Ancient Superstructures: Journey to Petra, The City Carved in Stone | Full Documentary

In the Middle East, between the Red Sea and the Dead Sea is Petra, a city nestled in the middle of the Jordanian desert in a vast rocky massif. Built over 2,000 years ago by a nomadic Arab population called the Nabataeans, many people consider it the eighth wonder of the world. It is an incredible place. The colors of Petra are just amazing. Petra is known for its huge facades, carved out of the rock, which are often used as scenery for films. But taking to the skies, we discover an archaeological site stretching 260 square kilometers with over 850 remains. A nomad city which still has many secrets to uncover. It's like a puzzle, a huge puzzle. You only have a glimpse of their glorious city. This city, forgotten for centuries, was rediscovered by a Swiss explorer 200 years ago. Today, archaeologists are still trying to complete its story and unravel the mysteries that elude them. So we need some good features of this area. With the help of the most modern scientific tools, they can probe the work of ancient builders. And these technologies are giving us real insights into the building methods. By examining Petra from different perspectives, from the infinitely big to the infinitely small, experts can detect details that are invisible to the human eye. It's very exciting. We're almost on the telephone to the past. Thanks to digital tools, the luxurious structures from Petra's golden age are brought back to life. This mythical city, which was at its peak in the first century BC, can finally reveal its true splendor. Petra demonstrates opulence. These cascading fountains and pools and really show it's basically showing off. A Las Vegas of its time. We will unlock the secrets of the sprawling desert city by examining its spectacular structures, starting with its most famous monument, Al Hasne. It is so dramatic because it's sculpted out of the rock itself. What techniques did the Nabataeans, a population of nomadic traders, use to build such tall structures without modern tools? How did their masons transform this raw stone into an architectural diamond? And how did they find water in the middle of the desert and create one of antiquity's most advanced hydraulic systems? It had water and that is Petra's secret. The main ingenious engineering of the Nabataeans was their hydraulic engineering. History, science, construction examined like never before. Petra reveals its secrets on every scale. To understand Petra, you first have to understand the secrets of this fascinating city's construction. It was built in the 1st century BC by a population of nomads, the Nabataeans. At the time, and still today, each visitor would enter by following this astonishing and winding natural passage known as the Siq. 1,200 meters long and in places less than 3 meters wide, it leads to this breathtaking building, Al Hasne. The builders constructed it here specifically so that everyone entering the city would arrive facing this facade. The outside was meant to be seen. This was to walk. For archaeological experts, this monument is a true enigma. Now, thanks to new technology, they can unlock its secrets. It's just mind-boggling the skill that went into it. The lines are so clean, the reliefs are so beautiful. And there's just no mistakes. If you look at the detailed carvings, not only of the figures, but the decorations, perfect. This incredible lavishly decorated temple is about 39 meters high, the equivalent of a 12-story building. But how did the Nabataeans carve a structure which is as monumental as it is perfect from solid rock? An interesting new thing when we see the Hasne is first, obviously, its scale. It's always been a mystery as to how they were able to construct these sculptures so high above the ground. In this arid desert, wood resources are practically non-existent. It is impossible to find sufficient planks to build scaffolding. So how did the Nabataeans raise themselves 40 meters from the ground in order to sculpt al-Hasne? To solve this mystery, archaeologists today combine traditional observation techniques with sophisticated technology, revealing details that the human eye can't easily perceive. Qais Tswesi is the site's technical curator. Thanks to this machine called a total station, he can take precise measurements of the building. Then he processes the data on a computer to build an extremely accurate 3D projection. Which gives us a very good accuracy, which is about 0.01 millimeter, which is very good for documentation. The 3D model lets experts examine the smallest details of the facade. But do any clues remain that could reveal the Nabataeans' construction techniques? Details like the remains of fastenings for scaffolding? The first feature which become apparent are notches on either side of the monument. Changing the scale, thanks to digital zoom, reveals the shape of these strange notches. They are deeper at the base than at the top. They appear to be shaped for a human foot. But did Nabataeans' builders use them to access their construction site? They could only have been cut after the building's completion, so the building was already completely sculpted. They weren't used for its construction. So these notches were hollowed out by raiders and not by the builders. Thanks to a careful examination of the facade and the digital model, archaeologists have also ruled out other false leads. For example, these small holes are not the remains of fastenings for scaffolding. They're bullet holes or marks from tools made long after the construction of Alhazne. To solve the enigma of its construction, experts have had to find other clues. Thibaut Fournet is a French architect and engineer at the CNRS. He has been studying one particular cliff for several months using a variety of techniques. This cliff is the site of a unique partially carved building known as the Unfinished Monument. This building is precious for archaeologists and architects because it allows us to have a snapshot of a building site halted during construction. And from this site, we can understand how the Nabataeans could sculpt a building on a cliff like this. The site allows Thibaut to analyze the ancient builders' work. This four-column monument was intended to run down the side of the cliff. Only the upper parts have been carved, which means that the stone cutters started from the top. But how could they have worked on such a steep cliff? Today, Thibaut wants to observe architectural features at the top of the facade. Just like the Nabataeans, he has also had to find a way to access the cliff. Thanks to this drone, the team can collect clues that were unattainable until now in order to process them in 3D. These previously unseen images, alongside traditional observations made 20 years ago, allow archaeologists to piece together the secrets of the Nabataeans' construction techniques. To sculpt this kind of structure, ancient stone carvers started by smoothing the rock, creating a flat wall which marked out the front of their building. We are on a raw, rocky facade. They started by making a flat surface, which was used as a board upon which the building was projected. On this blank canvas of rock, the Nabataeans hollowed out small terraces, which allowed them to sculpt the rock face in complete safety. They started from the top, then prepared a small platform and descended. And they had to finish the top of the building before going down the rock. They couldn't go back to finish it. After completing the upper parts, workers needed extreme attention to detail as they gradually descended, finishing with the foundations. The fact that they had to do the whole facade in one go, with no chance to restart or to repair a mistake, is something very courageous and a risk that nobody could bear today. In Petra, you need preciseness in engineering. That means if a mistake would go, that means the whole facade is destroyed. Researchers haven't found any plans or written records about al-Hasni, but they assume that the Nabataeans planned their construction to the smallest detail. The builders had to anticipate every volume, project the unbuilt structure onto the rock. And it's this ability to anticipate and plan the project beforehand, which is most impressive for an architect. At al-Hasni, the ancient builders erased all traces of their construction as they worked. But taking a step back, we can see evidence that this structure was also built using the inverted construction technique. To the right of the monument, these stairs, discovered at the same time as the site in the 19th century, allowed workers to access their construction site. But a mystery remains. How can this monumental facade stand, when it was impossible for the Nabataeans to test the strength of its foundations before they started building? The images taken by drone reveal an immense massif which overlooks al-Hasni. Hundreds of tons of rocks weigh down on the monument. How could the Nabataeans be sure that these structures wouldn't crumble underneath this immense weight? One of the principles of construction is that you have to transfer the load from its highest point down to the ground. And when you look at the sculpture, each of the elements, the weight is transferred down to the ground through the column. At first glance, the support element seems obvious. A monumental lintel held up by six immense columns cut directly into the rock face. But appearances can be deceiving. On 19th century engravings, we discover that one of the load-bearing columns was broken. It wasn't repaired until the 1960s, yet the building didn't collapse. The 3D model of the monument, built using data from the total station, reveals the solution to what holds up the structure. From the side, we realize that the lintel is built into the cliff. So are all of the columns, except for the two central ones. So it's the rock wall itself which is supporting most of the weight and not the façade which is almost like an optical illusion. The interior of Al-Hasnay confirms this hypothesis. Usually, it is closed to the public. But Kayse Tuesi has special permission to explore inside. Surprisingly, the interior of Al-Hasnay is much less spectacular than the outside suggests. Look from outside, very huge. And from the interior, small chamber. One master chamber, 10 meters high and undecorated, leading to three much smaller rooms. This transition from gigantic to miniature, between exterior and interior, shows that the Nabataeans knew about force dynamics and were careful to evaluate the risk of collapse. You need to triangulate any loading so that it can go over the structure and back down to the earth. And if you hollow it out to too great an extent, it would collapse and it would have brought the façade down with it. These rooms are small because they had to be careful that the ceiling, the quarry ceiling in a way, wasn't too wide, so that the rock can support such a large slab. These modest rooms pose question about the very nature of Al-Hasnay. Even today, experts still can't agree on its exact function. It is considered a tomb, or it may simply be a cenotaph, a building in memory of the deceased. Considering the site and the quality of the architecture, it was for someone important, probably a king. One thing is for sure, this structure was designed to impress. It is the first thing you see when entering the site. Thanks to the support of the cliff, the Nabataeans ensured that this spectacular monument would remain standing 2,000 years later. For centuries, Petra has survived many dangers, erosion, earthquakes, enemy attacks. But the city also has to withstand another major challenge, a hazard that's unexpected in the desert. From this blue sky, it's hard to believe how it's messed up during the winter season. Between October and January, the city can be hit by torrential rain. And the rain here always starts suddenly. The other side of this is that there is a danger of flash floods. In the event of a storm, rainwater flows between gaps in the rock and funnels towards the Siq. This natural gorge can transform into a furious torrent, threatening the entire city, starting with Al-Hasnay. Even in my 12 years here, there were two times where you saw the water roll into the center city and wreak havoc. That major boulders moved out loud and you realized that people needed to be evacuated out of the city. In November 2018, 4,000 tourists had to be urgently evacuated to avoid being trapped by devastating mudslides. But what did the Nabataeans do when faced with this danger? To answer this question, archaeologists first look for clues at the Siq, the narrow gorge that allows access to the city. The Siq was the weakest point of entire infrastructure of the city of Petra. So they had to protect this paved road from destruction. During the first excavations in the 19th century, archaeologists discovered an amazing passage carved through the rock. A tunnel, three meters wide and five meters high, leading to a canal. We are now in the middle of one of the most impressive hydrological structures in Petra. This tunnel was partially cut in the rock 2,000 years ago. This tunnel was associated with another diverging dam that was constructed not far away from it. And both structures were constructed in order to manage the water floods and to prevent them from entering the city. The dam was built near the city's entrance, diverting water towards the tunnel and away from the Siq. An ingenious solution that prevented the destruction of Petra's jewel. Other dams discovered later completed this protection system. Ahmad Hassan is an expert in digital archaeology. By taking thousands of pictures of the protection system's remains, he has been able to make 3D digital models of 36 individual sites. We're standing in one of these dams, which is a clear example about how the Nabataeans used to protect their cities from a flash flood. Today, most of the dams have been restored and returned to service. Nabataean system was re-erected by the Jordanian Department of Antiquities for the protection of people, tourists going into Petra. It's the first time ever in history of archaeology that I did archaeology, not only for curiosity, but to get to know how this system worked and whether it's possible to restore it, that it works again. And that's what I did. It works again. The system still functions and is proving to be an ingenious feature. Because the Nabataeans faced another major problem, how to supply the city with water all year round. It has 200 millimeters of water a year, whereas in Western Europe, we have 900. They needed to harness that. There is no life without water. How did the city's 30,000 inhabitants obtain fresh water? And how did they cultivate the land to feed themselves? The water at Petra has a reliable source up in the mountains. But it is some distance from the city. Satellite imagery reveals that the closest waterway, the Ein Musa, is situated nine kilometers away. So how did ancient engineers provide drinking water to the city's inhabitants without a nearby water source or regular rain? The first answer can be found at the dams built to protect the city from flash floods. So the Nabataeans built this, first of all, to protect the city from the flash floods and then to use the water, to collect the water, because they were very careful and wise to collect every drop of water. Every drop of rain that trickled down the rock face was channeled and directed into water tanks where it could be stored. There were as many as 200 tanks in the city. Many are still visible to the naked eye, some from high up, others like this one from the ground. But there was a problem. The water that collected in these tanks wasn't drinkable. It was not only water. It was dung from the birds. It was branches from bushes. It was dry leaves, God knows what. And then the water had to be collected and to still down. And then all the material fell down to the bottom. In 1999, Uli Balward carried out tests on the rainwater. The results allowed him to understand how the Nabataeans obtained drinking water. They stored water in a first pool and waited for the waste to sink to the bottom. Then the water flowed into a second pool, followed by a third, controlled by a system of valves. From the first to the next to the next. And at the end of this filtering process, the water was really clean. This filtration system made the water cleaner for drinking. But how did it reach the city? The answer can be found here in the Sikh. On either side of this narrow rocky gorge are shallow grooves that Ahmad Hassanah thinks were carved by the Nabataeans. One diverted rainwater, the other channeled in spring water from the neighboring mountains. Actually, there's a complicated system of pipelines to provide the water for every single house. So every single house in Petra get the water like our days. Thanks to his meticulous digital work, Ahmad was able to compile details of all the discoveries made by previous archaeologists. It allowed him to build a complete map of the complex Nabataeans' hydraulic system. 29 kilometers of pipes and five aqueducts that channeled an astonishing 46 million liters of water into the city each day. A change of scale allows us to discover other elements of this stunning hydraulic system. Some of the rock-cut channels retain the faint traces of terracotta pipes. If we look at close here, we can see that the edge of this pottery pipe and also we can see how this pipe connected together. The Nabataeans developed a system of pipes that was highly advanced for the time. Terracotta pipes were embedded into the rock, fixed in place with mortar, and the joints sealed with a waterproof coating. They made first a clay like the paste for bread and then they wrapped this clay paste around a wooden cylinder and fixed the two ends together and then they were firing these enormous pieces. These terracotta pipelines preserved the quality of drinking water. But the Nabataeans faced another major problem. How could these pipes withstand enormous pressure created by the steep slope between the source at an altitude of 1300 meters and the center of the city 500 meters below? Inside the pipelines with each percent of gradient, the pressure inside the pipeline increases by one bar. We know from tests that these terracotta pipes could bear an internal pressure of nine bars. With a slope between four and five percent, terracotta pipes won't explode, but they still remain 80% filled with water, allowing for a regular flow. This is exactly the angle of the channels discovered in Petra. Through experience, the Nabataeans had managed to determine the perfect angle for their pipeline to withstand the pressure, another feat for the era, while their system of aqueducts and valves let them adjust the flow rate in their hydraulic network. Now, the city could be supplied with sufficient water all year around. In Petra, the population, which grew to 30,000 people, could rely on being able to drink eight liters of water a day. They could rely on about 40,000 cubic meters of water per day, which makes it really incredible. It means that they had water not only for the normal domestic uses, but also for public uses. The Nabataeans figured out how to transport an incredible volume of water into the middle of the desert. But what could they use this abundance for? In the heart of the city, the study of ruins next to the Great Temple reveals something astonishing. At first glance, these piled slabs and pebbles don't suggest the presence of water. The first archaeologists even thought this was a marketplace. But in 2013, a study of soils on a microscopic scale made it possible to make an incredible discovery. And through the archaeologists' analysis for sand samples, they proved that those samples, they have seeds of palm tree, which gives us the idea that this garden was full of palm tree as the main tree for the garden. Amongst the grains of sand, scientists found remains of fossilized seeds and plants, which meant that the Nabataeans had mastered irrigation to create lush gardens. And that's not all. By analyzing the ground with georadar, researchers have detected the presence of an enormous pool. Instead of a market, there used to be an incredible aquatic complex here, which today's digital imagery makes it possible to recreate. Above the garden, there is this unique swimming pool. And this swimming pool is about 44 by 25 meters, and about two meters the depth. It's actually like the size of Olympic swimming pool nowadays. An Olympic-sized swimming pool, one century before Christ. It seems excessive in a desert where water is so precious. But the abundance of water was key to the Nabataeans establishing their power in the region. Water was used to serve the city's splendor. You have to imagine how impressed the caravans that arrived in Pesra would be by the water and the way it was almost wasted just for glory. The mastery of the water and then creating these cascading fountains and pools, it's basically showing off. The Nabataeans were originally nomads, caravan drivers. They knew that in the desert, water is the most important source of wealth. And they took full advantage of it. In terms of Nabataean economy, it relied heavily on trade. The main trade that brought the most money was the trade of incense and myrrh. In ancient times, incense and myrrh were luxurious, rare and expensive goods. Petra became so rich due to its location at a strategic crossroads. All of the commodities from Arabia and therefore Yemen passed through this kingdom. Also India, the Red Sea, all of these commodities arrived in Gaza or Alexandria and flooded the Mediterranean market. They were just at a junction between the Roman world, which had the means and a taste for luxury and the source countries where the goods were cheap. And thus, this economic somersault made them a lot of money. Today, archaeologists and historians believe that the abundance of water and luxury in the city were key to the success of the Nabataean people. Petra demonstrates opulence and people would be drawn from great distances to come and marvel at this society and also to spend money there. It was a financial Mecca, a Las Vegas of its time. The city stood out as a place of opulence. It was the Dubai or Las Vegas of its time. In Petra, everything was made to dazzle visitors, al-Hasni as well as other monuments. This entire valley was densely built. It was covered entirely in buildings, large public buildings, the impressive remains of which we can see, but also between them, every side was occupied by domestic architecture of very high quality, which dates from the era. You have to imagine that we're in the city center in ancient times, so you have to recreate in your minds the colonnade streets from antiquity moments here and there. Today, scientists are studying the remains of 850 buildings from every angle. They want to understand how the city developed over the centuries. These constructions have suffered the ravages of time much more than those which were carved into the cliffs. However, thanks to new technology, experts can reveal their full splendor. In the heights of the city, at the summit of Jabal Qutat, a French archaeological mission has taken an interest in these ruins, which were first discovered at the start of last century. So this is the place? Yeah, that's the place. I think we should stand here. Archaeologists have long believed that this was a place of worship. To verify this theory, Thibault Fournet is focused on recreating the entire building. He will use drone photography to achieve his goal. I need mainly pictures on this main monument, from this angle to this one, on different flights from the top and from the front of the cliff. By employing a drone to photograph these remains from many different angles, photogrammetry can be used to visualize the dimensions and volumes of the ruined buildings and reveal their architecture. All these photos gives us an overall coverage of the structure and photogrammetry software and reconstruct a kind of digital copy of the remains. So this digital model of its current state allows us to have a truly complete description of the remains, a description upon which we can base our restoration and test assumptions about the restoration of architecture on the basis of a three-dimensional model. Three-dimensional recreations in conjunction with excavation work has revealed a building measuring 225 square meters and composed of seven rooms. And the research shows something surprising. This wasn't a place of worship, but an incredible spa hanging from the cliff. Bathers would make the most of the comfort of this thermal bath, as well as an incredible view of the entire city center. It had to be on the cliff edge, not only for the bathers to make the most of the view of the city center, but probably also to be seen. Thanks to these digital models, Thiebaud could even reconstruct the exact path taken by bathers who visited this luxurious spa. A bather would come this way into a courtyard, then enter the first room of the thermal block, the frigidarium. This room was fitted with a cold swimming pool here, a kind of collective pool, and from here the bathers would continue towards the first heated room, the tepidarium, and from there they went into the caldarium, the structure's main room, which was fitted with two heated pools. And these pools were supplied by a reservoir, which supplied the whole structure with water. According to Thiebaud, this layout is typical of the thermal architecture from Latin civilization. The Nabataeans must have built these baths under Roman influence. Historians believe that these invaders took over Petra without a fight. It became a Roman province in 106 AD. Today, scientists are trying to understand how the Romans influenced or even changed the city's architecture. It is the study of a building's architecture which allows us to understand how the buildings, which had a function in the Nabataean era, were reused by the Roman power. And there are many emblematic edifices which allow us to follow the Roman occupation. When flying over the site, traces of Roman presence are evident, like this impressive ancient amphitheater, 33 rows of seats, and a 50-meter orchestra area holding 5,000 spectators. But other sites can only be uncovered through the development of complex investigative techniques. Shall we go up and set up there? Yes. For several years, French experts have been investigating one of Petra's most important monuments, the Caseralbin Temple, literally meaning the castle of the pharaoh's daughter. This building was constructed by the Nabataeans around 30 AD. For me, it was built from there. There was the same kind of overhang. It all fell down. We'll have to check the colored areas. But where did all those big blocks there come from? The monument has been severely damaged over time. On top of earthquakes, the Caseralbin was subjected to deterioration due to wind erosion. Being a sandstone is a very freeable material. We can deduce that the building was inexorably destroyed progressively. François Renel leads a team of 10 archaeologists supported by 10 Jordanian workers. They are meticulously excavating the ruins of the temple. The team is hunting for traces of the Roman building works. We make the most of the gaps in the slabs to comprehend its chronology and the different phases of construction present. The works carried out for several years show us that there are two phases of stairs, a first monumental phase, which is linked to the first phase of the temple's construction during the Nabataean period, and a second which corresponds to the embellishment of the temple within the framework of a much vaster imperial program during the second century AD. The Romans added their own materials to the top of the limestone and sandstone of the Nabataean buildings, materials like white marble, which they considered more precious. Experts believe they changed this Nabataean place of worship into a temple dedicated to the glory of their gods. We see that the Romans reclaimed the sanctuary, kept it in its Nabataean state, let's say, gradually renovated it, rebuilding parts of the stairs. But while repairing the Nabataean building, they added to the west of the structure a new monumental building, a monument with an altar, which is most likely dedicated to Roman emperors. The team have carried out a detailed topographical survey of the site, allowing them to recreate the dimensions and volume of the Casa Ralbent. Then modeling software could bring these ruins back to life, recreating in three dimensions the structure as it looked 2,000 years ago. Thanks to the Roman Empire, Petra experienced glory days once again. But without the architectural genius of the Nabataeans, these constructions would have disappeared long ago, wiped out by earthquakes. To understand how, it's necessary to take to the sky. Satellite imagery reveals the very particular geographical situation of the site. Petra was built over a seismic fault. The city is on the boundary of two sliding plates, the African plate and the Arabian plate. In this area, two continental plates are moving. One is shifting below the other and the other one is shifting over. As the plates slide over each other, they can cause particularly violent earthquakes. We think about Petra, we should always remember that there were many earthquakes that influenced the city's history. The biggest one was in May 363 and devastated the center city of Petra. Here we can see the devastating effects caused by an earthquake, which took place between the 3rd and the 4th century, where we have an upper part of the temple, which fell like these columns, like these parts of a ledge. Archaeologists have studied the Khazar al-Bint in an attempt to understand how it could survive centuries in this highly seismic zone. Within the partially destroyed walls, they found a clue, the remains of wood. Some experts think that this is the key to withstanding tremors. Their engineers, they understood that they needed to deal with the tectonic problems. The Nabataean engineers had invented one of the first anti-seismic measures in history. They have this idea that this piece of wood in the middle of the building should control the cracks, the vertical and the horizontal cracks within the building. Experts think that the framework of wooden beams, made of a more flexible material than the stone around it, absorbs the energy of earthquakes, combating the effects of stress in the walls, the raising of foundations and even twisting. Somehow it's controlling the balance of the building, which makes a smooth movement in the bulks of the building. And this technique is actually approved. This is a successful technique. The Nabataeans had risen to the challenge posed by earthquakes, and they even took some benefit from the phenomenon. Because it was seismic activity that created the city's exceptional scenery. In Greek, Petra means stone, and it is this extraordinary raw material that gave its name to the city. But what is so particular about it, and how did the Nabataeans manage to work with it? It's not possible to understand Petra without understanding its geology. The nature of the stone in Petra determines its present appearance. Petra is carved in the natural rock, so it is important to understand the raw material. Seismic activity allowed the raw building materials used by the Nabataeans to rise to the surface. This is a sedimentary rock called sandstone, with very specific properties. Petra is surrounded by mountains that are made of igneous rock, but Petra itself is sandstone. Sandstone is formed by layers effectively under the sea. And that means this sandstone is not like granite, a stone which is strong and compact, but it's only like cement. The grains are still the same as they ever were, but they are glued together by line. When you observe sandstone under a microscope, you realize that it is composed of millions of compressed grains of sand. This characteristic explains how the stone is so easily cut. To work it, the Nabataeans could resort to relatively basic techniques. Some of their tools even left marks that are studied by scientists today. What's interesting is that all the walls still have the marks of the tools which they were cut by. And in particular, here's the cut which we call ornamental. With a pick, the stone mason cut all these lines parallel, which allowed the creation of a surface, which plays with light. It shows one way the Nabataeans enhanced the beauty of their landscape. When flying over Petra, the variety of color in the monuments is impressive. The colors and the design in the bedrock itself, it's very, very impressive. A next door knee range, which stretches from yellow to purple, as well as pinks, oranges, and reds. This is the only place where sandstone becomes art. The colors of Petra are just amazing. You cannot stop looking or gazing at the facades of Petra. The builders understood that to embellish the city, they could utilize the incredible range of colors that the sandstone naturally offered. All different colors of sandstone of Petra are visible, each single layer to represent the different chemical composition, depending on the typology of iron oxide, which is present inside. When sandstone forms, it preserves a record of the different types of sand deposited over time. And one element is particularly spectacular. There's so much embedded quartzite in the sandstone, it refracts the light. And so you get these different colored grains within the sand, which through this reflection makes all these different colors and different bands, which brings the sculpture to life. It would be dead without that geological composition. It's the composition of the sandstone, in part thanks to quartz, that gives the city its splendor. The Nabataeans used the colors of this rock to great effect, but they also suffered from the stone's weaknesses, because the strength of sandstone varies greatly. By choosing to carve their structures here, the builders were taking a big risk. The rock has to be good quality. In particular, there can't be any of the faults which are unique to sandstone, which is a rock that is made of different layers that can easily detach. It's likely that the instability of the rock accounts for the number of construction sites found abandoned here. We now have investigation tools which allows us to know the nature of the rock and its possible flows before we begin. And that would probably have avoided what we see in several places of Petra, unfinished tombs. Even today, the nature of the stone poses a threat to Petra. A team of scientists from UNESCO, led by Giorgia Cesario and Giuseppe del Monaco, have the delicate task of securing this site and safeguarding this jewel of the desert. All the monuments are affected by similar factors like wind and water erosion. It's really fundamental for the people who take care of the site to understand this concept in order to act and to prioritize. The most fragile area is the Siq, the passage through which half a million tourists pass each year. In May 2009, just in the place where I am, a small stone fell from the top of this cliff, which is one of the highest points in the Siq itself. Unfortunately, it happened in the night and so we didn't have any injuries for the visitor. But in May 2015, a big block fell from this cliff. It happened around at 11 o'clock when people were passing through. Today, new technology like GPS sensors give advanced warnings of this type of accident. 11 dangerous spots have been identified at the Siq and Giorgia's team, aided by rock climbing specialists, have installed nets in the areas where the rock is most likely to crumble. Then, the team installed a highly sophisticated surveillance system. On the top of the block, we put a deformometer and a tiltmeter to measure the inclination in time. This device monitors the rock 24 hours a day, sending alerts to the UNESCO team who can intervene in the event of danger and evacuate the Siq. Through fieldwork and thanks to digital models, scientists today are trying to bring the ancient city of Petra back to life. For centuries, the city was abandoned and even the Nabataeans' architectural genius wasn't enough to ensure they would be remembered. This ancient people disappeared in the 7th century, prompting the fall of this superb city. Why did the citizens of this prosperous and busy city abandon Petra and flee? First of all, it was the diversion of caravan driver commerce via the city of Palmyra in Syria. There was an economic decline. The cause of Petra's lost status can be traced 500 kilometers away to the city of Palmyra's rising economic importance. But another disaster played a role too. The big turning point when damage started again was definitely the earthquake of 363 AD. All the water system, the cisterns and the aqueducts were damaged heavily and were no longer reconstructed in the future. The ending of Petra, it will finally come when they no longer maintain those water access points. There is not enough rainfall there to sustain agriculture on the normal basis. Even the ingenious anti-seismic measures of the Nabataeans weren't enough to save the city from destruction. But some scientists have suggested another theory that in 400 AD, the city fell victim to a different tragedy. By analyzing the ground of the town's main road, they discovered the presence of unusual white sand totally different to the local red or orange sand. There was a very irregular event which deposited white sand in amongst the buildings of Petra. And that could only have come from an area some distance away from Petra. So it was a monumental flood which happened and it could well have destroyed the entire population, certainly in the centre of the city. But today, this theory is disputed because signs of life and even subsequent building work have been discovered in Petra. Several discoveries of urban sites from the Byzantine era show that after the 4th century earthquake, life continued. So life went on, but it did not go on as rich or as strong as before. These Byzantine churches with their famous mosaics are the final glorious chapter of a city which is thought to have finally sunk into oblivion in the 7th century AD. The lost city of Petra was only rediscovered in the beginning of the 19th century by a Swiss explorer. Since then, this marvel of architecture has been observed from every angle and on every scale by hundreds of scientific missions revealing new parts of the city's history. By exploring the site in areas visited by tourists, we're still discovering more architectural remains whilst coming out of the ground sculpted ruins which have never been seen or interpreted. We know only 6% of the town so that gives you an idea of the work we have left to do. We only have glimpses of the Nabateans, we only have glimpses of their glorious city, capital city. Piece by piece, the secrets of the city's construction have been revealed through meticulous work on all scales but mysteries still endure. So for now, this pink city of the desert will remain the perfect playground for archaeologists, architects and hydraulic engineers all passionate about the incredible heritage left by the Nabateans.