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00:00Angkor Wat, the largest temple on the planet, built by the ancient Khmer civilization who
00:15ruled the tropical forests of Southeast Asia a thousand years ago. It's a masterpiece
00:21surrounded by the ruins of a huge metropolis hidden in the jungle. It's one of the most
00:27remarkable architectural and engineering achievements in all of human history.
00:31Now a team of intrepid archaeologists is using pioneering technology to reveal this
00:38ancient temple city is the core of something even bigger. Concealed for
00:44centuries beneath the rainforest canopy, they are exposing a lost world almost
00:50twice the size of modern-day France. This is one of the most stunning monuments in
00:55the ancient world. How did the Khmer keep a tight grip on their vast
01:00jungle empire? Why today does it lie in ruins reclaimed by nature? This is where it
01:07gets risky. To hunt for answers, our cameras have exclusive access to follow
01:14investigators as they use drones and helicopters to scan the jungle with
01:20lasers and embark on ground-breaking expeditions to reveal the astonishing
01:27world of the Khmer civilization. This is incredible. It's huge.
01:31And to expose how the ancient Khmer rise to rule the jungle, we digitally rebuild their gigantic temples,
01:44forms. We decode their engineering secrets and reconstruct their ancient society to reveal the true scale of this lost empire.
01:56Angkor, Cambodia. Once home to the mighty Khmer civilization who ruled these jungles for 500 years.
02:17Today, Damien Evans from the French School of Asian Studies in Paris and Sarah Klassen from Leiden University in the Netherlands are on a mission to rediscover this lost world.
02:31In our picture of the Khmer Empire, we have a lot of blank spaces on the map.
02:34So far, they've found traces of Khmer cities not just in the rainforests of Cambodia, but hundreds of miles away in what is now Laos, Thailand and Vietnam.
02:47It's a 400,000 square mile territory connected by ancient highways and roadside temple communities.
02:55Now they turn their attention to the origin of the materials that built the empire's spectacular stone monuments.
03:04At the Khmer civilization's capital, Angkor, the empire's most iconic temple soars above the tree line.
03:12This is Angkor Wat. Here, damaged corners expose the work of ancient architects and lays bare a confusing mystery.
03:23Angkor Wat is a masonry marvel. The platforms it rests on are made of laterite, a rough stone with an open honeycomb structure that makes it both light and strong.
03:38The superstructure on top is made of smooth sandstone blocks, which could be carved into intricate shapes and fitted with millimetre precision.
03:50Some blocks weigh over 1,500 kilograms and the Khmer stacked them up to 60 metres high to make the central lotus tower.
04:00Overall, there are nearly 10 million stone blocks in this temple alone, with no quarries anywhere near Angkor.
04:09Where did the Khmer find this much sandstone?
04:12The sandstone used to build Angkor Wat is believed to be the same rock used in other temples across the region.
04:24It hints at a single mega quarry that provided stone for hundreds of Khmer constructions.
04:31Clues to the quarry's location could lie in a mountain range 80 kilometres from Angkor.
04:38Archaeologist Damian Evans has spent over a decade scanning the jungles of Southeast Asia.
04:45He uses drone and helicopter mounted lasers known as Lidar to hunt for traces of the Khmer civilisation hidden beneath the trees.
04:55Damian investigates the area that surrounds a temple called Beng Melia.
05:00His Lidar scans here reveal a sandstone rich landscape scarred with pits and strange markings.
05:08Damian believes that these could be the remains of a vast mountain quarry.
05:13To examine the site from the ground, he takes a dirt road deep into the jungle with Chia Sochiet from the Apsara Archaeological Authority.
05:23The road is terrible. Absolutely shocking.
05:26Sochiet has studied Beng Melia for decades and knows the region better than anyone.
05:35But a recent monsoon deluge has made this remote site a serious challenge to reach.
05:43I see a lot of farmers around here so we have help if we get stuck.
05:51I'd say it's about 50-50 at this point.
05:54I'm going to go check that out.
05:56The road is too muddy for Damian's Jeep.
05:59So the team need to find another way to get to the suspected Khmer mega quarry.
06:03This stuff here is lethal. It's like clay. You sort of get in there and that's the end of you.
06:09It gets a bit risky up ahead. I don't know about this.
06:14Damian and Sochiet enlist the help of a local farmer and his tractor.
06:18To power their way across the rutted terrain.
06:31Led by the LiDAR map, they leave the road and make their way through the jungle on foot.
06:37Half a mile from the road, the terrain falls away dramatically.
06:41They discover a rugged rock face.
06:50It's not natural, this place.
06:52You have this sort of rolling gentle slope of this rocky outcrop and then all of a sudden it falls away.
07:00Just need to be careful. It's kind of snake country.
07:03As they clear away the vegetation, evidence of the quarry emerges.
07:10It's huge. It's very big.
07:12It's well hidden, actually.
07:15The mountain looks like a regular cliff face, but an expert eye can identify traces of stonework.
07:23Here, you see the wall of the quarry. It's very big and very high.
07:27It's very easily identifiable by the shape of the stone, so these kind of right angles and kind of things here don't really appear in natural formations.
07:38And also, if you look closely, you can very clearly see these chisel marks as well.
07:43The markings carved into the stone reveal the Khmer mined this wall for sandstones centuries ago.
07:49This rock face is vast, but it's not large enough to provide the Khmer with enough sandstone to build hundreds of temples.
08:02The dense foliage makes it hard to see the scale of the quarry site.
08:08So Damien examines his lidar data, which strips the jungle bare.
08:14This quarry here is just the tip of the iceberg.
08:16Looking at the lidar data, you can very clearly see that this isn't a collection of small, independent quarry sites.
08:23It's actually one gigantic quarry field.
08:25It's had entire hillsides torn out in order to produce these blocks that are needed for building the temples at Angkor.
08:32This sprawling canyon is one of the largest quarries ever discovered in the Khmer Empire.
08:38But to prove the Khmer used this quarry to build their temples, Sir Chiat finds a piece of sandstone.
08:46He wants to compare it with the blocks that make up the nearby Bengnelia temple.
08:56It's impressive. Extremely impressive.
08:58Sir Chiat and Damien hunt for a piece of clean stone to make the comparison with the piece from the quarry.
09:05Most of the blocks have been discolored by wind and rain over the centuries.
09:10But as they clamber through the ruins, Sir Chiat finds a sandstone pillar underneath a lintel, protected from weathering.
09:16If we look at the sandstone column here, and we compare it to the piece of sandstone that we collect from the quarry,
09:25it looks almost the same if we compare the chimichore composition and also the color.
09:31It means that it comes from the same quarry.
09:35Bengnelia was built using the stone cut from the massive local quarry site.
09:41Though the temple's entrance tower has collapsed,
09:44the ruins reveal the durability of the stone.
09:48It's interesting how the stone is such high quality that even when it falls from that height,
09:51it doesn't smash apart or anything. The blocks are still intact.
09:55The Khmer builders made use of this superior sandstone to build hundreds of temples across the region.
10:02But some of the stones are huge, weighing up to 1,500 kilograms.
10:07How did the Khmer engineers haul these massive pieces of stone to build Angkor Wat over 80 kilometers away?
10:18To transport stones from the mountain quarry to the city via a natural river is a long journey.
10:25And boats must fight against the strong current.
10:29LIDAR maps reveal the Khmer dug clever canals and skillfully linked them to natural rivers.
10:36This heavily engineered transport network shows how much effort the Khmer put into getting the stone where they needed it.
10:44Even today, the Khmer mastery of water and stone is breathtaking.
10:5940 miles out of Angkor, Damien and Sarah examine a monumental sandstone bridge along the main artery heading out from the city.
11:08This is a busy and bustling bridge right now, but it's actually also an ancient bridge.
11:13Modern traffic crosses this river using a structure built by the Khmer engineers around the same time as Angkor Wat was constructed.
11:23Centuries ago, these 54 stone arches carried processions of merchants, oxen and elephants.
11:33Such an impressive piece of engineering.
11:35The fact that it's still functioning as a road supporting a highway probably 800 years or so after it was built is really impressive.
11:43Thousands of stone blocks were used to build this massive bridge.
11:48It stands as lasting proof of how the Khmer used their unrivaled stone working industry to expand their empire.
11:57Previously, what we see in the Khmer empire is that stone masonry constructions were reserved for the gods.
12:03What we see, however, in the 12th, 13th century is all of a sudden stone being used for everyday construction for things like bridges, dams, canals.
12:12This is a really interesting shift.
12:13It shows that they were devoting more and more resources to try and consolidate power over ever farther regions away from the centre at Angkor.
12:20In the 12th century, a wave of roadway expansion saw scores of bridges constructed along ancient highways leading to Angkor.
12:30Back in the capital, the epic engineering projects continued.
12:35The deaths continued.
12:36Sarah and Damian's laser surveys expose how the Khmer landscaped the entire city to sustain a growing population.
12:50Scanning Angkor with LiDAR revealed stone construction was just the tip of the iceberg.
12:56Ancient engineers carved canals across the city and dug thousands of ponds into the landscape that filled up during monsoons and provided the Khmer with drinking water.
13:13It's a water management system on a vast scale that could hold billions of litres of rainwater.
13:24Thanks to their incredible urban engineering, archaeologists believe the Khmer population skyrocketed to 900,000 people by the 12th century.
13:34But this population boom created new challenges for the empire's god kings.
13:41They needed to sustain vast jungle cities in a challenging monsoon climate.
13:53For six months of the year, the rainy season strikes with devastating floods.
13:59Before the dry season brings crippling drought.
14:02To maintain the city's water-based lifestyle all year round, the Khmer must have found a way to keep their ponds filled to the brim through the dry season.
14:15Huge reservoirs on the edge of Angkor appear to provide the solution.
14:21But there are no canals linking these reservoirs to the city.
14:25So how did they work?
14:27Vegetation's overgrown right now.
14:29Yeah.
14:30End of the wet season.
14:33Sarah investigates with David Brotherson from the University of Sydney.
14:38He's an expert on the Khmer's water systems.
14:42A short jungle trek brings them to the shores of the largest reservoir at Angkor.
14:48It's known as the West Barai.
14:50It's huge. It's so enormous. It's hard to imagine it's actually man-made.
14:58The reservoir's immense scale is difficult to examine from ground level, so Sarah and David launch a drone.
15:05We're at the northwest corner right now and you can see from this angle just how massive this reservoir is.
15:20It's over five miles long.
15:21Built 1,000 years ago, the rectangular-shaped West Barai is still the largest hand-carved reservoir in the world.
15:31At maximum capacity, it can hold up to 53 billion litres of monsoon rain.
15:36The Barai's sheer size and proximity to Angkor is the secret to how it kept the land wet without using canals.
15:46The huge volume of water slowly seeped through the reservoir's earthen banks into the ground.
15:53This filled village ponds and paddy fields all year round.
15:58A massive source of water like this would have been so important to sustain the population of Angkor.
16:06Similar enormous reservoirs have been found across the empire.
16:11As the monsoon rains fell, each reservoir collected enough water to keep the surrounding landscape moist.
16:17The engineered environment really allowed the rice farming agricultural system to flourish and with that reliable crop surplus year in year out,
16:28that is what basically was the foundation for this incredible capital here at Angkor.
16:34The Khmer tamed the extreme tropical climate to store water through the dry season.
16:40They kept their ponds and canals in the city full and nourished thousands of acres of farmland.
16:47The ancient Khmer were masters of holding onto water in the dry season.
16:52But how did they survive when the monsoon turned their roads into rivers?
16:56Khmer settlements at the edge of Angkor faced unique challenges from extreme weather due to their location.
17:18Even today, the monsoon breaks the banks of a massive lake south of Angkor called the Tonle Sap,
17:24completely flooding the communities that live around it.
17:29By studying the population that resides here, Sarah can investigate how the Khmer overcame the same problems a thousand years ago.
17:38She takes a boat heading south from Angkor in search of a remarkable village.
17:45This is Kompong Pluk.
17:48We're here on the edge of the Tonle Sap, which is one of the largest freshwater lakes in Southeast Asia.
17:55During the wet season, it actually doubles to twice the size.
17:59As a result, the local communities have adapted to this very unique environment.
18:06At first glance, the village appears to be floating, but it's rooted to the ground.
18:11In the dry season, the waters evaporate to reveal its engineering secrets.
18:19The houses are built on incredibly long stilts.
18:25The stilts run into holes that are driven deep into the mud.
18:29This provides a stable foundation for the homes above.
18:40Still going.
18:42Wow.
18:44This is pretty amazing.
18:46All right.
18:50It's about 20 feet.
18:51You can see that the stilts are still pretty high above the water, so it must look amazing during the dry season.
19:04The clearance from the water's surface means that all the buildings remain safe even during the most extreme monsoon flood.
19:11They've completely mastered this challenging environment, and water isn't viewed as this thing to be protected and guarded against, but you can see that they really live with it and among the water.
19:30The engineering skills that we see here are probably pretty similar to what was practiced and developed during the ancient and quarian period,
19:36where we know a lot of these ancient houses were also built on stilts.
19:43The Khmer used large-scale engineering to turn their climate to their advantage and support vast urban populations in a challenging environment.
19:53But as their cities grew, their wealth made them a target for rival states.
20:08To keep authority, kings had to quash rebellions at the edges of their empire.
20:13In the Middle Ages, the key to military supremacy anywhere in the world lay in one vital resource.
20:24Iron, to make sharp, bladed weapons.
20:27Iron must have been immensely important during the Angkor period.
20:30But archaeologists investigating Angkor Wat find very little evidence of ironworks, and very little evidence of iron itself, until they notice something strange about the walls of the main temple.
20:48In the walls of Angkor Wat, there are deep fissures between the stone blocks.
20:52They are the work of looters, who dug into the ramparts to look for metal.
20:58On the blocks are cavities that once held iron clamps to bind the stones tightly together.
21:08The evidence reveals the Khmer had enough iron to construct Angkor Wat.
21:12Did they use this invaluable metal to build more than just their flagship temple?
21:26Damien and Sarah load up a 4x4 and venture deep into the jungle, 200 kilometers east of the city, to investigate a temple that holds a long association with metalworks.
21:40Here, they hope to find rare evidence for the scale of iron production.
21:46Things made of metal tend to be recycled and reused and melted down and turned into something else, rather than just kind of discarded into the ground and left for us to find centuries later.
22:00They forge their way across ancient roads and dirt tracks to reach their destination.
22:05This is Praia Khan Kampong Suai.
22:10The name translates to Temple of the Sacred Sword.
22:15They must stick to well-trodden paths.
22:18In the 1970s, the Khmer Rouge regime controlled this region and hid landmines in the ground.
22:26This area was mined extensively during the Civil War and a lot of those landmines remained on the landscape.
22:41The Khmer Rouge demolished the ancient structures before they left.
22:45But there's at least one benefit to the temple's state of disrepair.
22:59This piece of metal sticking out of the stone might look like it's relatively recent, but it's actually part of the ancient structure.
23:05The broken walls reveal the stonework here is filled with iron crampons.
23:12It's very rare to find these iron crampons in situ like this because the metal was so valuable that they've been stolen and taken from these locations over time.
23:23The fact looters have neglected these crampons could suggest iron might be common in this area.
23:29We know that there's a source of iron around here, so this crampon probably came from not too far away.
23:40If iron is abundant here, the Khmer could have used it to make weapons too.
23:45Sarah and Damien examine the LIDAR map for clues.
23:50They're looking for evidence of the slag heaps that widespread iron manufacture would have left behind.
23:56The site they're sitting in stretches over five square kilometres and is the largest complex the Khmer ever built.
24:05One of the most remarkable things about this place is the scale of what was achieved here.
24:11It's amazing how many features are scattered across the landscape here, but actually in a very organised manner.
24:17As they examine the data more closely, the same repeated shapes jump out.
24:22Small mounds on the edges of reservoir banks.
24:27So if you zoom in here, for instance on the edge of this reservoir here, you have this kind of huge round thing embedded in the wall.
24:33So those are the remains of some kind of industrial activity.
24:37Field reports conducted in 2018 suggest these mounds could be iron smelting sites.
24:43Clues on the ground indicate the Khmer produced iron in this ancient city.
24:51But these smelting sites are too small to account for the iron needs of a great army.
24:59If the iron to power the military comes from this area, the Khmer must have sourced it outside the walls of this distant temple.
25:06The team need to examine the wider region for clues.
25:11As the warm jungle evening draws in, they down tools for the day.
25:17But there are no hotels in this remote outpost.
25:21Luckily the village chief invited us to spend the night at his house.
25:25So they've been kind enough to cook up some dinner for us and I'm pretty excited because it smells very good.
25:30The next morning they travel to the foothills of a nearby mountain where archaeologists have found some unusual looking mounds.
25:48This is Phnom Dyke. The name translates to Iron Mountain.
25:53Guided by the LIDAR map, they proceed on foot away from the road.
26:00Half a mile across grassy fields, they investigate what looks like a huge exposed pile of grey coloured stones.
26:08Oh wow, look at this.
26:10It's not even a hill. It looks like a hill, but it's just a giant mound of industrial waste basically.
26:15Damien is convinced these are the remains of a massive iron working operation.
26:24Looking closer, he discovers the proof he needs.
26:29On the ground he finds a ceramic pipe.
26:33It's known as a touillère and it's an integral part of a blast furnace used to smelt iron.
26:39Given the size of it, this is about the biggest touillère I've ever seen.
26:41We can imagine that the scale of what took place here must have been really very impressive.
26:48The metalworking sites at Phnom Dyke are so big they could have supplied the Khmer with enough iron for their vast army.
26:56But historical records suggest it's not the Khmer who held the knowledge of how to extract the iron from the ore.
27:03Sarah meets members of a local tribe known as the Khmer.
27:10The Khmer are an Aboriginal tribe whose ancestors inhabited the region many centuries before the Khmer.
27:18Today, these tribal leaders hold some of the last knowledge of how their ancestors smelted Khmer iron over a thousand years ago.
27:27Iron is so crucial to the Khoi, they pick a villager to devote his life to it and become an iron shaman.
27:41While the smelters feed a furnace with bellows, heating it to 1,000 degrees, the shaman performs magic and praise to the gods.
27:53So that when they break open the furnace after 15 hours, the gods release the precious iron from the ore.
28:00The Khoi used witchcraft to produce iron on a massive scale to supply the Khmer in Angkor.
28:11The vast supply from the Iron Mountain gave the Khmer soldiers all the metal they needed to win great wars.
28:19But how did the Khmer warriors defeat rising numbers of enemies?
28:23Ancient scriptures suggest that by the dawn of the 13th century, the Khmer empire feared attack like never before.
28:43Extraordinary evidence for this new found threat comes from a temple on the empire's far western border with neighbors, the Ayutthaya.
28:58It's early morning and Sarah and Damien are headed 160 kilometers from Angkor to a remote temple in the far northwest of the country.
29:16After a long journey, they arrive at the overgrown site.
29:22This is Bhantieh Chhmer.
29:27Every time I come here, it's just amazing.
29:32Faces carved into towers across the site reveal the first clue this temple was built in response to a growing threat.
29:41These face towers are exactly the same as some that we see at Angkor.
29:45So that means that this temple must have been built by Jayavarman VII.
29:48King Jayavarman VII is the Khmer's most powerful monarch.
29:55During his reign, he rapidly expanded the Khmer empire, but faced conflict from his fierce rivals to the northwest, the Ayutthaya.
30:04Sarah explores deeper into the central tower enclosure.
30:11The stonework here could provide a glimpse into the mindset of the great king at the time he builds this temple.
30:18Parts of this temple are completely in ruins, but this probably didn't happen in one catastrophic event.
30:26Probably what we're seeing here is the result of a rushed construction.
30:30And we think that's because it was built in a hurry to establish this spot on the edges of Jayavarman VII's empire.
30:36On the temple's exterior walls, violent war imagery dominates huge, intricately carved scenes.
30:45Here we have another massive military relief. It's taking up this entire wall. It's crazy.
30:52Over here, let's take a closer look.
30:54This part of the relief is actually depicting part of a civil war or the ending of the civil war.
31:01And you can see that the rebel heads are being presented to the king.
31:05Almost every exterior wall of this complex seems to be covered with military imagery and stories depicting military battles.
31:14Damien examines the carvings by the main gate of the temple.
31:22The size and number of military wall reliefs is far greater here than any other temple in the Khmer Empire.
31:34It's an incredible artwork on this wall, but it also serves a political purpose.
31:39It's designed to remind people of what happens when they cross the king.
31:41designed to remind people of the power and the might of the king Jayavarman VII
31:46and of his ability to project power even over very far and remote places like we are here at Bhante Chema.
31:53Damien believes that this temple could serve a military purpose.
31:58They study a LIDAR scan of the landscape outside the temple walls.
32:03We don't see any of the city blocks with house mounds and house ponds.
32:06So that suggests this is maybe a slightly different type of occupation, maybe more temporary.
32:12What becomes very clear from the LIDAR data is something quite striking actually,
32:17is that this place is completely different from all of the other ancient Khmer cities that we know.
32:22The signs point to Bhante Chema being a military site, erected in a hurry to deter and defend threats from the Ayutthaya.
32:30To my mind that all kind of adds up for evidence for this temple as being a garrison temple,
32:37as a strategic location where perhaps a force was based in order to guard the western flank of the empire during this period.
32:45Garrison temples like this, filled with soldiers, helped establish the Khmer's fearsome military presence.
32:52The Khmer infantry carried tough shields and were armed with deadly weaponry like the mighty Pekir.
33:03They attacked with battle-hardened elephants who carried archers on their backs.
33:08Their artillery used lethal wheeled crossbows that decimated enemy troops.
33:18And they even fought battles on water, their warships filled with soldiers armed to the teeth.
33:25Sarah wants to investigate how much of a threat the elite Khmer soldiers pose to the Ayutthaya forces.
33:39She meets with a pair of blacksmiths who specialise in forging the Pekir.
33:45While they're using modern equipment, the scene is probably not too different from what it might have looked like over a thousand years ago,
33:51when the ancient Khmer were making weapons out of iron.
33:59One blacksmith uses a bellow to pump air into the furnace to keep the fire raging.
34:06While the other manipulates the lump of iron as it roasts to over 1,000 degrees Celsius.
34:16It's been about 30 minutes and he's almost finished shaping the iron.
34:19Once shaped, they plunge the blade into water to harden the metal before sharpening it to a fine point.
34:33Finally, Sarah gets a feel for the finished blade in her hands.
34:38It's been less than an hour that it took them to make this blade and it's incredible.
34:42It's really heavy and it's really sharp.
34:45If the threat of war arrived, the Khmer could have assembled a vast production line of blacksmiths to fashion weapons for the entire army.
34:57With the access to iron and all of these workshops creating blades, you can just imagine how formidable the Khmer Empire would have been.
35:03Thanks to their access to iron and rapid weapon making expertise, the Khmer could react to surprise attacks extremely quickly.
35:15For 500 years, the Khmer were a military superpower without rival in Southeast Asia.
35:25But in the 15th century, archeological evidence suggests the Khmer's fortunes changed dramatically.
35:31This mighty civilization crumbled.
35:33Why did the Khmer empire fall?
35:36Damian believes that as the Khmer's wealth and control over resources grew, they became the envy of rival states.
35:49As it solidified its control over this very diverse and far-flung collections of peoples in this area, it encountered new threats and was challenged from the outside.
35:59Without written evidence from this period, discovering exactly what happened is a serious challenge.
36:12But experts have found remarkable evidence that the kings of Angkor feared an attack at the spiritual heart of their empire, the great Angkor Wat.
36:29David Brotherson has discovered mysterious holes in the boundary walls of the temple that he believes could reveal the Khmer were preparing for battle.
36:40There's thousands and thousands of small ones, about 2 or 3 inches in diameter.
36:46And then there are much larger ones, 6 to 12 inches in diameter.
36:50David measures the distance between the holes to try and understand why the Khmer drilled them into the walls.
36:57The first one's about 8 feet.
37:03By measuring their spacing, we discovered there was actually a very consistent pattern all the way around the enclosure.
37:10David thinks the holes held long wooden beams that formed a giant wooden defensive platform on top of the walls.
37:20To protect the temple of Angkor Wat, the Khmer had to deface it.
37:24They drilled reams of holes into the top of the walls, into which they inserted wooden poles to build tall palisades that formed a shield against enemy missiles.
37:37Between the gaps in the palisades, they carved holes into the inside of the wall.
37:42These most likely helped to support a wooden scaffolding from which Khmer soldiers could discharge arrows to decimate attacking troops.
37:52When Angkor Wat was built in the first half of the 12th century, obviously defence wasn't at the top of their list of priorities.
38:02In the following centuries, something changed.
38:04They came back to places like this and they scrambled to shore up their defences and to install these wooden fortifications in order to better protect these areas from invading forces.
38:12The changes made to the architecture reveal how Angkor prepared for a major invasion.
38:2415th century statues and carvings suggest an Ayutthaya attack on the city does finally arrive.
38:30Thanks to this improvised wooden defence, Angkor Wat survived destruction.
38:40But much of the city that surrounds it was demolished.
38:43The Khmer Empire was on its knees.
38:47Enemy forces pushed down through modern-day Vietnam.
38:51And in Khmer cities, tension erupted between Buddhist and Hindu followers.
38:55But there are signs at Angkor that the final blow to the city didn't come during wartime.
39:02Sarah believes the final cause of Angkor's demise was a dramatic change in weather that overwhelmed the city's complex water systems.
39:13When the water management system isn't functioning in the way that it was engineered, it becomes increasingly difficult to sustain large populations here.
39:20Sarah and David explore what remains of Angkor's hydroengineering.
39:29Six kilometres north of Angkor Wat, they examine an ancient Khmer bridge.
39:37You can see just how wide this bridge was.
39:41But it's largely in ruins now and there's no water flowing underneath it anymore.
39:45Archaeologists believe that this destroyed bridge is evidence of a huge climate shift.
39:52We know there were massive monsoon storms in the 14th and 15th century.
39:56And the water from those floods may have washed this bridge away.
39:59The climate shift didn't only wreak havoc on the city's fragile water management.
40:04It also affected the river system that leads to Angkor.
40:07When the water flows, you know, outside of its original course, you can just imagine that there must have been a lot of destruction in the urban area, potentially a lot of flooding related to this one event that happened at the bridge here.
40:19The water management infrastructure the Khmer spent centuries building began to fail.
40:29Huge boulders blocked the canals and with an empire severely weakened by conflict, repairs may have been impossible.
40:37Evidence like this bridge suggests that Angkor as a city is beginning to crumble.
40:41During the periods of extreme drought, people may not have been able to grow as much rice and there may have been starvation.
40:48Widespread invasions combined with extreme weather to force the Khmer people from their Angkor homeland.
40:54But where did they go?
40:57190 kilometers south of Angkor, in the small city of Udong, soaring memorial towers built from the 17th century mark the graves of the descendants of Khmer kings.
41:13As the Khmer empire crumbled, many escaped to set up a new Khmer capital in this region known as Longvek.
41:20The final refuge of the Khmer monarchy.
41:24Even though it's covered by a modern village today, this is actually an area with a few hundred years of history as the capital of the Khmer kings right after the Angkor period.
41:34Damien launches a drone to hunt for traces of the ancient Khmer city hidden within the modern landscape.
41:43From the air, he can make out one of the last structures the Khmer kings built.
41:48You can see very clearly the remains of these linear features on the ground. These are the old walls of the city and you can see the sort of linear depressions right next to them which are very clearly the moats as well.
42:01These walls are far smaller than anything seen on the scale of Angkor Wat.
42:06This is a very large city but there's just not evidence for the kind of density of occupation or the intensity of occupation that we might expect if there was a huge exodus of people from Angkor.
42:16The Khmer leaders' slow migration left the city of Angkor to collapse over generations.
42:27Neighboring kingdoms absorbed the remaining Khmer people. They adopted new customs and ways of life.
42:33By the end of the 15th century, the Khmer civilization was a shadow of its former self. Cities were abandoned and nature began to creep into the void.
42:46Angkor too began to crumble.
42:48Jungle trees grew rapidly and pushed over stone walls with their thick branches.
42:57But today, much of the city survives partially intact.
43:02Its decay halted before complete annihilation.
43:06Incredibly, there are signs that parts of the temple owe their survival to the very force that also threatens to tear them down.
43:25Nature.
43:26At Angkor Wat, Damien explores a gigantic sandstone arch that stands interwoven with a huge tree.
43:37These tendril-like roots come down over the top of the temple and make their way into and among the stones at the bottom.
43:44And so you can see here that all of this is the root system of this one gigantic tree.
43:49Known as a strangler fig, these plants seed themselves in the canopy of a host tree, while the roots crawl down to the forest floor, piercing their way through cavities in the temple stonework.
44:04But in many places, the thick vines hold the temple together like natural scaffolding.
44:11Without the roots, many of the structures may have collapsed centuries ago.
44:16Across the city, the jungle growth manipulates the Khmer architecture, breaking it up in parts but holding it up in others.
44:27What you see here is a kind of paradox.
44:30You have a tree that's in a certain sense holding this part of the temple together, even as its roots, its tendril-like roots, crawl their way through the temple into the foundations and between the stones and contribute to the instability of the temple overall.
44:43The result is a spellbinding harmony of ancient stone and wood.
44:50For hundreds of years, many parts of the Khmer Empire were entirely engulfed by jungle.
44:58Cities are overrun with invasive plants and vines.
45:01They're magnificent, they're iconic, it's amazing to see the jungle encroaching into these kind of religious spaces.
45:09However, of course, longer term, there are risks to the temple, risks of damage from leaving this kind of tree in place.
45:16Left like this, nothing would remain standing forever.
45:19To be able to study these buildings, archaeologists need them to remain in good condition.
45:27So today, dedicated teams of workers are undertaking a mission to save these temples from complete annihilation.
45:36Sarah heads to a mountain temple in Angkor, known as Phnom Bha Keng, to see how they do it.
45:48The temple is located on a hilltop one mile northwest of Angkor Wat.
45:52When the Khmer kings constructed it in the 9th century, this combined Buddhist and Hindu temple was the centrepiece of the city.
46:03But centuries of erosion have weakened the stonework.
46:07Chiyam Pali runs a project designed to repair the structure piece by piece.
46:12I've worked here for 20 years.
46:15One moment I have a hundred workers working two projects.
46:24Pali and her team carve the blocks using traditional tools similar to those the Khmer used centuries ago.
46:32But thankfully, they have one big advantage over the ancient people.
46:37They use a crane to lift the stones into position.
46:40You can see when we lift the stone up, the track is...
46:46Go up, up.
46:48The stone on the track, the track is down.
46:52So very heavy.
46:53Wow.
46:54That's why we use a tower crane to lift.
46:56Yeah, could you imagine carrying it up there by hand?
47:00The team hoist the blocks 13 metres into the air, to the workers waiting on the bamboo scaffolding above.
47:07They lower the stone into position and chisel away the edges to help the blocks slot securely into place.
47:18This is a massive puzzle.
47:19You have to figure out how to insert all of these blocks and make them fit perfectly.
47:23It's time consuming work, with each new block requiring over an hour of sculpting to fit into position.
47:34But from Sarah's high vantage point, it offers a unique insight into the construction techniques of the ancient Khmer.
47:41This is really, it's really interesting because it's so clear.
47:44You can see exactly how they've built this mountain temple into the bedrock and then created these corners using sandstone blocks and ladderite blocks.
47:53So this mountain temple is literally part of the mountain.
47:56Pali and her restoration team's incredible dedication revolutionises what we know about the Great Khmer Kingdom and helps preserve these ruins for another thousand years.
48:10But this isn't the end of the story. It's just the beginning.
48:14With each new project, investigators discover more about the astonishing world of the Khmer civilisation.
48:22It's thanks to the pioneering work of archaeologists that knowledge continues to grow and sheds more light on the people who forged this incredible jungle empire.
48:33It's structures like this that are hidden in the jungle that inspire archaeologists to do this work.
48:38The long-term trajectory of history lies in the traces that they've left inscribed into the landscape here in the Angkor area and really all across Southeast Asia, across the Khmer Empire.
48:52The rise of the Khmer civilisation was unlike any other medieval empire.
48:59From their monumental capital at Angkor, powerful kings reigned supreme.
49:05They built lavish temples, inspired by mysterious cults, they used revolutionary engineering to defend their cities, which stand strong centuries after the empire falls.
49:21The Khmer civilisation built one of the ancient world's greatest jungle empires.
49:27The Khmer civilisation built one of the full-time
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