Original subtitles

In Peru, in the heart of the Andean mountains,

built at an altitude of 2430 metres

on a rocky ridge surrounded by steep, rainswept cliffs

stands the city of Machu Picchu.

Machu Picchu is the most impressive living testimony

of Inca civilization.

It's absolutely breathtaking.

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 civilisation'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."

As an architect, I really wish that I could have 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?

What are the construction secrets behind these unshakeable 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!

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 Inca Empire.

Exploration, science, construction.

Examined 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 this immense site

that is well known to the world,

the breathtaking city, of almost 200 buildings,

built in 1450 at the height of the Inca Empire.

The nuclear part of Machu Picchu is what we see,

but within 5 kilometres there are other sites that probably,

in the master plan of the Incas,

were going to be integrated into 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 focussed their research

on the heart of the citadel, the area's most recognisable 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 will have to see it from very high above.

To comprehend the extent of this famous Inca sanctuary,

and better understand the archaeologists' ground work

First, it is necessary to get a bird's-eye view,

and use satellite imagery to visualise 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 traversed 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.

On a yearly basis, we find new roads or new segments of roads

during 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.

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 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.

Nowadays, after the latest 2014 discoveries,

we have nine roads that arrive to Machu Picchu.

They are all connecting Machu Picchu

with all the other settlements in the area,

and, within the sanctuary borders, we have more than 300 km 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 visitor access.

Or entrance ways once fitted with doors

to control access to various sacred sites of the city.

Here, we are very close to the sun gate, Inti Punku,

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 would have had two lintels.

Having a double jump door

means that you are entering into a more important or sacred space.

Safe passage was an important part

of the society surviving in that landscape.

The network of roads 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 runners who would take messages back and forth,

and they travelled hundreds of kilometres a day.

It's quite amazing.

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.

There are 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.

A satellite scale makes it possible

to visualise the exact location of Chachabamba,

a gateway to Machu Picchu positioned in a valley

7 kilometres from the heart of the city.

The site exposes the vast size of this historical park.

But it is 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.

Dominika Sieczkowska is an archaeologist

from the Centre of Andean Studies at the University of Warsaw in Cuzco.

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 experts.

Several days ago, the archaeologist and her team

scanned the area using LIDAR.

Now, she is back in the field to uncover mysterious structures

that the device has located in the jungle.

Structures not possible to locate so quickly with the naked eye.

For aerial LIDAR technology to be useful to Dominika and her team,

she works with researchers who first analyse the data,

then send her the precise geographic coordinates

of any structures which have been located.

In addition to these geographic coordinates, Dominika has received

an infrared image of the structure scanned by the LIDAR.

The image provides clues

about the shape of the structure she is trying to locate.

The wall Dominika has located today will soon be excavated

and can then be 3D modelled 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.

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 the function of the different areas.

And we can say that Machu Picchu is a multi-function site.

The exact nature and number of functions

that Machu Picchu 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.

The most impressive part of Machu Picchu

are basically the astronomical observatories.

It was very important for the Incas

to observe closely these astronomical phenomena.

It's not only because of the ritual calendar of the Incas,

but also it was related to the moment of planting and harvesting.

Like many farming civilisations,

the Inca organised 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 Huayna Picchu mountain,

which has never been revealed to the general public.

This place is El Mirador of Inkaraqay.

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 archaeologist, Fernando Astete.

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,

archaeologists 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 archaeologists 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 next year

to run a more advanced investigation

that would finally unravel the mystery of this building.

To see further than the human eye,

the archaeologists teamed up with Polish researchers,

to organise the scanning of the entire site.

Their aim was to model it and expose its function.

Without scanning or modelling,

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 mountain side,

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 Ziolkowski and Kosciuk,

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 that had 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 Yanantin,

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 modelling offers us a unique opportunity

to recreate the past and discover what the Inca saw 500 years ago

when they looked through the openings of this building,

which is now known as the Inkaraqay observatory.

What we have here is an instrument

that the Incas will use to watch the June solstice

and also the Pleiades.

The orientation of the southern opening

allowed for the perfect study of the rising of the Pleiades,

a group of stars whose brightness,

and position in relation to Mount Yanantin,

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 let in the sun's light

precisely when it rose over Mount Yanantin,

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 the El Mirador structure,

and what we have here is the two orifices.

How these things function...

The priest astronomers will, in this case in the left orifice,

they will not look directly, because...

looking directly into 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 Pleiades,

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 god.

Here 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.

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 going to wash your civilization down the hillside.

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 2 metres 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.

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 run-off.

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

dedicated to build Machu Picchu, it's 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 armour around the citadel,

covering the steep mountain slopes.

But it's by probing the depths

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 overly 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 the foundations

for an effective drainage of rainwater

that have made Machu Picchu survive for hundreds of years.

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 realise where the threat comes from

and then to address that threat through design and 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 LIDAR and 3D modelling,

allowed them to identify and uncover new plots of terraces

in the lower part of the mountain.

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 2400 metres,

with an uninterrupted supply of water, for more than a century?

Solving this mystery is the challenge for Alicia Fernandez,

an archaeologist specialising in water.

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.

This shows a level of planning that you would talk about with

modern day megastructures.

Now, the source, which motivated the construction of the city,

and supplied it with water for over 100 years,

has been located at an altitude of 2458 metres,

on the eastern side of the mountain.

A unique source,

which is at the origin of an incredible hydraulic system.

The water system that has been discovered at Machu Picchu,

shows that there was running water all the time.

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 centimetres wide.

This channelling system

allowed the city to be supplied with 150 litres of water every minute.

200,000 litres of water a day for the citizens of Machu Picchu.

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.

It was a way of maintaining a clean drinking source for the people,

which enabled the civilization to flourish.

Throughout the city,

there's this whole canalisation,

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 the 17 fountains in the urban area.

The inhabitants of Machu Picchu had access to fresh water all year round.

Enough to fulfil 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 2000 mm of annual rainfall in Machu Picchu

are enough to have agriculture of corn 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 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 night for the crops.

In Machu Picchu, they have worked in tune with nature

rather than against it.

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 civilisation.

Machu Picchu is actually the most complete Inca site.

It is the most impressive living testimony of Inca civilization.

When you are 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 theorising today

how they may have done it.

As an architect, I really wish I can have 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 unshakeable walls

have to base their research

on archaeological 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 Victor Carlotto,

took an interest in the site's topography,

as it was 500 years ago when the Inca first 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 depths of the Earth.

Granite is an igneous rock.

It's a rock that comes from the centre of the Earth

in a molten form.

But to understand how this granite travelled

from the depths 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 Andes,

the longest continental mountain range in the world,

up to 800 km wide, stretching over 7000 km long

and peaking at an altitude of 6900 metres.

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 this 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.

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 transform the mountain

with this organic architecture

that doesn't break nature, but blends with it.

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 agree 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 an amazing achievement, and probably unrepeatable.

Fernando Astete is the director of the Machu Picchu archaeological park.

He has been studying the secrets of this area for many years.

Now, he has developed a theory

that has become widely recognised 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.

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.

Once again, the use of 3D models

has complemented the investigators' research and observation work.

It allows them to better visualise

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 a wedge.

Then they placed a 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 harmonised,

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 a regular 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.

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 their goal,

it's a unit moving all together.

The architect, Andres Adasme,

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 and 1650, a long way ago, right?

But both, the walls are here, just like the way it was.

In 1950, a terrible earthquake devastated Cuzco,

80 kilometres 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, rounded stones.

And the idea is that it works like a skate,

so it moves all together, and it's not cracking.

Otherwise, if it's steady and really strong,

it will crack with the earthquakes.

In this case, it's moving as the earthquake is happening,

making the stones support 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.

The main sources for understanding the Incas

come from Spanish chronicles,

and there is 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 depths of the tropical jungle,

the hidden mysteries of Machu Picchu have slowly been unravelled.

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 'khipu'.

A system currently being studied

thousands of kilometres 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 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 Urton has already been successful

showing us information that was recorded on 'khipus'.

If they are able to decipher or translate what it means...

then that would 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 100 years,

and will undoubtedly continue to do so for a long time.

Captions C SBS Australia 2020