The Secret of Machu Picchu – Lost City of the Incas | Full Documentary

In Peru, in the heart of the Andean Mountains, built at an altitude of 2,430 meters on a rocky ridge surrounded by steep rain-swept cliffs, stands the city of Machu Picchu. Machu Picchu is the most impressive living testimony of Inca civilization. It's absolutely breathtaking. It's a moment of wow of emotion. Frequented by over 800,000 visitors each year, this Inca city, built in the middle of the 15th century at the height of the Inca Empire, remains in the eyes of many this civilization's most amazing and enigmatic urban creation. From an engineering point of view, we just have to stand back and say, that's really impressive. So as an architect, I really wish that I could have like a time machine and go back in time and see how they did it. Today, with the help of new technology, experts can turn back the clock by probing the invisible and exploring what is hidden beneath and around the city to expose its secrets. To understand the magnitude of Machu Picchu, we'll have to see it from very high above. What we don't see is that 60% is underground. How did extraordinary Inca civil engineering overcome the most violent natural threats and transform this extreme terrain into the empire's most famous city? It is the challenge of man against nature. Settling here was extremely risky. What are the construction secrets behind these unshakable buildings? What do these iconic terraces hide? What amazing astronomical engineering are these massive stones concealing? And what is left to be discovered about Machu Picchu in its most impenetrable areas? Machete. It's here, isn't it? Thanks to advances in technology, Machu Picchu is revealing itself to today's experts in extraordinary new ways. Surveying the depth of the mountain, scanning from above and exploring the ruins, they will share their most recent discoveries and allow us to enter areas of the site never before seen by the public. It's unique and it's the only one of its kind still standing up in the whole Incan empire. Exploration, science, construction. Examine close up and from space alike, Machu Picchu strips down and reveals itself on every scale. Declared a UNESCO World Heritage Site in 1983, the incredible sanctuary of Machu Picchu covers 32,592 hectares of mountainous slopes, peaks and valleys embedded in the heart of Peru's tropical rainforest. But it's just one small part of the cement site that is well known to the world. The breathtaking city of almost 200 buildings built in 1450 at the heart of the Inca empire. The site of Machu Picchu itself is 32,000 hectares. It's an enormous site and each visitor covers only about 700 meters. The nuclear part of Machu Picchu is what we see. But within five kilometers, there are other sites that probably in the master plan of the Incas were going to be integrated to Machu Picchu. Today, most people only know about a tiny fraction of the entire Machu Picchu site. For more than a century, since the rediscovery of the city in 1911, explorers and scientists focused their research on the heart of the citadel, the area's most recognizable part. But now investigators have the means to carry out their studies and focus their research on areas which have remained invisible to the eyes of its visitors until today. The goal of this exploration work is to solve a series of intriguing mysteries. The first one being, what secrets are hidden in the remote terrain surrounding the city? What is the true extent of this extraordinary sanctuary? In order to understand the magnitude of Machu Picchu, its position and its relationship with all the other sites, we'll have to see it from very high above. To comprehend the extent of this famous Inca sanctuary and better understand the archaeologists' groundwork, first, it is necessary to get a bird's eye view and use satellite imagery to visualize Machu Picchu's position on different scales. From above, we discover Machu Picchu at the summit of a mountain with the same name, then the mountain at the heart of the Peruvian Andes, and finally, its location within the Inca Empire and the empire's gigantic network of ancient roads which traverse no fewer than six modern-day countries, from the south of Colombia to the tip of Chile and Argentina, weaving through the most important Inca sites. Experts believe that the number of roads linking a city to this immense network could reveal the site's importance. This is why a large part of recent excavation work at Machu Picchu has been devoted to the exploration of new segments of road which would link the city to the rest of the sanctuary and to the empire. It was once thought that Machu Picchu was isolated, but then the road network was discovered. In a yearly basis, we find new roads or new segments of roads to do in our explorations within the sanctuary borders. And it is absolutely incredible. It is a network that spans the entirety of the empire. To find and clear these new trails in the heart of the rainforest, the site's official archaeologists first rely on traditional methods of work. They start by collecting information about the potential presence of sections of road by studying historical documents and interviewing the local population who know the terrain well. Once this information has been collected and areas of special interest identified, they swap traditional groundwork for new technology which operates at an aerial scale. This is LIDAR, a technology which uses airborne lasers. LIDAR helps us tremendously. Without it, we would have to roam the jungle blindly with a machete, without truly knowing if there is something to be found underneath the trees. LIDAR allows for reconnaissance work before exploring the ground. Fixed to a plane, helicopter or drone, the device scans an area defined by experts. It emits light beams that it sends towards the ground. These beams pass through tiny gaps in the vegetation. When they reach the ground, they are reflected back to the transmitter recording the terrain's contours. LIDAR makes it possible to confirm the presence and precise location of sections of road. The use of technology is very important. This exploration work and archaeological excavations allow us to discover new structures. Nowadays, after the latest 2014 discoveries, we have nine roads that arrive to Machu Picchu. Everyone knows the Inca Trail. It's the most traveled path. But others exist, which lead to the city from other places. They are all connecting Machu Picchu with all the other settlements in the area. And just within the sanctuary borders, we have more than 300 kilometers of roads that interconnect everything as a spider web. And the roads weren't just for everyday use. You had to have special permission to be on them. Not everybody was allowed to enter Machu Picchu. The discovery and clearing of trails has revealed the presence of numerous control posts built to filter access to the city. Ingenious checkpoints located at regular intervals, such as these suspension bridges from which the Incas could remove wooden logs to deny access to visitors. Or entranceways once fitted with doors to control access to various sacred sites of the city. Here, we're very close to the Sun Gate in Tipuncu. But before we get there, we have this double-jump door. This is what we call a double-jump door. It's two levels of the door, and probably this will have had two dintels. Having a double-jump door means that you're entering to a more important or sacred space. Safe passage was an important part of the society surviving in that landscape. The network of roads that the Incas created across very hostile terrain was an information highway akin to the internet of our own age. They were mainly for military use. And of course, you had the runners that would take messages back and forth. And they travelled hundreds of kilometres a day It's quite amazing. So it was a means of communication between all corners of the empire. Exploring this network of highly protected roads is allowing the site's archaeologists to understand the true significance of this now-famous citadel. A city which is beginning to look more and more like a megalopolis, as additional sites within the sanctuary are discovered. Machu Picchu is not alone in the wilderness. It's actually part of a network of Inca sites. There's still a lot of things that we are starting to understand about Machu Picchu. So in the last five years, we have been intensively excavating not only Machu Picchu, but other sites within the historic sanctuary borders. By exploring this network of sites, investigators have unveiled a new gateway to the citadel, a site called Chachabamba. The ruins of this site were first discovered in 1940, but now investigators are undertaking a new campaign of archaeological excavations. We are at the site of Chachabamba, at the entrance to the Machu Picchu Historical National Park. A satellite scale makes it possible to visualise the exact location of Chachabamba. A gateway to Machu Picchu, positioned in a valley seven kilometres from the heart of the city, the site exposes the vast size of this historical park. But it's excavations on the ground that allow archaeologists to determine what links this site to the city. They have discovered a remarkable concentration of 14 fountains, which shed new light on the importance of Machu Picchu within the empire. We think that these fountains were used by visitors to purify themselves before entering the city of Machu Picchu. Dominika Sikskowska is an archaeologist from the Centre of Andean Studies at the University of Warsaw in Cusco. Today, she is leading a team to explore a new area of Chachabamba. She is hunting for other structures which could, like the fountains, offer precious information to the people of Machu Picchu. And she is also talking to experts. Several days ago, the archaeologist and her team scanned the area using litter. Now, she is back in the field to uncover mysterious structures that the device has located in the jungle. Structures which would be impossible to locate so quickly with the naked eye. Of course it saves time. I don't have to cover the entire jungle looking for structures that might not exist. Now, I only go to the places where we think something might be. We started our research fairly recently, but thanks to this new technology, we realized that previously mistakes were made. There were a lot of structures that we didn't locate correctly on the map. That's why we have to increase our exploring work to correct the cartography and planimetry of Machu Picchu. For aerial lidar technology to be useful to Dominica and her team on the ground, she works with researchers who first analyze the data, then send her the precise geographic coordinates of any structures which have been located. So this dot here is me, and the others are the structures that I have to get to. This way I can see how far they are away from me. In addition to these geographic coordinates, Dominica has received an infrared image of the structure scanned by the lidar. It's just there, isn't it? The image provides clues about the shape of the structure she's trying to locate. The photo that they sent me is this cloud of points, and it corresponds to the structure that we can see here. And here I can see that it has been more or less clean. The next step will be to dig and determine how and why this wall was built on this spot. The wall Dominica has located today will soon be excavated and can then be 3D modeled and studied in closer detail. This scanning and exploration work, which was started several years ago, allows researchers to improve and clarify the mapping of the Machu Picchu sanctuary, but it also allows them to model the explored areas, sites like the citadel itself or the additional sites like Chachabamba. This is an invaluable technological advancement for the understanding of this area. Today, experts have access to perspectives and scales of study, which would otherwise remain entirely inaccessible to the naked eye. For us, it is essential because it contributes to the knowledge of the site. We shouldn't limit ourselves to describing the monument. You have to explain it, understand Machu Picchu's function. As well as contributing to revelations about Machu Picchu's true scale, this new method of exploration is beginning to reveal answers to another key mystery. What was the function of Machu Picchu within the Inca Empire? In Machu Picchu, they've put a community on top of a mountain, and that suggests that they wanted something other than just to build a city or a community. It has something else which was important to the Incas. Thanks to the research that we have been doing in the past few years, now we understand a little bit more what was a function of the different areas. And we can say that Machu Picchu is a multifunction site. The exact nature and number of functions that Machu Picchu's had within the empire is still cause for debate amongst experts. But a recent discovery has finally allowed for agreement on one, Machu Picchu's role as an astronomical observatory. One of our most important discoveries concerned the sites for observing natural phenomena, which gave rhythm to city life. That is to say, the discovery of sites dedicated to astronomical observations. The most impressive part of Machu Picchu are basically the astronomical observatories. Machu Picchu was clearly a site for observing the sun's movements and for observing solstices and equinoxes. It is equinox. It was very important for the Incas to observe closely these astronomical phenomena, not only because of the ritual calendar of the Incas, but also it had relationship with the moment of planting and harvesting. Like many farming civilizations, the Inca organized their lives to the rhythm of the seasons. Observing the sky and the movement of the sun was essential to keeping track of the passing seasons and in anticipating the arrival of particularly violent rains in the region. Experts have long known about a few key sites in the city that were designed for astronomical observation. But two years ago, for the first time in Machu Picchu's history, investigators used new technology to uncover a remarkable site, unlike any other found in the empire. Machete. A unique observatory hidden under the tropical jungle vegetation on the north face of the Guayana-Picchu mountain, which has never been revealed to the general public. This place is El Mirador of Incaracai and it's basically the only Inca structure of its kind that is still standing. This unique site, which can now be located by using lidar to virtually clear the area of its vegetation, was spotted for the first time in 1980 by the archeologist Fernando Ostete. But due to a lack of means, it wasn't until 30 years later in 2012 that an excavation campaign was started on the site. Overlooking a foundation terrace, archeologists discovered the remains of walls with stones that had been cut particularly carefully and regularly. Preparation of the kind reserved for buildings of prestige and of high importance to the Inca. On the main facade, the archeologists also located two openings very different to anything they had discovered on other walls built by the Inca. So in this fine masonry wall, what we have is two orifices, one here and one over there. Convinced they had uncovered a structure built for a very particular function, they decided to return the following year to run a more advanced investigation that would finally unravel the mystery of this building. To see further than the human eye, the archeologists teamed up with Polish researchers to organize the scanning of the entire site. Their aim was to model it and expose its function. Without scanning or modeling, there would be no way for experts in the field to study the structure in its entirety. Its location on the steep slope of a mountainside makes it impossible to observe the building's layout and parts as a whole. So it was only thanks to a study of a virtual model that investigators could reveal the secret of its true function. With the recent investigations made by Fernando Astete and Professors Szukowski and Skociuk, that now we know for sure that this is an astronomical, a precise astronomical instrument. The unique perspectives given by the 3D model allowed Polish and Peruvian scientists to carry out a detailed study of the two openings which first intrigued them. They first discovered that the orientation of these two openings made it possible for the Inca to form a horizon marker from the sacred summit of Mount Yonantan, situated to the north of Machu Picchu. Then they reconstructed the ancient positions of celestial bodies to establish what exactly the Inca astronomers were observing through the holes. Today, 3D modeling offers us a unique opportunity to recreate the past and discover what exactly the Inca saw 500 years ago when they looked through the openings of this building, which is now known as the Inca-Racay Observatory. What we have here is an instrument that the Incas will use to watch the June solstice and also the plates. The orientation of the southern opening allowed for the perfect study of the rising of the plates. A group of stars whose brightness and position in relation to Mount Yonantan allowed the Inca to forecast the seasons. The northern opening allowed them to detect with incredible precision the day of the June solstice. Inca astronomers managed to orientate this opening so that it led in the sun's light precisely when it rose over Mount Yonantan, but only upon the exact day of the June solstice, As well as determining the extraordinary function of this structure, scientists have solved the mystery of how it was used. So we are in the inner part of El Mirador structure and what we have here is the two orifices, how these things function. The priest astronomers will actually, in this case, in the left orifice, they will not look directly because looking directly to the sun can get you blind. They will just see the projection of the light here on this other back wall. And for watching the plates, the priest astronomer will position himself here inside the niche and will watch through the orifice. It's absolutely breathtaking. They wanted isolation and they wanted to be closer to their goal. It's ideal for observations because we have what we call horizon astronomy, which means that the horizon is used as a reference with these mountains. In Machu Picchu, they have an incredible understanding of their environment. Technological progress and the tireless work of experts at Machu Picchu is revealing the genius of the Inca and exposing their extraordinary knowledge of the environment. Some of the most glaring proof of this knowledge is revealed in the construction of the city itself, built on a particularly hostile site with difficult access and extremely high rainfall. The climate in Machu Picchu was a real challenge for the Inca. Tons and tons of rain falling from the sky every single year. So you need to slowly release it. And if you don't deal with it in the right way, it's just gonna wash your civilization down the hillside. But it's also a complicated relationship. Clearly, we need water to survive. Both a deadly threat and an essential element to man, the problem of water has long been at the heart of explorers' research and it still interests experts today. How did the builders of Machu Picchu manage to both protect the city from this devastating element and exploit it to survive? How did Inca engineers manage to rise up and solve the challenge of water? Rainfall at the time when it was built is estimated to have been around two meters a year. Even today, floods and landslides regularly destroy homes and roads in this region. But Machu Picchu's buildings have survived for over five centuries. The Inca were affected by these destructive natural phenomena, but they studied the dangers in order to face them. We could learn a thing or two from the Incas. You know, the way that we design our cities today, we need to think about how we deal with excess water, how we deal with runoff. No written testimony by the Inca has ever been found. So it's only by searching for clues in the exposed parts of the city that experts have managed, piece by piece, to unravel the secret of how the city withstood the devastating threat posed by water. More than 50% of the effort destinated to build Machu Picchu is underground. If we look at Machu Picchu, we see buildings on the surface. What we don't see is that 60% is underground. 60% of that city is dealing with water and getting rid of it. The secret to the city's survival was found hidden underground, beneath the feet of its visitors. An ingenious underground drainage system, the visible parts of which are these iconic terraces. Surprisingly, the best way to measure the extent of this underground system is to take to the sky and scan the site on an aerial scale. From the air, we discover the incredible scope of these structures, a succession of staircase terraces of exceptional regularity. They act as an armor around the citadel, covering the steep mountain slopes. But it's by probing the depth that we discover the true genius of these structures, a natural drainage system, invisible to the naked eye, which allows the control of rainwater and the protection of the mountain from landslides. So this is an example of a retention wall of a terrace. Now, inside these terraces, what we have will allow rainwater to infiltrate slowly and it won't affect the structure. It was excavation work that allowed past experts to understand the function of these terraces and their stages of construction, stages which 3D imagery can now recreate. To construct these terraces, Inca builders started by erecting a retaining wall on the steep side of the mountain. Then they filled in the area behind this wall with four layers of different materials. What we have is big rocks, then medium gravel, then we have some sand mixed with fine gravel, and then we have the fertile soil. And then this just gets rid of the water in a controlled environment. The layers of different sized material stop the ground from getting excessively waterlogged, which would weaken it. It also regulates the release of water. The Inca constructed these terraces one by one, shaping this incredible staircase structure cut into the mountainside. It's a foundation for an effective drainage of rainwater that have made Machu Picchu survive for hundreds of years. Can you imagine? Building these terraces, it's monumental work. Machu Picchu's terraces are not just something decorative, they are something structural. If they didn't do that, if the Incas hadn't put these systems in place, they wouldn't have had a citadel to talk about. It would have been washed down the hill, it would have gone. That is one of their engineering skills, is to realize where the threat comes from and then to address that threat through design and through engineering. In the last 20 years, with the systematic excavations, we understand way more about how Machu Picchu was built. Excavations and traditional research allowed experts to determine the function of these terraces. But today, investigators are using new technology to gather fresh clues and uncover a new secret about the true extent of the city's foundation. Archaeologists investigating previously unexplored areas around Machu Picchu discovered something surprising. Contrary to long-held beliefs, the terraces weren't only built around the citadel, on the upper part of the mountain. Their exploration work using litter and 3D modeling allowed them to identify and uncover new plots of terraces in the lower part of the mountain. Actually, what happened these past few years during the work on the terraces is that we discovered that they reach all the way down to the river. The whole mountain is protected by these kinds of walls. It's the challenge of man against nature. Such a masterpiece on land, shaped like this. From an engineering point of view, we just have to stand back and say, that's really impressive. By unlocking the secrets of these iconic terraces, experts have uncovered the ingenious Inca engineering that protects Machu Picchu from the devastating threat of water. But now they face a new mystery. Once the threat posed by water was removed, how did the ancient engineers meet the second challenge, to conserve and exploit this essential element for survival? How did they supply a city, perched at an altitude of 2,400 meters, with an uninterrupted supply of water for more than a century? Solving this mystery is the challenge for Alicia Fernandez, an archaeologist specializing in water. The Inca engineers had to know the environment and the site's weather conditions perfectly before settling here, especially with the torrential rain. This has been thought out. Someone has actually gone to this site and thought, we need water to do this, we need water to do this. Before we build anything. They had to gather the most information possible beforehand to anticipate, and then build a suitable hydraulic system which would guarantee the longevity of the population and the construction of the city. This shows the level of planning that you would talk about with modern day megastructures. Their first mission before planning any structure was to find the water source. Now the source, which motivated the construction of the city and supplied it with water for over a hundred years, has been located at an altitude of 2,458 meters on the eastern side of the mountain, a unique source which is at the origin of an incredible hydraulic system. This is a sacred source which determined the place where the city was constructed. As you can see, the water is already filtered by the mountain when it arrives here. And once the Inca found the source, they were able to build a channeling system. The water system that has been discovered at Machu Picchu shows that there was running water all the time. The route of the canal adapts perfectly to the terrain topography. There is a 21 meter difference in level between this water source and the city's first fountain. It was perfect. Following the terrain's natural 3% incline, the water from this unique source was guided to the city built below. Thanks to a series of sculpted blocks of stone just 10 to 12 centimeters wide, this channeling system allowed the city to be supplied with 150 liters of water every minute, 200,000 liters of water a day for the citizens of Machu Picchu. Here, the flow rate is low because it is summertime. But in general, with the rains, the source is much more plentiful and there is much more water than this, a lot more. And the channels they've built, they're capable of taking twice the amount of water that the springs can provide, so they'll never be overwhelmed. And it was a way of maintaining a clean drinking source of water for the people, which enabled the civilization to flourish. So throughout the city, there's this whole canalization of channels which diffuse the water to outfall areas. To distribute water around the city, canal channels were branched and multiplied into an immense web capable of supplying water to each of the 17 fountains in the urban area. We are at the first of the 17 fountains that exist in the city. And all of the fountains registered in the sector share a similar design. They all have a water inlet, which can be opened or closed, and an outlet channel. The inhabitants of Machu Picchu had access to fresh water all year round, enough to fulfill their needs and to celebrate the sacred rituals which gave rhythm to city life. To reach the urban areas, the system of channels also crossed the agricultural zone, but it wasn't intended to irrigate the land. The agriculture there, they had no form of irrigation. They didn't need it. They had the rain. The gods would give them all the water they wanted. The more than 2,000 millimeters of annual rainfall in Machu Picchu are enough to have agriculture of current here. This channel is just for providing water to the urban sector. It just happened that the source of water is on the other side of the Machu Picchu mountain. And well, it has to cross the agricultural sector. The agricultural zone is made up of the iconic foundation terraces. Thanks to the underground drainage system, rainwater could irrigate these terraces without flooding them. A controlled flow of water supplied the layer of fertile ground and allowed for corn to grow. Also, most part of the agricultural sector is directly oriented to the east, to the sunrise. In this way, the terraces can capture most of the sunlight during the day and can generate a special atmosphere at nighttime for the crops. Here in Machu Picchu, they have worked in tune with nature, rather than against it. This relationship between architecture and landscape is exceptional. It's genius, the system, of not only getting rid of excess water, but equally delivering drinking water all year round. Thanks to a mastery of civil engineering techniques and a perfect knowledge of their environment, the Inca were able to overcome the challenge set by water and turn this threat into a strength while maintaining the breathtaking scenery that surrounded them. But with the mountain protected from landslides and a source of water located, another challenge awaited the Inca, the construction of the city itself. At the peak of the eponymous mountain, the city of Machu Picchu, built in the middle of the 15th century, sheltered between 500 and 700 inhabitants. With nearly 200 buildings and terraces carved into the stone, it remains the best preserved urban creation of Inca civilization. Machu Picchu is actually the most complete Inca site. It's the most impressive living testimony of Inca civilization. When you're looking to locate a city, a settlement, a community, you're looking at trade routes, you're looking at lowest crossing points on rivers, but in Machu Picchu, on top of a mountain, that is something special. We're still learning today or theorizing today how they may have done it. So as an architect, I really wish that I can have like a time machine and go back in time and see how they did it. Without a time machine or written testimonies, experts attempting to unravel the mystery behind the construction of these unshakable walls have to base their research on archeological and geological excavations. The first question is about the origin of the raw material used to build the city. Where do these stones come from? And how were they transported to a site so high up a steep-sided slope? To answer this question, geologists like Víctor Caloto took an interest in the site's topography, as it was 500 years ago when the Inca first arrived. We're in the western part of the Inca city of Machu Picchu. It's a sector we call the quarry. It gives us an idea of what the site looked like before the construction of the city. This is what it looked like when the Inca arrived. When the Inca first discovered the site, 80% of its surface was covered by these stunning geological formations. To solve the mystery of its origin, it is necessary to study it on different scales. First of all, on a microscopic scale, this rock revealed its nature to experts. It is granite, a magmatic rock originating from the depth of the earth. Granite is an igneous rock. It's a rock that comes from the center of the earth in a molten form. These granite rocks formed 250 million years ago and at a depth of over five kilometers. But to understand how this granite traveled from the depth of the earth to the summit of this mountain, we must take to the sky and view the area on a satellite scale. On this scale, we can locate the Citadel in the heart of the Andean Mountains, the longest continental mountain range in the world, up to 800 kilometer wide, stretching over 7,000 kilometer long and peaking at an altitude of 6,900 meters. It is the formation of this impressive mountain range which is at the origin of its specific geology known as granitic chaos. When the Andes were formed, these granitic rocks rose to the surface and broke apart. During the formation of the Andes over 200 million years ago, this magmatic rock formed at depth and slowly rose to the earth's surface. As it rose, it cooled and fractured. Then erosion of the upper layers of the mountain slowly revealed it at the surface. As torrential rainwater seeped into the gaps, it accentuated the cracks. Due to gravity on the steep terrain, the blocks finally separated, fell, collided and piled up, creating the rocky heap of granitic chaos discovered by the Inca upon their arrival. A disordered rock pile, giving the site an uneven and rugged relief particularly unsuitable for construction. The city was built on this chaos. Living here was a real challenge. But the Inca of Machu Picchu managed to profit from this inhospitable geological architecture by exploiting it as a quarry to build the city. It gave their buildings the impression of being a natural extension of the mountain. Machu Picchu is fascinating because of where it is located and how the Incas transformed the mountain with this organic architecture that doesn't break nature, but blends with it. What distinguishes Machu Picchu from other sites is the symbiosis between the architectural work and the nature surrounding it. What we wonder today is, how did the Inca take up this challenge and manage to build a city which has lasted centuries and is still standing today? With the question of the raw materials origin resolved, the next mystery is about the construction of these indestructible walls. This natural granite quarry offered the Incas an ideal building material as hard as concrete, but how did they work with it? How did they manage to cut it when historians agreed that the Inca used neither tools nor machines? Granite is a very hard material. One of the most difficult materials to actually shape, to work, to cut, and yet they chose to build a city out of it. Granite is a very, very hard rock and it's usually worked with iron. Without iron tools, to actually shape these huge blocks is just amazing. Without cranes, without machines as we have today, this is a really amazing achievement and probably unrepeatable. Fernando Astete is the director of the Machu Picchu Archeological Park. He has been studying the secrets of this area for many years. Now, he has developed a theory that has become widely recognized by his peers. According to him, the answers to the questions concerning cutting granite can be found on the site and explained on a microscopic scale. On this site, hematite, natural iron oxide with a resistance superior to granite, can be found. You see? It's easy to cut. According to Fernando Astete, the Inca used hematite as a natural hammer. By banging the two rocks together, they could cut blocks irregularly but with extreme precision. The engineers that were able to shape the granite into such irregular forms with such close bonding between the different materials is amazing. But once cut, how were these granite blocks assembled and fixed in place so they wouldn't move for 500 years? Once again, Fernando Astete has uncovered clues that help answer this question. He's discovered a construction site abandoned by Inca stone workers and still visible today. The site gives him an incredible opportunity to study the work methods of Inca builders. Here, we have a very clear example left by the Incas. We see that the top face of this rock has nearly finished being polished. The stone above it visibly still needs to be worked on. Once polished, the two surfaces are supposed to coincide. Here, we have a wedge holding up this block. If we took it away, this curve would coincide with this one and that face with this one. Once again, Fernando Astete has discovered the use of 3D models has complemented the investigators' research and observation work. It allows them to better visualize the different stages of ancient stone working. To ensure that the granite blocks fit perfectly together, the Inca first selected a block upon which they placed the wedge. Then, they placed the second block on top of the first and finished their cutting. The Inca sculpted the surfaces of the top block and bottom block until they matched. Once the shapes of the two surfaces were harmonized, the Incas took out the wedge. The two blocks fitted together and stayed in place without cement or mortar. And mortar is there to keep material apart, using irregular material to create a level bed so that you can create vertical structures. At Machu Picchu, they threw away the mortar. They didn't need it because they were so good at shaping the stones. And these unusual shapes, they're like a jigsaw. They're all built to fit the hole. And there's virtually no space in the joints. You can't put a knife or a piece of paper in there. And now, in times of earthquake, it's the buildings of the Incas that survive rather than the buildings that were built 10 years ago. They're all together working as a unit. And that's the goal. It's a unit moving all together. The architect, André Audazme, has been studying the walls of Inca temples for years. He wants to unravel their secrets. One of these mysteries concerns the resistance of these walls to earthquakes that regularly shake the region. We have already, in the last 200 years, two big ones. 1960, 1650. Long way ago, right? But both are still, the walls are here, just like the way it was. In 1950, a terrible earthquake devastated Cusco, 80 kilometers from Machu Picchu. The buildings, constructed by the Spanish conquistadors in the 16th century, crumbled. Whereas the Inca walls, built one century earlier, remained standing. Investigators trying to unlock the mystery of these invincible walls' seismic resistance have discovered new proof of incredible civil engineering techniques mastered by the Inca. In the first layer, in the first foundation, they have small stones, like circular stones, rounded stones, and the idea is to work like a skate. So it moves all together, but it's not cracking. Otherwise, if it's steady and really strong, it will crack with the earthquakes. In this case, it's moving, you know, as the earthquake's happening, making the stones are supporting one with another, and that makes that it works all as one element, right? Moving like this. The Incas were the best engineers of their time. They were on top of their technology in their lifetime. They didn't have paper, nor architectural plans. Everything depended on their great intelligence. They were truly incredible architects and builders who had everything in memory and worked with no written traces or drawings. The Incas didn't write anything, nor engrave information into stone. The main sources for understanding the Incas come from Spanish chronicles, and there's still a lot of things that we do not understand fully. Little by little, over the decades, through research and technological progress, from the heart of the city to the depth of the tropical jungle, the hidden mysteries of Machu Picchu have slowly been unraveled. And even if it still has many more secrets to expose, experts are far from running out of leads in their hunt for answers. The most unexpected one is based on a mysterious system coded using tiny knotted ropes found in different places over the Inca Empire called quipu, a system currently being studied thousands of kilometers away from the sanctuary at Harvard University. It is here that a distinguished American anthropologist and his student are on the cusp of deciphering it. Up until very, very recently, it was always believed that it was record-keeping. But recent research at Harvard University is starting to demonstrate that it's actually a linguistic system as well. The team of Gary Horton has already been successful showing us information that was recorded on quipu. If they're able to decipher or translate what it means, then that will be a revelation. Within these knotted cords, we effectively could be looking at the secret of the Incas, the secret of Machu Picchu. A secret with a quest which has motivated researchers around the world for over a hundred years and will undoubtedly continue to do so for a long time.