angkor Wat,

this one-square-kilometer
temple,

as being a religious temple,
a sacred space.

But were there any people
living inside Angkor?

We did not know yet.

We also tried to map the area,

and the vegetation was
too thick, was overgrown.

It's incredibly difficult
for archaeologists

to map the center of Angkor,
where all the large temples are,

because of
all the dense vegetation.

But now, with the help
of new technology,

archaeologists are trying to see

what has been invisible
for centuries.

That's where the lidar data
comes in.

Lidar is a powerful laser
technology that has recently

become a game changer
for archaeologists.

Lidar has been absolutely
revolutionary

for our field of archaeology,

because it allows us
to see the ground floor

underneath dense vegetation.

We acquire lidar data by putting
a drone or a helicopter

or a plane in the air
with the lidar device on it.

These devices send out
millions of pulses of light.

Most of those pulses
bounce off things

that we're not interested in,
like buildings or trees,

but some of them, critically,

reach the groundsurface.

What we do is,
we measure

the time
that it takes

for those groundreturns

to return to the
lidar device,

and, using those measurements,
calculate distance.

With that information,

we can then strip away
all the vegetation

so we can clearly see
these archaeological features.

The lidar scans revealed

the breathtaking size
of the city.

The data was spectacular.

All of a sudden, we could see

these elements
of the urban space

that were
completely invisible before.

The lidar data is like
the most amazing treasure map,

not because we're looking
for gold or statues,

but because it allows us to ask
bigger and better questions

about what it was like
to live at Angkor.

The amount of detail

that the lidar revealed
about the landscape

is absolutely mind-blowing.

We were able to map
an additional 20,000 features.

The full scale of the city
of Angkor is staggering,

covering more
than 150 square miles,

about the size of Denver.

Hundreds of miles of roads

and a complex network
of waterways and canals

connected the city.

Hidden in the data were the keys

to knowing how and where
the citizens of Angkor lived.

Because Angkor is built
on a floodplain,

all of the features
were built on mounds.

So when we're looking
at the lidar data,

we're not seeing
ancient houses themselves,

but we're seeing the mounds
these houses were once built on.

This is Angkor Wat,

which is absolutely beautiful
in the lidar data.

So these are depressions
and elevations in the land

that we can very clearly see
in this lidar imagery.

But it's almost impossible to
see these features on the ground

because the vegetation
is just so dense.

All of these black dots
are house ponds,

and beside them
are usually house mounds.

Within the Angkor Wat
temple enclosure itself,

the lidar revealed
more than 200 of these mounds.

Using the lidar maps as a guide,

Piphal Heng set out
to investigate these sites

on the ground.

Those mounds are generally
habitation site.

When we saw a similar pattern
inside the Angkor Wat enclosure,

we started to excavate those
mounds.

Turned out that those mound
have residential debris:

ceramics,

Angkorian stonewares,

and trade ware from China.

So what we didn't know was,
those mound and ponds

were arranged
into a grid system.

That's when lidar came around.

So here we are
in the eastern section

of the Angkor Wat enclosure.

What I am standing on now,
what had been a house mound,

and because of the overgrowth,

we can hardly tell
the topography change.

But lidar map allow us
to pick up

just a slight topographic change
that allow us to identify

whether this area was a mound,
a pond to my right.

The lidar data shows
that we are standing

in an urban block
that is replicated

into other urban blocks

covering the entire
Angkor Wat enclosure.

You can tell
when you're walking around

that there's mounds there.

It's really forested,

but you can see that the
landscape undulates

quite a bit.

Archaeologist Alison Carter

has been collaborating
with Piphal

to try to decipher
what life was like

at the Angkor Wat complex.

The lidar in Angkor
was incredibly eye-opening,

because you just see that they
didn't just build Angkor Wat,

which is a spectacular temple, right?

Like, they were transforming
the entire landscape.

And transforming it
with extreme precision.

If you look
at the temple structures

and align them with the gates
of Angkor Wat,

you would see
that the grid system

was actually aligned
with the temple.

It was all
an engineered landscape.

The remarkable
urban design of the Khmer

extended to the even larger

royal complex, Angkor Thom.

You can see, this is Angkor Wat.

There's a huge moat
that's very visible

from the satellite imagery.

And then up here
is Angkor Thom.

And you can see
the moat of Angkor Thom.

But you really can't see
all of that detail

that becomes so clear
and obvious in the lidar data.

So you can just imagine,

when you enter Angkor Thom
through these magnificent gates,

it would have been a bustling
city on either side of you.

Again and again,

the lidar revealed
sprawling neighborhoods

around Angkor's
more than 1,000 temples.

Combining this with the finds
from excavations on the ground

and new technologies,

a team of archaeologists
is finally able to crack

one of the city's
biggest mysteries:

the size of Angkor's population
at its peak around 1250.

To answer that question

of how many people lived
at Angkor,

we compiled
all of the data

that we had--
C14 dates,

ceramic evidence
from excavation,

and we used some new,
cutting-edge algorithms

and machine-learning techniques

to try to model the development
of the city over time.

I was part of a group of people
that were working

on trying to understand the
growth of the city of Angkor.

If you compare,
this one is better.

That one's better,
it's a much bigger piece.

Yeah, so that'll be great
to collect

from this mound.

That's a really good example

of how we can bring in
good, old-fashioned,

on-the-ground dirt archaeology

with all of these
new technologies.

From our excavations, it seems

like there's just one household
or family per mound.

We use ethnographic data, then,
to estimate

that there's
about five people in a family.

Another important piece of data

was from inscriptions.

A lot of the temples
have foundation dates.

And that was really important
to understand

when they were built.

And inscriptions in two
of the larger temples provide

crucial clues
about the population,

which are of special interest
to archaeologist Andrew Harris.

They actually list
the numbers of temple staff.

These include government
officials, dancers, laborers,

and also, how many people
that the temple staff oversaw

in the surrounding villages,

numbering between 200,000
and 300,000 for both temples.

And then Sarah took that data,
the lidar data,

she used some machine-learning
algorithms.

We brought this all together

to try and create a model
for how Angkor grew.

The final estimate
from their calculations

was staggering.

From our estimates, we think,
at its height,

that it had
about 700,000 to 900,000 people

living
in the greater Angkor region.

That would have made it
one of the largest,

if not the largest, city
in the world at the time.

The discovery of Angkor's
true size

was a major breakthrough,

but it was all
the more impressive

because of Angkor's location.

Because the entire city
was built

on a water-soaked floodplain.

Every year,

the rainy season brings massive
rainfall and heavy flooding.

My family's connection
with Angkor runs deep.

During my childhood,

my grandparents
and my parents frequent

the pagoda in Angkor.

So I've been coming to Angkor
since...

Yeah, for forever.

Growing up here provide

a different perspective
on the water.

We have the Great Lake
to the south.

The lake level
change drastically

during the rainy season,

particularly
around October and November.

The great Tonlé Sap lake

often quadruples in size
in the rainy season,

flooding vast areas
of the countryside.

In the dry season,

nearly half the year,
almost no rain falls.

Why would the Khmer build
in a place

with such extreme swings
between flooding and droughts?

Water is incredibly important
for the Khmer Empire.

Almost everything
revolves around it.

And one of the most
important functions was

irrigating the main agricultural
crop of the empire:

rice.

The economy of Angkor

was underpinned
by rice agriculture,

which is heavily dependent
on a stable supply of water.

There's a strong relationship
between water, the floodplain,

wet rice agriculture,

and the early phase
of Angkor period.

As the Khmer Empire and city
expanded,

controlling the flow of water
was key

for their economy, trade,

and ability to feed
a growing population.

But how did they do it?

Visible remnants suggested
there had once been

a complex water system.

But it took lidar to reveal

the full scope
of the Khmer engineering.

So, with the lidar,
we were able to create this map,

which very clearly shows

the layout of
the water management system

and how water flows
into the city,

through the city,

and then,
how there are exit channels

to remove excess water.

The design was both ambitious
and ingenious.

A series of massive reservoirs
called barays

collected water
in the rainy season.

So here at Angkor,
we can see the large barays.

Here's the West and East Baray,

and then all of these
straight lines

funneling into the city,

these are
manmade water channels.

So this is rerouting water

from northern areas of Cambodia
into Angkor.

The water was captured
from natural rivers

and moved into storage
in these massive reservoirs.

Those barays were really the
centerpiece of the whole system.

The largest baray
stretched across

more than six square miles.

All of these features
are so big

that you can literally
see them from space.

That's the beginning
of Angkor's power--

water management.

Those reservoirs are
fantastically important.

They hold huge volumes of water,

which can be distributed
in the dry season,

if you want a second crop
of rice, for example.

So it really kind of
super-boosts your productivity

in those parts of Angkor
which are downstream

of those reservoirs.

And the system extended
far beyond the city itself.

The landscape around Angkor

is actually at a slight incline,
about one percent,

so the East and West Barays

can catch water as it comes
into the city, hold it,

and then redistribute it
through the different channels.

Taming the water was
a major feat

of urban engineering,

with hundreds of miles of canals
and reservoirs, all dug by hand.

But the floodplain also
created a major challenge

for an empire intent on creating
monumental architecture.

It's a bit of a difficult spot

for building heavy temples
like Angkor.

So in order to do this,

they had some really ingenious
engineering strategies.

How did the Khmer manage
to build

massive stone structures
on soft, deep soil

surrounded by water?

The first clue may be in the
choice of building material.

Hang Peou is the head of APSARA,
the organization in charge

of restoring the city of Angkor
and the surrounding area.

The priority of APSARA,

it's about the conservation,

how we can preserve the temple
without falling.

Before we start
to make the restoration,

we need to do research.

The highly decorated walls
of Angkor's temples are built

of fine-grained sandstone,

well-suited
for intricate carvings.

But appearances
can be deceiving.

Just under the ornate façade

lies the first secret
of Khmer construction.

Hidden within the walls
and foundation

are blocks of a rough,
porous stone called laterite

that can be even lighter
than sandstone.

In term of the weight,

it's less heavy.

The core inside
they build in the laterite,

and then they put

the limestone around
for decoration.

The whole temple's built
in that concept.

Using the lighter laterite

greatly reduced the load
on the soft ground.

But the stone temples
are still incredibly heavy.

The stone that forms Angkor Wat,

towering over 200 feet high,

weighs millions of tons.

And yet,
it has survived the wet terrain

for over 900 years.

There must be more
to the engineering.

But what?

Archaeologist Neth Simon
leads restoration teams

for APSARA.

Her excavations are focused
on understanding

the key elements
of ancient Khmer engineering.

As a result of the
excavation

to see the condition of the
foundation,

I was able to understand
the ancient techniques

in building the temple.

We observed that before
shaping the temple itself,

the ancient builders began
by digging down

to reach the natural soil.

They then began compacting
the soil inside.

The next step
involved filling the foundation

with fine, pink sand.

This was followed
by the laterite foundation.

Above it, they laid the base
layer using sandstone.

As any trip to the beach
reveals,

dry sand is soft
and shifts easily,

while wet sand,
closer to the water, holds firm.

Water and sand together

can create a solid base
for construction.

This could be one reason

the temples at Angkor
were surrounded by moats.

Another thing that we can see

is that all of these temples
have water features around them,

so each temple,
which is marked in red,

they tend to have moats
around them.

They built on the sand layer,

and we understand
that the sand layers

need a lot of the humidity

to make it stronger

to support the load
of the temple.

The moats were, and still are,

key to maintaining
the required level of moisture

beneath the biggest buildings.

The water in the moats provides
stability to the sand,

which allows them to hold

the heavy, heavy stone
structures up.

These innovations demonstrate

that the Khmer were masters
of hydraulic engineering.

As do the hidden features
that enabled

Angkor's massive reservoirs,
the barays, to function.

Cambodian archaeologist
An Sopheap and his team

are excavating a unique location

on the edge
of the Eastern Baray,

an ancient reservoir long dry
and now covered in jungle.

Right in front of
me is the East Baray,

a water reservoir
from the Angkor era.

These are the ruins
of an ornate stone pier

overlooking the baray.

The Cambodian government
hopes to restore the pier

and partially repair
the reservoir

to make it functional once more.

The first step is to understand
how both were constructed.

But while excavating the pier
and the reservoir wall,

they found
an unexpected surprise.

At the excavation site
I opened here,

we found the foundation
of the pier,

made from laterite rock.

Hidden beneath the wall
of the baray

is a laterite stone foundation

extending more than 50 feet
out into the reservoir.

This work is important because
it reveals a new discovery.

We had never seen a construction

with laterite sloping
like this before.

The next step is to determine
if this massive foundation

extends along the banks
of the reservoir,

beyond the area of the pier.

We opened another excavation
site ten meters to the south

and saw the structure
continues with four more steps.

But we only exposed
a small section

to confirm.

In the future,
we'll keep excavating

to see if the laterite structure

surrounds the water reservoir
or ends somewhere.

No one knows yet how far
this stone foundation extends

around the baray.

If this structure

goes around the reservoir
or part of it,

it would be a new discovery
for the Angkor area.

The pier itself

was a very special structure
for Angkor's Hindu god-kings.

This pier was used by the king

to offer alms at the temple
located at the center

of the eastern reservoir.

In the middle of each reservoir
was an island temple.

The barays were more than just

a brilliant piece
of hydraulic engineering.

Here's the East Baray.

And you can clearly see both
in the mapping

and in the lidar data

that there's a huge temple
in the middle of it.

These large reservoirs were
both functional and spiritual.

The kings of the Khmer Empire
played both

a political
and a religious role.

In addition
to being head of the army,

the king was also a king-god,

so head of the religious system,
as well.

Every king left an
imprint of himself

if he was powerful enough

to create a mark
on the landscape.

Dozens of temples here

are reflective of
the absolute power over nature,

over people,
and over the landscape

that they manifested
during their reign.

Over the centuries,

rulers built
larger and larger temples

as the Khmer Empire expanded.

Early in his reign,
in the 1100s,

King Suryavarman II
outdid all of his predecessors,

building Angkor Wat.

Angkor Wat was a huge project.

It would involve so many workers
and so many craftsmen

to be able to build it.

Dedicated to
the Hindu god Vishnu,

nearly every surface of
Angkor Wat was highly decorated.

Traces of paint found
on the carved walls and ceilings

hint at its ornate history.

All of the reliefs
on the temples

were originally painted
in vibrant colors.

Immense carved panels
with scenes from Hindu texts

run down vast hallways.

Thousands of priests, dancers,
and attendants

filled the temple
and its grounds.

It was a ceremonial center
on a grand scale,

demonstrating
the glory of Vishnu

and the power of the king.

Temples were not just
places of worship.

The kings were also using them

to demonstrate their power.

So they probably
were really acting

as this kind of billboard for
the king and the king's power

and putting his stamp
on the landscape.

Wealthy and prosperous,
the Khmer Empire

was an attractive target
for neighboring powers.

Carved scenes at Angkor
illustrate the story

of one major conflict.

In 1177,
the nearby kingdom of Cham

invaded Angkor
in a surprise attack.

To reclaim the city
and restore power to the empire

would take one of the strongest
of the Khmer kings,

Jayavarman VII.

In the year 1177,
the Chams conquered Angkor

and occupied Angkor.

Jayavarman VII made it his vow
to reconquer Angkor.

And part of this is depicted

through various campaigns
of warfare.

This is probably the most
elaborate of those campaigns,

and it involves a naval battle.

What we can tell here
is that one, it was intensive.

It was violent.

You could see the people
falling overboard.

Most of them have been
stabbed or dead or whatnot.

And a lovely crocodile

eating a poor Cham
who's fallen overboard.

So, through
a series of campaigns

lasting several years,

he was able to eventually
vanquish the Chams.

With the enemy defeated
and the Khmer back in power,

Jayavarman VII would usher in
Angkor's golden age.

During his reign, he gave back
to the public in many ways.

He constructed hospitals

and he built a number
of different temples.

This is a scene of a hospital.

Here you have women
giving birth, making medicine.

This was a major point during
the reign of Jayavarman VII.

He built, I believe,
102 hospitals

across the Angkorian empire.

Alongside his support
of public health,

Jayavarman carried on
the tradition

of his Khmer predecessors--

monumental construction.

When kings came into power,

they all had specific mandates
that they had to accomplish.

And a lot of this
revolved around temple building.

Angkorian kings
had a undocumented habit

of trying to one-up
their predecessors.

If you think
about Suryavarman II,

he built the world's largest
religious monument, Angkor Wat.

Jayavarman VII left the largest
architectural footprint

on the Angkorian landscape

of any monarch
in Cambodian history.

The pinnacle of his reign
was the construction

of Angkor Thom,
an enormous complex

more than five times
the size of Angkor Wat.

It is surrounded
by eight miles of moat,

and at its center
stands a temple different

from any built before or after--

the Bayon.

The Bayon is not
a Hindu temple--

Jayavarman was a Buddhist.

All of the elite temples

up until the reign
of Jayavarman VII

were considered to be Hindu
temples of various deities.

One of the most interesting
things

about King Jayavarman VII

is that he switched
the state religion

from Hinduism to Buddhism.

Hindu worship involves
a pantheon of gods

and observation of rituals
set out

in the Vedic scriptures,

while Buddhism focuses
on enlightenment

through the teachings
of the Buddha.

The Bayon temple towers
feature 216 enigmatic faces

that may contain
a hidden secret.

The faces on the Bayon
and the gate

are potentially a Buddhist saint

or they're the king himself.

And the reason we think
it's Jayavarman VII himself

is because a number
of the images that we know

of Jayavarman VII

look almost identical

to the face towers on the Bayon
and on the gates.

Following the reign
of Jayavarman VII,

ending around 1218,

Angkor was at the height
of its size and influence.

What was life in Angkor like
at its peak?

Few written descriptions
have survived from the Khmer,

but historians
have one detailed account.

In the 13th century,
the emperor of China

sent an emissary to Angkor.

Zhou Daguan was ambassador

of Mongolian-controlled China
to Cambodia.

He lived in Angkor
sometimes

between 1296 and 1297,
almost one year.

Zhou Daguan left us a journal,

and it's incredibly valuable
in terms of

the types of details that
he wrote about.

I'm not sure if heintended

for archaeologists
to read this,

but it sure provides
a lot of information.

Around the outside of the city
walls, there is a large moat.

The walls of the bridges
are made of stone

and carved into the shape
of snakes.

As an archaeologist, I refer
to Zhou Daguan constantly.

He talks about how poor people
lived in smaller houses,

and their roofs were made
out of thatch,

but richer people
would have bigger houses

and their roofs would be made
with ceramic roof tiles.

Zhou Daguan's journal described
scenes of everyday life.

Their litters are made
of pieces of wood

that bend in the middle.

A person sits in the cloth
and is carried by two people,

one at each end.

The parasols are made of a
strong, thin, red Chinese silk.

But how reliable
are Zhou Daguan's descriptions?

His descriptions of daily life
are actually backed up

by a lot of what we see on
the walls of the Bayon temple.

Zhou Daguan describes Angkor
as a bartering system,

and he describes a market day.

He describes how merchants,
mostly women,

would lay down their blankets
and sell their wares.

The local people who know
how to trade are all women.

Small market transactions
are paid for in rice

or other grain
and Chinese goods.

Larger in size
are paid with cloth.

There's these bas-reliefs of
people cooking and eating food,

and then, in our
archaeological excavations,

we find
really similar materials.

When we are excavating
an, an occupation area,

and you're, like,
"Oh, this looks

just like what's on the Bayon,"

like, you can really see

how these different sources
of evidence come together

to give you a more complete
picture of the past.

That record represents

a fantastic contribution

to our understanding of the life
of the city.

And at that time,

the king and the court
were very impressive,

the city was enormous,

and there was clearly
a lot of wealth floating around.

Above the gates are
stone Buddha heads.

One of them is decorated
with gold.

In the center of the capital
is a gold tower.

Zhou Daguan describes Angkor

as a very active,
very vibrant metropolis.

He talks about the significant
amount of wealth

coming out of the palace.

So, Zhou Daguan described

the temples at Angkor

not as these stone mounds
that they are today,

but covered in gold
and very clearly upkept.

But that upkeep would not last
much longer.

Zhou Daguan's record represents

the very kind of last gasp
of Angkor

as a spectacular, opulent,
thriving metropolis.

From that point forward,

things change dramatically.

This inscription indicates

that this temple
was the very last Hindu temple

that was dedicated at Angkor.

We actually have
an exact date for it.

It's the 28th of April,
1295 C.E.

Which was just a year
before Zhou Daguan showed up.

As far as we know, there are
no temples after this one.

As the 1300s continue,
Angkor starts to decline.

We start to see evidence
from different sources

that population starts to slide.

There are no more inscriptions
created, no more temples built.

Official written histories
of the Khmer

did not appear again
until much later.

There's a bit of a black hole
in the historical records

from about the 13th century
to the 15th century.

So, it's a big gap--
it's many hundreds of years.

What happened to bring an end
to centuries of prosperity

and monumental construction?

No big city like this one
is ever going to have

a single reason
for its start or its end.

So, in that context and
in the complexity of that story,

we can start to accept

that there's no linear,
simple, single explanation

for the demise of a place
like this,

but, rather, a tangle
of different explanations

that happen to coalesce
at a particular point.

The warm and humid environment
of the Cambodian jungle

works against the archaeologists
trying to shed light

on the declining years
of Angkor.

The rainforest does not help,

because almost every wooden
remain--

and the vast majority of
structures at Angkor

were built in wood--

deteriorate very, very quickly.

One of the most puzzling aspects
of Angkor today

is the complete absence
of human remains--

no bodies, no burials.

This is a very fascinating thing

that's baffled archaeologists
for a long time.

There are no funerary remains
until much later.

So, for 600 years,
one million people,

not a bone,
not a cremated remain,

not a funerary jar,

not a trace
of a funerary remain.

The lack of bodies,
human remains,

in Angkor's
archaeological record

is fascinating and frustrating
in many ways.

It's very rare for a city

which had 700,000,
a million people in it,

that there are so few bodies.

What happened to the bodies
of the ancient Khmer?

Well, Zhou Daguan, he talks

about different
burial practices.

The body is taken to a remote,
uninhabited spot,

where it is thrown down
and left.

After that, the vultures, crows,
and dogs come and eat it.

But the archaeologists at Angkor
are struggling to find

the location
of the "sky burials"

that Zhou Daguan described,

where the dead are left
to the elements and animals.

He said that they carry the dead
outside of the Angkor Thom gates

and then left it
outside the wall.

But when we look at lidar data
outside the wall,

would have been
just settlements everywhere.

Where was that outside the wall?

Burials do tell us a lot about
health and the individuals.

So, to find a graveyard, or
even to find cremated burials,

that would be phenomenal
for Angkorian archaeology.

But so far, we have not found
evidence of a burial ground yet.

Without the bodies themselves,

archaeologists are searching
for other clues,

hoping to find out,
when did everyone leave?

Dan Penny is focusing
on the barays and canals,

and the sediments
below the surface.

The sediment is accumulating

at the bottom of these
reservoirs, ponds, and so on.

They end up as beautiful
little traps for material

landing on the surface

and then settling
onto the sediment,

and then being buried by
subsequent layers of material.

And so it goes,
layer upon layer.

We can come along,

hundreds or even thousands
of years later,

and take these samples and find
this undisturbed material

which faithfully records
the conditions

that were occurring
when they were deposited.

The moats of Angkor Thom
are a fantastic archive.

They've been largely left alone.

So, we can use them as natural
archives of change through time.

This core goes all the way back
to pre-Angkor.

So, it goes into the alluvial
soil beneath the moat,

and we get the whole sequence
all the way

through the rise and fall of
Angkor and into the modern day.

After processing,

Dan studies the samples
from the sediment layers

under a powerful microscope.

So this guy here is a
pollen grain from a lotus.

This one is a sedge,

so it's another aquatic plant.

This is a huge chunk
of charcoal,

coming out of maybe
a domestic fire,

someone's fireplace,
where they're cooking.

Could be any source,

but it's invariably associated
with people.

Radiocarbon dating adds
another layer of information.

Most of the work
that I do with radiocarbon

is actually based on dating
the pollen grains themselves.

Each of these pollen grains

is about ten to 20 micrometers
in diameter.

A micrometer is a thousandth
of a millimeter.

So, they're pretty small.

The types of pollen grains--

found at different depths
in the core--

can reveal when the ancient
moats were well maintained

or filled with weeds.

When moats are being maintained,
you'll often see species

rooted into the sediment
at the bottom of the moat,

as opposed
to an unmaintained moat,

which is kind of completely
covered

by ferns and grasses
and other things.

Once management stops,

the moats will quickly cover
with vegetation.

We definitely find evidence
of the water systems

not being maintained.

Once they're abandoned,
they are permanently abandoned.

And so that represents a very
clear horizon for us to, to say,

"Hey, at this point,
this water feature

is no longer being managed."

What we are finding increasingly

from a range
of different variables,

charcoal among them,

pointing to a progressive
decrease

in the intensity of occupation

in the very epicenter of Angkor
as a city.

All of these things
are decreasing progressively

through the 1300s.

What could have happened
at Angkor in the mid-1300s

that would have caused the Khmer
to leave?

And the reality is that we
don't really know what happened.

There are a number
of different hypotheses,

and probably it was
a combination of all of them.

So, we need to cast our mind
to what other reasons

might there have been for people
to start leaving Angkor.

There's another story there
sitting underneath,

which is far more interesting
and far more important.

Angkor was not the only place
to suffer

a major population decrease
during the 1300s.

The bubonic plague--
also known as the Black Death--

that killed millions in Europe

came out of Asia
during this century.

The timing fits
the abandonment of Angkor,

but is there any evidence
of a connection?

Evidence of a pandemic at Angkor
would be revolutionary.

The effect of a pandemic

in a pre-industrial city
like this one,

which was massive
and had a huge population,

would have been catastrophic

and it would likely have led
to very rapid depopulation,

particularly by those people
that can move.

If there's evidence
of a pandemic.

I can assure you
there will not be.

Because if you, if you have
a pandemic here,

you will find bodies.

Right?

Because there are
900,000 people here at the peak.

If you had the plague here,
it would have been horrific,

and there would be no way that,
that people would have been able

to deal with that much human
remains in, in the normal way.

If it wasn't
a cataclysmic event,

could a slow decline

have been triggered from
within the Khmer Empire itself?

What changed for the Khmer?

A discovery
during the restoration

of Angkor's Ta Prohm temple

may provide a clue.

Ta Prohm was constructed

as a Buddhist temple in honor

of King Jayavarman VII's
late mother.

The first step taken by our team

was to conduct
an initial survey.

Our team discovered broken
pieces of a Buddha statue.

Once our team began digging
and cleaning,

more and more of the statue

began to emerge
from the ground--

more than 140 pieces in total.

Some of these sculptures
were buried,

while others were left scattered
around the temple grounds.

Ta Prohm was not the only temple
to see this kind of destruction.

So, we're at Preah Khan temple
here.

It was dedicated
in the year 1191

to Jayavarman VII's father.

What's on the left here

is a series of niches
where the Buddhas

have been completely hacked out.

And we think that this was
an act of religious violence.

What would have caused the Khmer
to turn against Buddhism?

We believe
that in the 13th century,

one of Jayavarman VII's
successors, Jayavarman VIII,

was responsible for this,

and shifted the royal cult

from Mahayana Buddhism
back to Hinduism.

This act was either due
to religious reasons

or even political reasons,

as a retaliation against
the reign of Jayavarman VII.

After Jayavarman VII's golden
era under Mahayana Buddhism,

and Jayavarman VIII's
Hindu backlash,

the Khmer religion changed
one last time,

to an older form of Buddhism
called Theravada Buddhism.

The Khmer Empire was undergoing

another major cultural
and religious shift.

King Indravarman III

essentially switched the entire
religious ideology and landscape

to Theravada Buddhism,

beginning during his reign
in the year 1296.

Chinese ambassador Zhou Daguan
arrived at Angkor

just as the empire moved
to worshipping

at Theravada Buddhist temples
called viharas.

Zhou Daguan saw this society

that was in transition
and changing.

Rather than more temples
being constructed,

it was now
vihara and monasteries.

There were still construction
activities after 1295.

The type of structures,
the type of temple,

changed
because of Theravada Buddhism.

Because Theravada Buddhism
only require a terrace

surrounded by boundary stones,

and a Buddha statues,

and then wooden
upper structures.

So that's very simple.

And that's what could drive
a lot of change.

The age of giant, ornate
stone temples was over.

The shift away from huge temples
and elaborate ceremony

not only meant
less construction,

but fewer monks, dancers,
and religious staff.

All this could have contributed

to a shrinking population
at Angkor

in the 1300s.

But would it have caused
the Khmer

to completely abandon
such a vibrant city?

Or was there another, fatal blow
to Angkor?

We don't, obviously, have
a historical record

of climate from Angkor.

There was nobody here
recording it at the time.

So what we have to do is look
for other sources of information

that can tell us that.

We exploited
some tree ring records

from the mountains of Vietnam

that tell us about rainfall,
in particular.

And they tell us
a really interesting story

about variations
in weather and climate

during the period
where Angkor is abandoned.

The study showed

a series of droughts at a time

when Angkor was vulnerable.

So those two droughts
occurred

from about the middle

of the 14th century,

so from about 1350 onwards,

and they lasted
for about two decades,

more or less, at a time.

So, they were really, really
severe-- quite profound.

Nothing like we have seen
in the modern era.

Angkor had survived droughts
before,

but this time
may have been different.

So, you have the sense
that Angkor is very successful,

but it's building itself
into a state of precariousness.

So, by the time it gets
to the middle of the 1300s,

and you're hit
with a massive drought,

and then a, a big wet period,
and another massive drought,

the whole system starts to crack
and come apart.

Fractured it, shattered it,
by eroding,

by sedimenting or infilling
canals,

and blowing out banks
and reservoirs.

Doing all sorts of damage
to the system.

You need to have the people who
continue to have that knowledge.

Without those people,
everything collapse.

The lidar data also reveals
some failures

in the water management system.

So here we can see
where a channel

cut through an embankment.

You can see
that this is going

right through the middle of
a densely occupied urban space.

So, this would have been
devastating for the people

that were living here
at the time.

And one of the other things is,

we can see that this failure
was never repaired.

At some point in the 1400s,

the city of Angkor
was largely abandoned.

Only a handful of farmers,
monks,

and religious pilgrims remained.

Over time, the jungle covered
the ancient heart

of the Khmer Empire.

For over 600 years,
the seasonal floods and droughts

ravaged the ancient monuments.

Today, Cambodians have
an ambitious plan:

restore the ancient
hydraulic systems

and bring the water back
to Angkor

and the surrounding area.

Our ancient ancestors

already designed and built

working water systems.

So as the younger generation,

our work is simply
to restore and rehabilitate.

We are combining the use

of ancient technology
that already exists

with our modern technology.

Restoring the channels
into the barays

to prevent flooding
and hold water

has been successful.

The Northern Baray is now full.

So is the West Baray,

holding more than
13 billion gallons of water.

The project to widen the canals
and fill the ancient moats

continues,

including work
on the moat around Angkor Wat.

In the same time,
that system allow us

to save the water
for the dry season.

It's impossible to have
this level today

if those system
are not put in place.

Even in the end of the dry
season, you will have the same,

nearly the same water level
in the Angkor Wat moat.

Angkor's legacy reaches
beyond Cambodia.

Angkor is a location.

It's also a representation
of a culture and a civilization,

the ways in which humans
can flourish

in difficult environments.

It represents a celebration
of the past,

and it represents a warning
to our future.

But for the Cambodian people,

the centuries of history
at Angkor

form a central part
of their identity.

Through the transition

from the Angkorian
to post-Angkorian period,

it's social transformation.

After the collapse of Angkor,
it became a symbol of power.

For the Khmer people,

Angkor is an important
cultural symbol,

a reflection
of the Khmer identity and soul.

All the local people want
to pay the respect to Angkor.

Angkor,
for the Cambodian people,

it's a lot-- everything.

The sacred aspect of Angkor

never left Angkor--
it is still here.

It's still here today.