[ Bird screeches ] -Hidden in Southeast Asia's jungle terrain lie the remains of the spectacular city of Angkor, once home to one of the greatest civilizations on earth.

-Angkor was a very vivid city, full of life.

-Angkor was the capital of the Khmer empire, which controlled much of the region between the 9th and 15th centuries.

The great temple of Angkor Wat, still the largest religious site in the world, stood in the heart of the city.

The stunning accomplishments of Angkor's great kings are clear to see.

But the period that preceded the foundation of their great city is shrouded in mystery.

Now, the discovery of an incredible hoard of stunning artifacts, unlike anything ever seen before, is providing surprising new clues about this early Khmer society.

Pounds of gold, silver, and precious stones in pristine condition.

-Oh, wonderful!

-An international team of archaeologists is searching for the origin and significance of this treasure.

-This is the first state of the wall, here.

-What will the treasure reveal about early Khmer society?

-It's quite amazing.

-The team will study each artifact... -Yes!

Today we are really very lucky.

-...excavate the site where the goods were found... -Ah, incredible!

-...use the latest technology to reconstruct history... -This is the photogrammetry of the collapsed temple.

-...and laser scan the jungle, revealing lost cities in astonishing new detail.

-Do you think that this city was the capital of a kingdom?

-Can the treasure open a new window into the early history of the Khmer people, creators of one of the most powerful empires to rule the medieval world?

♪♪ ♪♪ In the bustling city of Savannakhet in southern Laos, scientists and researchers from around the world are on their way to see a priceless hoard of early Khmer artifacts for the very first time.

The treasure is one of the most spectacular discoveries in the history of Southeast Asia.

-Ta ta, let's go... because we have a short time, so... -This carefully assembled team has been given just two weeks by the Lao authorities to analyze the mysterious treasure.

-How many minutes before to see it?

[ Chuckles ] -The team gathered here is really a dream team.

-I think it will be very interesting to look at it and try to understand what it could be.

♪♪ -The team also wants to find out more about the tiny farming village where the treasure was found.

Laos is a landlocked country in Southeast Asia.

More than a thousand years ago, it was part of the huge Khmer empire that stretched almost 400,000 square miles, with Angkor at its heart.

It was here, in the modern village of Nong Hua Thong, 25 miles from Savannakhet, that the precious artifacts were found.

The growth of the Khmer empire began around the start of the 9th century, coinciding with the Viking era in Europe.

Khmer builders would go on to construct not only some of the greatest temples in history, but also vast networks of canals and reservoirs for large-scale rice cultivation.

The completion of the Bayon temple in Angkor in the early 13th century marked a high point for the civilization.

The team hopes the treasure will shed new light on the earliest years of this sophisticated society in the centuries before the foundation of Angkor.

To ensure maximum security, the collection is kept in a secret location in a heavily protected vault.

♪♪ Inside the vault, the team is stunned by what they see.

-Oh, wonderful!

I've never seen before like this.

-The treasure is even more extraordinary than they anticipated.

-That's quite amazing.

Wow!

Having such a gold and silver treasure in front of your face is really impressive.

-We have eight kilograms of gold here and 18 kilograms of silver.

It's very huge.

Never, we've never had something like this in all of Southeast Asia.

-The exquisite artistry of these artifacts is unprecedented.

-This level of craftsmanship on such precious material, it's phhh!

It's -- It's far beyond what I encountered already.

-With so much to do in just two weeks, the team wastes no time setting up a laboratory inside the vault.

Their job is to study 60 major artifacts and 1,800 fragments.

Each one will need to be analyzed to establish a date and provenance, then measured, cleaned, and conserved.

Objects that are broken must be carefully reassembled.

-I try to match it, but some parts is missing.

-Yeah.

Good luck.

-Thank you.

-Every item is photographed so it can be studied after the vault is closed.

But for now, having the real thing is the best way to catalog every detail and hunt for clues hidden within each object.

-We are trying to solve where the objects are coming from, why they were buried into this place.

-The huge diversity of this collection adds to its mystery.

It appears to tell a complex story.

Not only are there many different types of objects, ranging from small bronze pieces to large prestigious gold artifacts, but some show foreign influences.

-This is very strange.

You know, this kind of design we can find in, uh... -Champa Kingdom.

Yes, yes.

-...Champa Kingdom.

It's beautiful, huh?

-Some reflect contact with the Champa Kingdom, located nearby in what is now Vietnam.

Other artifacts appear to have come from further away, like these silver plates from China.

-But we never find any piece like that.

It's really exceptional to find this kind of object.

-The rarity of these pieces makes them more difficult to trace, even for the world's top experts.

There are so few references the archaeologists can draw on for information.

But despite these challenges, Christine makes an early breakthrough.

-I thought it was a sort of diadem, but it's not this.

It's partly gilded, yes.

Only partly gilded.

Here, there is no gold.

I think that it is from Khmer origin, and what is very interesting is the two faces, uh, with a lot of teeth.

So, it could be ancient and be pre-Angkorian.

-Christine's hunch that the object is Khmer and predates Angkor is exciting for the team.

If she's right, it dates the treasure to a time in history that comparatively little is known about, a period when the Khmer civilization was on the rise.

The next challenge for the team is to learn more about the place where the treasure was found.

♪♪ The village of Nong Hua Thong is in southern Laos on a tributary of the Mekong River.

The team begins their excavation here to see if the history of the site sheds any light on the artifacts.

Leading the dig is top archaeologist Nicolas Nauleau.

-The main thing for us now will be to explain, in fact, why the treasure was here, all the context of the treasure, what has been before the treasure, what has been after the treasure.

-Nicolas has been given four weeks for this large operation.

Most of his excavators are villagers being trained on the job.

-We are a team of 60 people.

It's a pleasure to share the knowledge with the Lao farmers because they are very interested in their history, because it's the history of the village, also.

I hope we will find maybe some artifacts, some nice artifacts.

It's very exciting and it's just the beginning!

-Nicolas and his team soon make their first important discovery more than six feet below ground.

-It's bones.

It's part of an animal.

We have ashes from fires, different fires, and after they put the bones from their meal.

It's probably part of a shoulder here.

Also, we can see the shape here, it's a part of the -- of the mouth.

We have the teeth.

So, which kind of animal is it?

It's too early to say that.

It's quite rare, in fact, to find bones in this area.

In Southeast Asia, the sediment, the soil, is very acid, so the bones normally, they completely disappear.

It could be 100, 200 AD, maybe.

It's possible.

It's very exciting.

-Finding evidence of such early human habitation is significant.

It shows this village could have roots that stretch back nearly 2,000 years.

The first Khmer people lived in small settlements centered around ponds.

They were rice cultivators who venerated spirits in the sky.

But starting in the 5th century, as ideas from India, like kingship and Hinduism, began to permeate society, the social structure of Khmer villages began to change.

The team hopes their excavation here will provide new insights into what Khmer society was like in the era they suspect the treasure dates to -- before the foundation of Angkor.

The treasure was found near the bones but higher up in the layers of earth, meaning it was buried later.

The way in which the objects were buried could also provide the team with important information.

To find out, one of the archaeologists, Viengkeo, is on his way to meet the farmer who discovered the cache.

Farmer Khieow Phommajack says his first encounter with the treasure happened in a dream, when a monk showed him a field full of gold.

-[ Speaking Lao ] -A short while later, he was out in his field digging when his spade struck something hard.

-[ Speaking Lao ] ♪♪ -Now that they know how the items were found, the archaeologists believe they were purposefully buried.

The team focuses on finding out who might have owned the treasure and why it was buried.

♪♪ In Savannakhet, inside the treasure vault, the priceless gold artifacts are being examined.

With such elaborate construction and decoration, they are the most likely objects to offer up hints about who this collection could have belonged to.

And the most impressive of the objects is the jewel-encrusted gold box.

-Did you see already something like this?

-No, no.

We have never seen such a very beautiful masterpiece.

-Do you think -- Do you have an idea of the provenance of this?

-I think the object itself could come from somewhere in this region of Laos.

-For the moment, we don't know at all the date of the gold box.

That's the problem for us.

I tried to find some comparison on bas-reliefs, on books, and I didn't find anything.

It's totally unique.

-For the box, I have no idea.

I never saw such a box before like that.

-The design of the box is of particular interest.

It features an unusual fusion of religious motifs represented by different birds, which may provide clues to its origin.

-You can see here some Garuda.

-The Garuda is an eagle that carried the Hindu god Vishnu.

-On the top, these are some birds, take the soul to the paradise.

This is Buddhist.

-This other bird is known as a hamsa, a type of swan that represents reincarnation in the Buddhist religion.

Buddhism and Hinduism arrived in Southeast Asia around the 4th century, brought by travelers from India heading up the Mekong River.

The dual religious motifs suggest the box may have been made in a place and at a time when these religions co-existed.

Studying how the box was made may provide additional clues about how technically advanced that place was and even who the object was made for.

-They were hammering a gold sheet from the inside to make these shapes on the outside.

But they also added the gemstones in rows and then put a separate band of gold across the back of them to keep them in place.

The craftsmen were experts.

-With such impressive craftsmanship, could the box have been made in a workshop supported by the wealthiest patrons in society?

-I think that the gold box is unique, it's for king.

-Could this box be from a royal collection?

The exquisite fabrication of an object typically made of terra-cotta suggests that's the case.

-Very, very beautiful, I've never seen this.

First time in my life.

-They are quite sophisticated objects in terms of techniques, made of precious metals, gold and silver.

They were clearly linked to kings or high dignitaries.

-So, it could be the treasure of a king.

Yeah, could be, but we have no archaeological evidence for the moment.

That's why the excavation continue on the site of Nong Hua Thong.

-Back at the dig site, the team has made another important breakthrough that may support their theory that the treasure could have belonged to an elite group of people.

They've discovered the remains of an old temple complex, and Christine has come to have a look.

Khmer kings built temples to show their devotion to the gods.

Could this temple mean the site was once a royal enclave?

And is there a link with the treasure?

-Maybe there is a connection?

-Maybe.

If there is a connection, it's good for us.

-I hope, yes.

Lucky?

-We will be lucky.

-The style and size of these large bricks date the temple to the 7th century, about 500 years before Angkor reached its peak.

But the team is even more excited about the discovery of an artifact found inside the temple site.

-Incredible!

Totally similar that we found in the treasure of Nong Hua Thong.

-The team has found a small cymbal, almost identical to one found with the collection back at the vault.

-A sort of bell.

-Okay.

Cymbale?

-Bell.

Cymbale.

Une petite cloche.

Usually there is two.

-Khmer cymbals like this would have been used in a temple as part of a religious ceremony.

The cymbal's location could, in turn, help determine its age.

-So, can you show me the exact place?

Ah, you find directly on the ground?

-Yes.

-It was a little bit higher?

-On the top.

Yes, a little bit higher.

-Knowing where the cymbal was found, the team is now able to date it to roughly the 7th century.

Along with the temple, it suggests the presence of a royal or religious elite here in the pre-Angkorian period, at the time the treasure might have been made.

But further discussion is cut short by looming clouds.

-I hope we will not have rain here today, because otherwise it will be a mess.

The rain we have here in this area, it's a huge rain, and I'm afraid that the walls, with the sand, that they will collapse for sure.

-Maybe we need to cover the trench?

-Yes, maybe we need to cover the trench.

-The team must cover up the dig site as quickly as possible.

-Take some small tools, but take the plastic also to protect the occupation layer, the bones.

So, the trench will be full of water.

If it rains a lot, the bones will completely disappear.

The rain, yes, the enemy of archaeology, in fact!

[ Thunder crashes ] -The rain may have caused a setback on site.

But at the vault in Savannakhet, there's been an exciting breakthrough.

While sifting through the 1,800 fragments, Donna's found one that appears to have something intriguing on it.

-Ooh!

Ooh, look, you've got an inscription!

-Oh, really, a character?

-Look.

Yeah, yeah, yeah, yeah!

Yeah, look!

-I didn't even see it.

Christine, you can come?

-Oh, yes!

-Inscriptions are a key source of historical evidence that can reveal the use, origin, or even the creator of an artifact.

-This one is in old Khmer.

It is quite rare to find an inscription like this, yes.

-It's a major find.

Only about 1,400 inscriptions from the 900-year period spanning the rise of Khmer civilization through to the eventual decline of Angkor have ever been discovered.

The team is eager to see if any other parts of the inscription can be found.

-Ah ha ha, haha haha, look, look, look!

-It seems Christophe has been lucky, but do the two parts of the inscription fit together?

-No, no, no, no.

-No, no, no.

-No.

On the other side.

-Ahhh.

Oui!

-Yes!

-Yes, yes, yes, that's it.

-Yes!

-Yes!

-Today we are really very lucky.

Two fragments back together, and we have some part of one fifth inscription.

-Very much incomplete.

-Very much incomplete, but anyway.

-Another inscription, this one in Sanskrit, can be seen on the rim of a small silver bowl.

-Interesting object.

-Inscriptions from the Khmer civilization were either written in old Khmer or in Sanskrit, a classical Hindu language often used for religious texts.

-If it's poetry, it's Sanskrit.

If it's just prose, it's Khmer.

-In addition to studying the inscriptions, the team is also analyzing the design on various items to help date them.

Christine has turned her attention to a silver plaque with a particular Hindu god represented in a unique way.

-We are sure that it is Shiva, because he holds in his hand the rosary.

Also, because he has a hair bun, and there is two worshippers.

This kind of plate, I can give a date.

It's coming from the 8th century.

So, this was easy, and it is Khmer art.

Totally sure, because it's the same iconography.

-Dating the plaque is a major breakthrough.

Christine is now confident that the artifacts in the collection were made from the 7th to the 9th century.

The era is known as the Early Kingdoms period, when Khmer village chiefs were beginning to claim the right to kingship and create independent states.

-There was a lot of small kingdoms and a lot of small city-states, which has a small territory all around the main city.

-Could the location where the treasure was found at Nong Hua Thong have been an early city-state, rich and powerful enough to have such exquisite objects?

The team has found additional evidence to support this idea.

In Nong Hua Thong, at the exact location where the treasure was discovered, the archaeological team has now found a large structure below the ground surface.

-This is the first state of a wall here.

-Layers of ash near its bottom provide evidence that people lived inside the wall's enclosure.

-It's very interesting to find occupation on one side.

-The discovery of this large wall further supports the team's suspicion that Nong Hua Thong wasn't always the small farming village that it is today.

♪♪ To better understand the layout of the site, the team is taking to the skies for an aerial survey.

They're using lidar, a game-changing technology.

Millions of laser beams are fired from above, and by measuring how long it takes the light to return, the team can map lost features in the landscape.

This is the first time this area has been analyzed using this method.

With the tree cover digitally removed, the ghostly outline of a forgotten world appears for the very first time.

This is where the treasure was found -- where the wall discovered by the archaeologists joins with another.

The two walls created a protective boundary more than a mile and a half long.

The team estimates that a site this large might once have housed nearly 8,000 people.

A lost city revealed for the very first time.

The lidar scans show the outline of a potential royal residence.

Its scale and shape suggest it could be the home of a local king or chief.

The team had no idea an ancient city like this existed in the area.

And they're keen to investigate further.

-Do you think that this city was the capital of the kingdom, a very rich city?

'Cause there is so many objects in the treasure.

Eight kilograms of gold and 18 kilograms of silver.

-Yes.

-So many objects, more than 200 objects.

-The team's hypothesis is that the valuable collection was stashed where the two walls meet because it was a memorable spot.

-You see, Viengkeo, the treasure was hidden in a special place, very easy to find again.

-Yeah.

-It has been hidden here to be protected, maybe.

-That's for sure, yeah.

-If the treasure was buried to protect it, it's possible the city was in danger.

During the Early Kingdoms period, city-states and local kings were often at war.

And here, high up on the bank above the river, Christine finds evidence that Nong Hua Thong may have been attacked.

-We found projectiles or cannonballs on the top of the cliff.

We see very well from here, it is a high level here, and they are heavy.

It's terra-cotta, so it's the right size.

So, it is very easy, you see, to use it.

-To throw against the enemy.

-To throw against the enemy the cannonball.

Maybe they used catapult to try to save the city.

And before the attack of the city, the king wanted to preserve the treasure, so they put all the gold and the silver and buried it in the earth to protect it and to save it.

But maybe they didn't win, because nobody came back to take the treasure again.

That's why we found it.

-It's only a theory, but even if it could be proven, a big question would remain -- Why would this small city have had such precious objects?

The archaeologists think it's more likely that such splendid items would have come from somewhere larger or wealthier.

And the riverbank now offers up a clue as to how that might have happened.

-Look, this one is clearly visible.

-Archaeologists have found what appear to be hand-chiseled post holes.

-They are quite 40 centimeters of diameter.

-Each hole would have been carved to fit wooden posts.

From the way they're made, Christine estimates they could date to the 7th century.

They stretch all the way to the water.

-I can see them doing a line like this.

-Judging by the number and size of the holes, it's possible that this could have been a large pier or wharf.

-And it's connected directly to the city... -Yeah.

-...and quite directly to where we found the treasure.

So, you see, Viengkeo, maybe the treasure was coming... ...with a boat and was hidden here, and maybe it's not the original place of the treasure.

Even if it is an ancient city.

-It's a hypothesis, but it leads the team to consider why the treasure might have ended up here.

The ancient city at Nong Hua Thong is situated in a strategic location.

Its position close to the Mekong River placed it on the main north-south trade route for the Khmer throughout history.

♪♪ -The Mekong River, it's very important to the life of the people.

It's considered the sacred river.

A lot of very important kingdoms, very important cities are located along this river.

Mekong River is the main road in the ancient times.

Of course, the kings at that time, they could be traveling by boat in between their palaces, their capitals, upstream to downstream.

-Inscriptions from the pre-Angkorian period reveal that some capital cities were richer and more powerful than others.

Could the treasure have been brought to Nong Hua Thong by a wealthy dignitary from another kingdom?

In the local language, Nong Hua Tong means the "Pond of the Golden Barge."

For the team, the wharf, in addition to the incredible diversity and artistry of the precious objects, suggests that the treasure might not have been made where it was found.

Their theory is that it could have come from a larger city-state with sufficient resources to support a highly skilled workshop.

Locating where that place might be is the next task.

♪♪ Back in Savannakhet, where the treasure is being studied, the experts are trying to work out where it might have come from.

By looking at the objects' possible uses, they're hoping to find some new leads.

The silver bowl appears to be a religious ceremonial object.

-We know that is a gift from a rich donator.

It was for the worshipping of Shiva.

-The inscription on the bowl shows it was donated by a very important person, potentially a king, to be used in a temple.

-It's very important for kings to give a lot of gifts to the gods.

-It was common practice for Khmer kings to build temples.

The connection to the gods sanctified the kings' right to rule and entrenched social hierarchies.

-The power of the king is more or less justified by the power of the gods.

-The more powerful the king, the larger the temple, the more exquisite artifacts it might contain.

The design and sheer value of the collection suggests it may have come from a major royal Khmer court.

And given the discoveries the team has already made, Christophe has a hunch where that might be.

-The number-one suspect to have this kind of object would be, actually, the temple of Vat Phou.

-Vat Phou is located 220 miles down the Mekong River from Nong Hua Thong.

It is the most venerated religious site in Laos.

♪♪ Every year, on the full moon of the third lunar month, thousands of worshippers flock to Vat Phou for a vast religious ceremony.

Over three days, processions of chanting monks pay homage to their gods at this holy site.

Although Buddhist today, the temple's roots are in Hinduism and stretch back nearly 1,500 years, long before Angkor reached its peak at the start of the 13th century.

Vat Phou temple sits at the foot of Mt.

Phu Kao.

This distinctively shaped sacred mountain was believed by the early Khmer people to be the physical embodiment of the Hindu god Shiva.

Vat Phou was built in this sacred landscape to represent a virtual heaven on earth.

Most of the surviving structures were built from the 11th to the 13th centuries.

They cover nearly a mile, rising to a natural terrace.

The holiest temple building is situated here, overlooking the Mekong.

For the research team at the vault, the sacred site of Vat Phou has all the hallmarks of a place that might have been able to afford the exquisite artifacts found 220 miles away at Nong Hua Thong.

Christine also suspects that there might be similarities between the iconography on the treasure and the designs on the temple itself, and she's heading there to find out.

-So, it could be a connection for some object between the treasure on Nong Hua Thong and Vat Phou.

-Matching iconography on the treasure to carvings at Vat Phou could help link the two.

Christine compares photos of the treasure with the carved lintels above the doorways in the main temple.

-On several objects of the treasure there is a face, which is very recognizable.

This is the head of Kala.

-According to Hindu mythology, Kala, the god of death, asked Shiva if he could eat people.

Shiva refused and, as a penance, made Kala eat himself.

-Yeah, you can see the teeth and the nose.

-This is not the only artifact that shows a visual connection with the carvings here.

The silver plaque is almost identical to lintel carvings depicting life around Vat Phou.

-In the middle, you have Shiva sitting and hanging the rosary, with the hair bun, between two worshippers.

Shiva is the god for the hermits.

And we know that there was a lot of hermits living all around in the mountains, because it was the sacred mountain, the mountain of Shiva.

-But while the iconography on the treasure and the temple match, the timeline doesn't.

The main temple here in Vat Phou was built long after the treasure was produced.

-The plate of Nong Hua Thong is more ancient than the lintel of Vat Phou, but it is exactly the same iconography.

-To support the theory that some of the artifacts in the collection may have originated in Vat Phou, the team now needs to establish if there was an earlier temple here, at the time the treasure was made.

Lao architect Vissa Chanthaphasouk is an expert on the earliest history of Vat Phou.

Her goal is to find out when different parts of the temple site were built.

-Part of my work consists of doing a survey with the new technology.

-She's using a technique called photogrammetry, where thousands of photographs are digitally amassed into a 3-D image.

-I will start from here.

-Okay.

-Surveying the site is a major task, best performed from the air.

-Okay, let's go.

-The first thing that we see is that Vat Phou is not alone.

So, a lot more remains of Khmer structure all around the sacred mountain.

-The aerial survey allows Vissa to identify ancient ruins that were once part of the landscape.

Photogrammetry will then enable her to reconstruct and possibly date some of these sites.

Today, Christophe is joining her to investigate a collapsed temple within the main temple complex.

-Uh, is that supposed to be the door or what?

-Yeah, we think that it's a door.

We did photogrammetry of this part, and I tried to do reconstruction.

-Okay, fine.

Let's look at your computer, then.

-Yeah.

So, this is the photogrammetry of the collapsed building we are sitting on and the whole wall slab, so we got the height of it.

-Vissa has taken the dimensions from the photogrammetry and a drawing made by an earlier architect to reconstruct the temple for the first time.

-This is the reconstruction of the buildings that we are sitting on.

-That's really, uh, brutal architecture.

-The lack of ornamentation suggests the temple could be from the pre-Angkorian era.

-In terms of dating for this building... -Yes, it's clearly the oldest building remaining... -In -- -...just -- yeah, in -- in Vat Phou.

-Christophe believes this unadorned temple could date back to the 7th century, the same time period as some of the earliest artifacts in the treasure.

-It's clear, then.

The shrine in Vat Phou could have had this kind of object.

-The team now suspects the cache of artifacts might have belonged to a king, who may have donated it to an early temple at Vat Phou.

They also have a hunch there may have been a local community of metalworkers making religious artifacts at Vat Phou.

Bronze, made from tin and copper, was used in the 7th century to fabricate ceremonial artifacts, like the cymbal found in the collection.

Historical reports suggest the early Khmer people were mining copper in the Vat Phou area.

To find out where, an archaeological team led by metal expert Brice Vincent is going in search of a huge sandstone cliff, where a copper mine is rumored to be.

-You see there is a cliff at 400 meters.

-And to climb up there, it's how many hours?

-Now it could be, maybe, two hours and a half.

-Only four other copper mines have been found in the whole of Southeast Asia, so this is an important expedition.

-So, great day.

Great day.

-The team will be heading up the mountain through jungle terrain.

To maximize their chance of success, they plot their route using a lidar map.

-We will follow this way, and after that we will follow the river, okay?

-So, two hours climbing to find copper mineralization.

It should be -- should be fine.

Okay, let's go!

-If the team finds evidence of mining, it would support the idea that the Vat Phou area was a center of metalworking, the kind of place where the artifacts might have been made.

-Here we are.

-Yeah, so we follow the riverbed.

-Maybe two kilometer more by walk.

-The archaeologists are following in the footsteps of a French geologist who claimed to have found evidence of mining here in the 19th century.

Brice has an old map and a drawing from this long-ago expedition.

-This was, yeah, the drawing made by Delaporte in 1866.

And, uh, so I don't know if it was his imagination, but it looks like what we have around here.

♪♪ -The team may be treading a path used by early Khmer miners who went in search of the precious metals used in the treasure.

-Copper was precious, rare, and so in ancient time, miners or kingdoms were looking for copper.

-After three hours of hard climbing in the extreme heat, the group catches a glimpse of the huge cliff where they think the mine might be located.

-Okay, so here we are.

Almost!

♪♪ When we arrive on the site, we discover this monumental cliff.

-Whoa!

-The cliff is... -It's amazing.

-Very impressive, actually.

-Having located the cliff, the team forges on.

They must find the mine and return before dark.

-Over there, so on the top, you have a copper vein.

You can see the copper.

Of course, the access is difficult.

-More exploration reveals exactly what the team is looking for -- signs of ancient copper mining.

-Here you can see traces of tools -- tool marks.

So, here it's very obvious, you can see the copper ore.

So, they really try to access.

-In addition to the tool marks and copper, Brice has also discovered a man-made cave.

-This is quite -- quite amazing.

It's a spot of what we call fire-setting, and this is unique in Southeast Asia.

-Fire-setting is a traditional mining technique.

The rock is heated.

Then cold water is poured on it, causing it to crack and expose the minerals inside.

-So, it's another evidence of copper mining activities.

-Brice is keen to take samples.

Chemical analysis will establish whether or not they contain copper ore. -So, we tried to take a sample here, but it's kind of hard to access.

The rock here is quite hard, so... -We guess this was an ancient copper mine from the pre-Angkor period.

From the time of Vat Phou, this resource was known and exploited here.

-The mine confirms the archaeologists' suspicion that the local population would have had access to some of the metals found in the treasure.

A place with these resources would be well-placed to serve the needs of a wealthy elite.

An incredible carving in the Vat Phou museum reveals the presence of kings here, stretching back centuries.

The Devanika stele was carved in 450 AD, around 300 years before the treasure was made.

It's the oldest-known Sanskrit inscription in Southeast Asia.

-Devanika was the king of kings, coming from far away.

That, he is saying, "I am Maharajadhiraja."

And he say that the mountain of Shiva was known since the antiquity.

He say, "I come here because of the mountain of Shiva."

So, he wanted to be as close as possible to the divinity, to the god.

The Sanskrit is very, very nice.

It's a beautiful poem.

It's very well written.

-From this foundation stone, the team knows that King Devanika started to build his royal city in the 5th century.

It was located here, on the banks of the Mekong River, just 3 1/2 miles from the temple site of Vat Phou.

Today, there's nothing to indicate that this was once a royal Khmer enclave.

To understand the layout and size of this first royal city, the team has carried out a lidar survey.

It's the first time this area has been mapped using this method.

The lidar scan reveals King Devanika's royal city in extraordinary detail.

A vast 2 1/2-mile moat surrounds and protects the city.

This rectangular enclosure was the first of its kind in Southeast Asia.

It's a shape that would later be echoed in Angkor.

A walled inner sanctum may have been where the elite lived.

Adding houses to the lidar map creates an image of how full of life this Khmer city might once have been.

King Devanika called his divine city Kurukshetra.

Although it was founded in the 5th century, inscriptions show that this first royal city was still thriving at the time the treasure was made, between the 7th and 9th centuries.

Could the treasure have been crafted by artisans living locally, made for a king to use at the nearby Vat Phou temple?

♪♪ -Would that be fabricated by any workshop here in the city in Vat Phou?

Yeah, it's highly probable, because actually they had the customer next door, actually, the priests and the kings.

-In Kurukshetra, Christine and Viengkeo have found more evidence to support the theory that some of the treasure could have been produced in this area.

An unusual shape on the lidar map has piqued their curiosity.

-And this one is very interesting, because it is the only one that's circular.

And with the moat all around.

Mm-hmm, this is very interesting.

-The team believes this temple might have been one of the most important within the city, so holy it could only be approached by boat.

-It's surrounded by the moat.

-Yes.

-So, no one can access this monument.

-The shape is significant.

It suggests that this was not a Hindu temple.

-This round shape, we think that could be a double stupa, so a Buddhist structure.

There was not only Hindu temple and also Vishnu temple, but also Buddhism inside the ancient city.

-The suggestion that Buddhists and Hindus lived harmoniously here in the pre-Angkorian period is in keeping with the design of the gold box, which reflects both religions.

♪♪ The investigation into the treasure has given the team a greater understanding of the early Khmer society... ♪♪ ...a society that would eventually go on to create the wonders of Angkor.

♪♪ And it all began here in the sacred landscape of Vat Phou.

-We have been working for years in Angkor trying to understand the origin, and Vat Phou is an amazing case for that.

-Vat Phou is the beginning of the Khmer civilization.

♪♪ -In Savannakhet, it's coming to the end of the last day in the vault.

The treasure must be carefully packed away.

The team will return again in 10 months to continue their investigation.

-Can you turn the lid around?

-Each object has to be wrapped in special acid-free paper to preserve it.

-There is definitely an art to packing.

Our whole goal, as always, is to preserve it for generations to come.

Ahh, beautiful.

Should be in perfect condition by the time -- when they open it up again.

-It's not just the artifacts that have to be packed up.

The whole laboratory must be cleared out.

-There is only 40 minutes, and it will be very short to finish everything.

-As the room is emptied, the team has time to reflect on their work.

-It's weird.

Strange, strange feeling.

-We -- Really, we do a lot of work in only 10 days, and we can be happy.

-They have learned a great deal.

The team's research into the treasure has taken them from a farmer's field... to Vat Phou, the spiritual heartland of the first Khmer kings.

The kings ruled their domain from Kurukshetra, the administrative capital, at a turning point in Khmer history -- when a collection of small city-states merged into a single, more powerful kingdom that started an empire.

The treasure reflects this exact period in history... revealing the power, wealth, and ambition of the Khmer people... who would go on to build one of the greatest cities on earth -- Angkor.

♪♪