Peru Journeys into the Ring of Fire

As a geologist, I'm fascinated by rocks.

But it's not just where they've come from, or what they're made of.

It's the fundamental role they've played in shaping human history.

In this series, I'm travelling around the Pacific Rim, on a quest

through some of the most dramatic and exotic locations on Earth,

to discover how civilisation's been moulded by the rocks beneath us.

It's brought me to the rugged, vertical landscape of the Peruvian Andes,

home to the highest peaks of the Western Hemisphere,

and some of the world's most unpredictable and destructive climates.

It's a land shaken by earthquakes and devastated by volcanoes.

Yet 500 years ago, this inhospitable terrain was mastered by the Incas.

The Inca civilization built a magnificent empire

equivalent to Ancient Rome's, in one of the most hostile places on the planet.

I'm going to journey through modern Peru, from coastal desert

to oxygen-starved peaks.

I want to uncover the geological challenges that they faced,

and discover the secret of the Incas' triumph.

On the eastern edge of the Pacific Rim, the South American continent

is shaped by the 7,000-km long Andes mountain range.

These mountains were the spine of the Inca Empire.

To grasp the implications of this,

I've come to look at a hole in the ground.

If that stone keeps falling down to the deepest part of this great gash in the landscape,

it will have dropped more than 4,100 metres.

That's almost 13,500 feet.

This is the Colca Canyon.

The deepest canyons in the world

lie crumpled in the heart of the Peruvian Andes.

It's so difficult to get a sense of scale in this vast space,

but to give you an idea, over there,

that white patch clinging to the other side of the canyon

is a village.

The Colca Canyon is twice as deep as the Grand Canyon in America.

It's home to these magnificent condors,

the largest birds of prey in the world.

Coping with the heights and chasms of the Andes was just one part

of the geological challenge that confronted the Incas.

Along the line of the Andes, there's a chain of dozens of active volcanoes,

all capable of erupting.

There's one volcano up there, Mount Hualca Hualca.

And over here down in the canyon, you can see the vertical columns

of a massive lava flow. What's more,

the ground beneath me is frequently shattered by terrible earthquakes.

I find it extraordinary that the Incas not only survived in this volatile geological setting,

they thrived in it.

More than ten million people lived under Inca rule,

in villages and towns nestled amongst the Andean peaks.

I want to know how they constructed buildings that survived the violent seismic activity of this region.

How did they grow crops to feed their millions on such steep slopes,

in such a hostile climate, and at such high altitude?

How did they manage to link up the far-flung corners of their vast empire,

an empire that sprawled over nearly a million square kilometres of geological extremes?

By 1530, the Inca Empire covered not just most of what's now modern Peru,

but much of what became Bolivia, Chile,

and even parts of Argentina and Ecuador.

A realm ruled by emperors

who claimed divine consent from the Inca Sun God.

Within its borders lay an extraordinary mix

of ecological and geological zones,

from snow-capped mountains to arid deserts.

The remains of the great Inca Empire

can be found even here, rising from the sands of Peru's Pacific coast.

What marked the Incas out from all the Andean civilisations

that had preceded them was how they mastered highland and lowland.

I'm hoping that by looking at some of these ecological zones,

I'm gonna get a better understanding of how they did it.

And I'm gonna start down there, with a journey along Peru's great coast, on the Pan-American Highway.

My first task is to find out how the Incas were able

to keep their empire fed, when challenged with such unforgiving terrain.

Lucio is our driver on this journey, and the landscape he reveals

as we drive alongside the Pacific is unique.

Almost all coastal Peru is the same,

bleak desert, little rain, and, just as it was 500 years ago, mostly impossible to farm.

But for the Incas, this barren coast was still a vital resource.

And it was all down to geology.

Parallel to this road there lies a deep oceanic trench that channels

unusually cold water along these shores.

This cold water sustains conditions perfect for an abundance of fish.

In fact, these are the richest fisheries in the world.

500 years ago,

the vessels that fished Peru's waters couldn't be made out of wood.

Trees are rare on this desert coast - it's just too dry.

But the Incas had a way around this shortage - their fishermen

made boats out of reed.

The original design, unchanged since Inca times,

is still used in a few places today.

These are the Caballitos de Tortora - Horses of Reed.

Hola, mi nombre Iain.

-Mi nombre Luis.

-Hiya. This is it. Caballitos.

-Grande!

Very "grande", isn't it? Gosh, amazing.

..Could we? ..Looking forward to it.

You must be very strong.

Is it cold?

The trick of it, as Luis attempts to explain,

is to always point the nose of the Caballitos towards the oncoming waves.

Get caught side on and you find out how cold the waters offshore really are.

Luis and the fishermen here need only a couple of trips a day

into these bountiful waters to sustain their communities.

In theory, there's enough fish off this coast to feed an empire as large as the Incas.

Perhaps they didn't need to bother with those inhospitable mountains after all.

The coastal people here had the knowledge

and the skills to fish these waters and make the most

of what these arid shores have to offer.

But there was one problem - El Nino.

For centuries now, Peruvian fishermen have noticed

that every few years, the waters offshore get warmer.

Usually it happens around Christmas,

so they started calling this warming El Nino - The Infant, after the Christ child.

We now know that the knock-on effects of El Nino are global,

from flooding in California to drought in Indonesia,

but Peru's Pacific coast is where the phenomenon originates.

It starts with a weakening of the normal coastal trade winds.

We're not really sure why it happens, but we know the result.

The waters out there get warmer by about 10C,

distorting weather around the world.

The impact in Peru is devastating.

The warm El Nino waters change the balance of life

in the seas - plankton die out, and fish levels plummet.

The Incas would have faced this problem too,

making reliance on fishing alone far too risky.

For a sense of the El Nino effect in Inca times,

I've come to the modern Peruvian fishing port of Chimbote.

In 1972, 20% of global fishing took place in these waters,

then came the worst El Nino in recorded history.

It wiped out nearly 90% of Peruvian fish stocks,

and just as it was getting back on its feet,

the fishing industry here was hit by another severe El Nino.

Many of the boats in Chimbote now spend several months of the year becalmed and inactive.

There's no way to predict when this El Nino effect is gonna happen - it can arrive

at intervals at anything between two and 11 years,

and it can last up to 18 months.

So for the Incas, depending on coastal fishing alone

would have been a flawed strategy.

To feed their empire, they had to find ways of exploiting

their natural home in the inhospitable mountains.

And that would be no easy task.

I'm heading eastwards, and up, into the Andes,

to Peru's fabled White City, to discover how the Incas did it.

As we drive inland and uphill,

I'm becoming aware of just how difficult this place must have been for the Incas to farm.

There are steep slopes, poor soils, and the weather here

is just as unhelpful as it is down on the coast.

In Peru, each altitude zone has been given its own name.

As we ascend between 500 metres and 2,300 metres,

we drive through the Yunga zone.

But we're heading higher still, into the Quechua zone.

This is our destination, Arequipa - the White City.

Buildings here are made from a white volcanic rock called sillar.

Above this thriving modern city,

the perfect cone of the volcano Mount Misti.

As legend has it, Ancient Arequipa

was named by an Inca emperor on his way back from a war of conquest.

Exhausted, he arrived here and said, in the Inca language,

"Arequipe", which means, "OK, let's stop here".

Now that's a good idea. Arequipa is, at 2,300 metres,

the start of the Quechua zone which extends upwards to about 3,500 metres.

That's about three times the height of Mount Snowdon in Wales,

which for those that live around here is a small hill.

In Inca times, most people lived in this Quechua zone.

Successfully cultivating the land at this altitude was a matter of life and death for the Incas.

To find out how they did it, I'm meeting up with Juan Ossio, one of Peru's most eminent historians.

Although I'm a little anxious about the Inca delicacy he's lined up for lunch...

So tell me more about this.

Well, you know, the name of this is quwi, and it has been known now

in the wider world as a guinea pig, a delicacy for the Incas.

This was an extremely tasteful course.

I normally have my guinea pigs on a wheel - I've never had them round.

So what were the challenges about growing food in this Quechua zone?

Well, the main problem, I think,

is the unpredictability of the weather.

Either severe drought that may produce frost, or if you have an excessive amount of rain,

the problem is that the crops that were grown by the Andean people may get rotten.

-So either too little rain, drought, or too much rain.

-That's the problem.

It seems a very fragile environment. How did they manage to manage it?

-The solution to sort out this problem was to develop land terraces.

-Land terraces?

-Land terraces.

In the countryside around Arequipa,

we can see how that Inca solution has transformed the landscape.

Everywhere, there are terraces...and terraces...

and terraces...

..and terraces!

It's said that the Incas approached farming with weapons in their hands and prayers on their lips.

To them, agriculture was like warfare - a victory claimed

by disembowelling the Earth.

So, Juan, these terraces are absolutely stunning.

-They are indeed beautiful.

-But how do they work, what's their job?

Well, they accomplish different purposes.

One of the purposes was to face the unpredictability of the weather,

another one was to expand the agricultural production, because, as you can see,

the slopes are very steep so this is a good alternative for making those slopes productive.

Apart from that, there is the problem of erosion as well.

-So, what, they stabilise the slopes?

-It stabilises extremely well.

So in terms of the unpredictability of the weather, what? Do they capture the rainfall?

Not the rainfall, but the water coming from the mountains.

-So the mountain streams feed these?

-The mountain streams, exactly.

Irrigation like this, using water diverted from streams, is much more reliable than rainfall.

As in Inca times, maize - that's corn - is the staple crop grown on these terraces.

Juan has asked Don Geraldo, a local farmer, if we can attend

an Inca ritual still performed when maize seeds are selected for planting.

This is a private ritual -

a practice that's widespread here, but rarely seen by outsiders.

There is no crop so important as maize - the seed is so important

that is treated very specially.

-And this is Chicha?

-And that is Chicha.

Chicha, a beer brewed from maize, is at the sacred heart of the ceremony.

He is offering to each of the mountains.

And then to the maize.

And now he spills some to the ground,

-because that's the Mother Earth receiving too.

-Pachamama.

-That's it, Pachamama.

-And then he drinks, obviously.

-He has to drink the whole thing.

-The whole thing?

-The whole thing, yes.

Wow.

It's your turn.

Thank you very much,

-thank you. Muchas gracias. Is it strong?

-No, it's very soft.

This one is mild.

It's quite strong, actually!

Ah! Muchas gracias, muchas gracias.

I read, I don't know if this is true, but in the Inca times the emperor had these virgins of the sun

that chewed the maize and fermented the chicha.

That is true, because saliva helps to ferment the chicha

and it's something that has continued even to our times.

Do you think this chicha was chewed by these ladies?

I doubt, I imagine that this chicha has been prepared in a different way.

Good, that makes me feel a little better!

Well, now they continue with the selections of the seeds

and that is a job performed exclusively by women.

Men are never allowed to touch seeds.

-To touch the seeds.

-They are not allowed.

Farming these slopes is a constant struggle.

To thrive, maize needs a warm and humid climate and a relatively long growing season.

Neither are assured at this altitude.

Every year, the crops are menaced by drought, while frost remains a constant threat.

As with the fishing on the coast, the Incas couldn't guarantee

that the terraces would provide for their vast population in any given year.

They needed to exploit every possible food source, wherever they found it.

So Lucio and I are off again, heading uphill.

As we drive higher, the mountains have become

even more hostile.

At this level, maize can't grow, and the soil is difficult to terrace.

Yet Inca farmers still saw great potential in these highlands.

We've now entered what's called the Suni zone,

which lies between 3,500 and 4,000 metres.

About three times as high as the UK's tallest mountain, Ben Nevis.

The temperature here regularly dips below zero - shadows from passing clouds can reduce it

by 15C in a moment.

But the Incas had a very special crop that meant

this Suni zone gave them some of their most productive land.

To find it, I'm off shopping.

What strikes me about the markets here is the amazing range of produce.

In fact, it's thought that Andean inhabitants were the first to cultivate as many as half

of the agricultural products the world grows today.

Their inventiveness,

a direct response

to the challenging landscape around them.

And in the Suni zone, one crop thrives...

the humble spud.

The potato was first cultivated in the Andes

and there are hundreds of different varieties in these mountains.

Because it grows underground, a tuber like the potato

is protected from the bitter night-time cold at this altitude.

Hola. Que es esta?

Olloco.

Olloco. OK, ah, tres. Tres.

-They look nice. Esta?

-Oca.

-Oca.

OK, oca. That looks fantastic.

-Look at that. Que es?

-Maca.

Maca, maca, this is. I hear this is an aphrodisiac.

Bueno, mucho bueno, eh? Dos, I better have.

Try two...cinco soles.

Muchas gracias. Adios.

By capitalising on the sheer diversity of produce, from maize and other crops in the low valleys,

to potatoes and related tubers up here,

the Incas mastered survival at up to 4,000 metres. But they didn't just stop there.

Some of the most important land was to be found higher still

and to get there, we'll have to abandon the car,

and use a more traditional means of getting around the Andes.

I've persuaded Lucio, our driver, to continue as my guide.

-These animals must walk slowly.

-Right.

There's a bit of a traffic jam here.

Ah, you use this, do you?

The Incas never used wheels for transport.

As I hike up the rocks and ruts of this mountain trail,

I'm beginning to understand why.

We're gonna climb higher into the Puna zone, 4,000 metres

and above. Few staple crops can grow in the Puna,

but this zone was vital to the Incas.

The reason, these pack animals - "yamas".

Or as we say in English, "llamas".

These animals were at the heart of the Inca economy.

The Puna zone is where the llamas like to graze.

They love the tough ichu grass

which grows at this altitude.

Llamas were a vital supply

of leather and wool.

They were even used as an offering to the Inca Sun God -

every month, hundreds were sacrificed to appease him.

They were crucial for carrying cargo.

The Inca army used supply trains of thousands of llamas.

And when they were no longer needed for porterage, they were eaten.

I've an idea for a menu tonight,

one of these llamas, he's got some tubers that I bought at the market.

It's an, um, I don't know, one of the white ones, I think. OK.

Fortunately for our woolly friends, tonight's meal will be strictly vegetarian.

-So what's in the pot, then?

-Well, we have some oca.

-Oca, OK.

-HE CHEWS

Oh, that's nice. It's quite sweet.

-Yeah, you put some sugar, it's better.

-Sugar with potato?!

-Yeah. Here we have some maca.

Ah, I've heard, I've heard about this.

-Yeah.

-I'm not sure I should, you know there's like five men and eight llamas and I'm taking maca.

-Is it, is it really an aphrodisiac?

-People say that it is.

Should I try this maca? No, he say no.

Lucio, maca, they're all saying no.

I don't think I should try this.

'The tubers and the potatoes on the menu tonight

'are high in carbohydrate - an excellent source of energy, and I'm going to need it.

'Tomorrow, we're gonna attempt a difficult climb to the roof

'of the Inca world, a place higher than I've ever been in my life.'

It was really hot out there today, but it'll get very cold tonight -

temperatures outside the tent are gonna drop to -5 Centigrade.

So I'm all wrapped up, and I'll see you in the morning.

We've climbed to about 4,300 metres,

which is about 14,500 feet.

If I seem a little breathless, it's because, at this high altitude,

the air is beginning to get much thinner.

Every breath I take feeds less oxygen to my brain

and I'm in danger of suffering from altitude sickness, or hypoxia.

The symptoms are headaches, shortage of breath,

nausea and dizziness.

I mean, I feel all right, but I've certainly lost my appetite

and I don't think it's to do with last night's meal.

But I do feel weak and the thing is, left to its worse,

hypoxia could put me in a coma.

But there was something remarkable about the Incas,

and it's been passed on to their modern descendents.

Compared to a lowlander like me, Andean people have hearts and lungs

which are proportionately larger and take in more oxygen,

adaptations shaped by their mountain home.

For our guides, evolution has been influenced by geology.

Lucio, how are you feeling?

Good. I'm OK.

-See, these guys they're just bounding up.

-Yeah.

There may be another secret to Inca resilience,

and it's nothing to do with biological adaptation.

Lucio, I think its time for those coca leaves.

-Really?

-Yeah, I got a terrible headache.

-OK, I'm going to bring you some, I have some.

'This is not cocoa as in chocolate, but coca, as in cocaine.'

-Chew it.

-Can you make cocaine from this? In Britain, this is illegal.

-No, it's legal here.

-For cocaine, you must to mix with chemicals.

-So it's...

This is natural, so it's legal here in Peru. You put it in your mouth

and after, you start to roll it.

-Roll it?

-Exactly - roll it.

-It's not gonna blow me away, go crazy?

-No, it's like a coffee.

-OK. Like a coffee?

-Yeah.

-OK.

-I'll have a coffee.

-You want more?

-More?

'Coca leaves are a stimulant.

'A rolled wad of chewed leaves will provide my body with an energy boost

'and the very act of chewing for several hours

'can accelerate the digestion of high-carb foods, like last night's potatoes.'

I've been chewing coca leaves for about three hours.

My mouth is completely numb,

but my headache's gone and I'm feeling kind of energetic -

surprising, because this is the highest that I've been in my life.

We've just passed the top of the Puna zone, at 4,800 metres.

We're now higher than any peak in Europe.

Because it's warmer here, near the equator, we're still below the snowline.

Above this point is the Janca zone.

Even the Incas had no need to venture up there.

No crops would grow,

and there was no possible grazing for the llamas.

This is as high as I go.

On my journey from this coast up into the mountains I've learned

that these altitude zones are incredibly challenging to farming,

the climate, totally unpredictable.

The Inca solution was to spread the risk.

They realised that if they exploited a number of different altitudes at the same time,

it would not matter if one failed -

the others might remain unaffected and keep the empire fed and stable.

The trick was to think vertically.

The Incas didn't just understand this, they actively studied it.

We know this because of the existence

of one the most remarkable archaeological sites in Peru.

And thankfully, it's not up here, it's down there,

where there's more oxygen.

This is Moray.

At first glance, these terraces,

a succession of perfect concentric circles,

seem like the remains of a religious site.

But this was something far more surprising -

500 years ago, this was a lab,

a greenhouse.

It's a relic of the Incas' agricultural prowess.

Here, the Incas discovered that they could recreate

the range of soils, temperatures and other variables

found throughout the Andes.

This was an experimental station

where they could see which crops could survive where.

And it was also a place where they could develop new strains to suit specific altitudes and conditions.

In just this one small area,

there could be as many as 20 ecological zones.

All made possible by the particular geology of the site.

The permeable limestone underneath soaks up the surface water,

and because it's slightly acidic, it dissolves the rock

leaving behind these holes, or craters in the landscape.

The concentric nature of these bowls lets them capture sunlight and shade

in a way that duplicates huge temperature variations.

Just 30 metres separates the top and bottom terraces,

yet a 16 degrees centigrade difference in temperature has been recorded.

Down there, growing conditions replicate the Quechua zone at around 2,600 metres.

Where I'm standing, it's more like the frosty Puna zone at 4,000 metres.

The Incas probably measured the temperature differences

by placing water in containers to freeze on the terraces overnight

and monitoring the rate of thaw in the morning sunshine.

What amazes me as a geologist

is how the Incas turned this rugged topography to their advantage.

The challenging Andean landscape was the problem,

but it also provided its own solution.

The essence of survival here was variety.

By experimenting with crops and exploiting different altitudes,

the Incas found ways to grow food for their millions.

In fact, they could even produce a surplus.

This was an amazing achievement,

but it only solved part of their geological challenge.

They needed to develop a storage and distribution network for their food.

But how could they manage to do it in a place as rugged as this?

I've come to the village of Ollantaytambo to find out.

'It's a good place to look for clues because it's a bit like a walk back in time,

'and taking it with me is Peter Frost, an English historian

'who's made Peru his home for 18 years.'

So what's so special about this Ollantaytambo?

Well, its unique really because it's the only Inca settlement with an Inca town plan,

the Inca streets, the Inca courtyards, in many cases,

which are still inhabited.

Each of these blocks would have been an Inca cancha, as they call them,

like a courtyard with a number of buildings facing inwards,

each of these inhabited by different members of the same kinship group.

Ollantaytambo gives us a real insight

into how the Incas organised their villages,

and because the site is so well preserved, it reveals exactly how they stored their surplus food.

The system that was developed by the Incas made it safer -

they weren't so subject to famine and disaster,

which is one of the ruling factors in the Andes.

They stored their agricultural surplus

in a vast network of storehouses that were scattered throughout the empire.

-There's one right there on that mountain, you see?

-Er, no.

-It's just above the trees over there -

there's three lines, three rows of storehouses there.

-Precariously balanced on the rock face, that's where they put their warehouses?

-Yes,

they built them in high, inaccessible places for various reasons.

Firstly it was a good place to keep produce fresh

because it was windy and cool.

It was also inaccessible, so unauthorised people couldn't get to it very easily.

And another is perhaps to make it very prominent

so people are always aware that these storehouses are there for their benefit.

-Like a big neon sign saying, "Inca will look after you", or something?

-In a sense, yes.

'So Inca storehouses acted like shock absorbers

'to stabilise their empire,

'providing access to food in times of poor harvests, adverse weather and famine.

'But without a means of distribution, these storehouses were of little use

'and for that, the Incas needed roads.'

It seems inconceivable in the difficult terrain of the Andes,

but the Inca Empire was bound together by a 40,000km road network.

And as the Incas hadn't developed the wheel, these roads were designed for travel by foot.

They had sophisticated drainage

and kilometres of steps were built for the slopes,

all done with simple bronze tools.

These roads could be used to transport food

and were a key means of administering the empire.

State officials travelled along them, and so did information.

Special couriers called chaskis would transmit messages

across the empire by running relays along roads like these.

They operated from stations at regular intervals

and could cover 240km, that's 150 miles, each day,

which is hugely impressive given this rugged landscape.

Chaski were trained from childhood to remember verbal messages perfectly.

Flawless accuracy was almost guaranteed

because they would suffer severe punishments if errors crept in.

Verbal messages were not the only thing the chaskis relayed -

they also carried these pieces of knotted string, called khipus.

These were enormously important to the Incas

because they didn't have a written language.

They used the knots in these khipus to record statistical information

vital to the running of the empire.

For example, how much was contained in each of the storehouses

in times of emergency.

It seems the Incas had developed a system

that mastered the difficulties of building an empire at high altitude.

But unfortunately for them,

extreme weather and topography were only part of their challenge -

all their structures had to survive the frequent earthquakes that strike this volatile region.

Recent natural disasters in Peru can give us a feel for what confronted the Incas 500 years ago.

I'm back on the road with Lucio to show you what I mean.

We're travelling northwest to the tragic market town of Yungay.

FESTIVE MUSIC PLAYS

En route, we encounter an colourful roadblock, a fiesta.

In the Andes, they're a frequent and raucous celebration of life.

On Sunday 31st May 1970,

Yungay was celebrating an unusual fiesta of its own.

It was the first day of the Mexico World Cup finals

and Peru had qualified.

Yungay sprawled in the shadow of Huascaran,

at nearly 7,000 metres, the highest peak in all the Peruvian Andes.

That afternoon, tragedy struck Yungay -

a massive earthquake shook the central part of Peru.

Measuring 7.8 on the Richter scale,

it was the equivalent power of 400 Hiroshima atomic bombs.

The quake caused death and devastation in many cities.

In Yungay, however, the worst was yet to come.

The earthquake had triggered an enormous landslide up there on Huascaran.

The slide took just three minutes to travel the 18 kilometres

from up there to Yungay,

covering the town in a layer of rock, ice and mud 70 metres deep.

According to official figures,

13,000 people lie buried beneath my feet.

This satellite image shows Yungay before the earthquake.

And this, after -

the town submerged under the landslide's white mass.

This may look like a rock, but it's the top of a church spire,

all that emerges from the now solid layer of mountain debris.

This is what a bus looks like when crushed together by a landslide.

On Huascaran, you can still see the scar

where 50 million cubic metres of the mountain just sheared off.

There were a few survivors.

One small group had been visiting the local cemetery, on this hill.

What happened here was the worst earthquake-triggered landslide in recorded history.

Earthquakes occur because the planet is covered in a patchwork of plates

floating on a plasticky interior.

Two of these plates - the Nazca and the South American -

have thrown up the Andes as they've crashed together, deep below us.

Peru suffers from serious earthquakes every few years.

They're caused by that Nazca plate pushing underneath the South American one.

As the plates collide, friction makes them stick,

the pressure builds until suddenly, they slip.

The result is earthquakes, landslides

and terrible devastation.

The Incas knew all about the geological risks of the region.

And while there's little they could do about a mountain shearing off,

they had to design buildings capable of withstanding massive quakes.

There's evidence of how they achieved this back here,

overlooking the Inca village of Ollantaytambo.

These monumental stones were in part an Inca response

to the violent seismic activity that shook their empire.

They're incredibly well carved -

you can't even slip a coin or a piece of paper between them.

And there's no need of mortar. As you go up the wall,

the stones get smaller, which makes this structure incredibly stable.

And if you look at where the joints meet, look, you can see they're staggered, not straight.

It's like an interlocking jigsaw that minimises lines of weakness

and all of this helps to make this construction earthquake-proof.

Windows, doorways and most walls slope inwards

to make them stronger and more flexible when a quake strikes.

But how did they carve out these perfectly cut edges without steel?

And how did they transport those huge stone blocks without the wheel?

Apparently, there are some clues at the bottom of this ramp.

And on hand to tell me more, historian Peter Frost.

Hi, Peter, so what we got here?

-It's a tired stone, as they call them here.

-What's a tired stone?

It's a stone that never made it from the quarries to the construction site.

So does this give us an idea on how it was carved?

Yeah, we have to get on the top really to see what they've been doing here. Hang on.

-You see these pockmarks all over the surface of this stone?

-Yes - to me these aren't natural.

No, those are the tell-tale signs of Inca use of hammer stones.

-This is an Inca hammer stone made of hematite.

-Oh, iron ore. Wow, its heavy.

Very heavy, very hard,

and very effective in carving away the surface of a softer stone.

-So they would just peck, peck, peck, rock on rock?

-That's it.

-Must have taken them absolute ages.

-Not as much as you might think -

because there are people who've duplicated this work in Inca quarries,

that in fact it doesn't take as long as you'd imagine.

I mean, this is enormous, how did they actually transport it?

They were coming across open country here from the quarries

and if you have enough people and enough rope, you can do it.

And the Incas did have this huge reserve of manpower,

they were never short of people.

Couldn't they have used cattle to pull it?

-They didn't have cattle.

-They didn't have cattle?

Peter, I'm gonna give you back your hammer stone and say thank you once again.

That was very useful, thanks.

-I'll gonna finish carving this stone.

-Are you? Well, don't ask me to drag it up the hill. Bye!

Although the Incas had been dealt a truly harsh hand by geology,

they were ingenious enough to fight back and thrive.

But theirs is a story with a bitter twist.

To show you what it was, I've come to the Inca capital, Cuzco.

This was a centre of Inca power and wealth.

From here, the Inca emperors, believed to be the divine descendents of the sun,

ruled their vast empire.

Cuzco is a bustling, modern city,

but there are still traces of its glory under the Incas.

Some streets are flanked by extraordinary foundation walls

that have withstood centuries of earthquakes.

The proud Inca empire was built supremely

to cope with the challenges of its geological setting.

Here were a people who knew how to farm the mountains, administer a rugged terrain

and build magnificent roads, storehouses and buildings

to withstand the shaking of the earth.

They might have thought, what could possibly go wrong?

In the end, of course, their undoing was down to a product

of the giant geological cooking pot beneath their feet...

gold.

It's hard to believe that these shiny precious trinkets started off as a metal-rich stew

injected by superheated waters up into the hearts of mountains

and later liberated by eroding rivers

to form particles or nuggets of pure gold.

The Incas loved gold, not as money, but for making beautiful objects.

For them, it only took on value when crafted into ceremonial jewellery, figurines

and adornments for tombs and temples.

In 1532, Francisco Pizarro and his Spanish conquistadors arrived in Peru

and they came in search of gold.

While the Incas had mastered the landscape,

they were unprepared for the threat of the Spanish.

Within a few years, they were a conquered people,

their glory extinguished.

But one extraordinary treasure escaped the Spanish

and it's not here in Cuzco.

The climax of my geological journey is an archaeological treasure

that lies at the end of a three-day hike, or from Cuzco, a four-hour train ride.

Recent discoveries at Machu Picchu have given us a new insight

into the true nature of the Inca relationship with the mountains.

All the trains go to Machu Picchu here.

'Archaeologist and explorer Dr Gary Zeigler is coming with me.'

This is quite a crowd, isn't it?

-Something like 500,000, I understand, go to Machu Picchu.

-500,000?

Can you believe that? That's a year.

'In the last few years, Gary's expeditions have uncovered two major Inca sites

'once hidden in this region. He's a real-life Indiana Jones.'

It was here, 85 years ago, that another American made one of the finds of the century.

-So tell me about the discovery of Machu Picchu.

-Oh, it's a wonderful story.

Hiram Bingham, an intrepid American explorer in the year 1911,

was travelling down the Urabamba River Valley looking for ruins

and he stopped at a small place there, what's below Machu Picchu now.

There was a family actually farming on one of the terraces at Machu Picchu.

-He offered a reward, they took him up and he found the site.

-So it's easy?

-Piece of cake.

-So how do you do it today?

-Well, we tried hi-tech, we tried satellite imagery

we tried thermal imaging from an aircraft,

but it's all come down to swinging a machete.

-Old style.

-That's right.

Gary is going to show me something in Machu Picchu

that these tourists won't find in their guidebooks.

There's something hidden in its design,

an ancient geological code that's only now being unlocked.

High above the Urabamba River, deep in cloud forest,

lies breathtaking Machu Picchu.

Wow, look at this.

Is this not fantastic?

This is the classic view, isn't it?

-It is.

-The clouds sweeping in over.

It really captures the mystery and the romance of it.

-Can you imagine how it looked in Inca times?

-I haven't a clue.

Imagine the thatched roofs covering everything,

imagine these walls being brilliantly white

with decorative plants and flowers around.

-It must have been ablaze with colour.

-Undoubtedly.

Until recently, the importance of the mountains that embrace Machu Picchu

has not been fully appreciated.

It's these peaks that are the key to deciphering the Incas' geological code.

Everywhere Gary takes me, he points out carved rocks.

He's picking out replica stones,

designed to echo the shapes of the mountains behind them.

I'm starting to see replica mountains everywhere.

Well, look at this. This is one of the best replication stones I know of at Machu Picchu.

Look carefully at the shape of the rock,

look at the two arms coming off of it,

It seems to closely represent the mountain in the background,

which is Cerro Yanatin.

And that sloping bit at the back is the sloping back wall of the peak.

Exactly - as close to the canyon on the right as you can get.

-That's amazing.

-It's amazing.

They certainly intended to make a replication of the mountain in the background.

What this suggests then is that Yanatin was something important in mountain worship

at least to this part of the site and people passing by here,

and so they took the boulder, they enshrined or made a roadside shrine

of a replica of the peak behind.

'Inca religion venerated not only mountains,

'but boulders and rock formations.

'Their architecture sought to work in harmony with the surrounding stone.'

And here's this great carved stairway.

-This is all rock?

-All solid rock.

-Wow.

Carved out of the native bedrock.

As we come up on top of the highest point here,

there's something I'd like to show you that's fantastic.

Look across the plaza,

look at this wonderful example of native rock blended into the construction.

-It's beautiful.

-That's one of the finest examples of Inca architecture

in the entire Inca empire.

Fantastic, how they've blended that in there.

Since Hiram Bingham's rediscovery of Machu Picchu,

sun worship and solstice ritual were believed to be the main religious rites here.

After all, we know the Emperor claimed descent and authority from the Sun God.

But the new focus on the site's geology has changed that.

Now, Machu Picchu appears more like an altar to the rocks.

Iain, here we are. This is my favourite place

and probably the most sacred point, most important point of Machu Picchu.

And it's interesting that Bingham identified this or named it the Intihuatana.

Intihuatana means, in some sort of description,

"hitching post of the sun". Others have called it a sundial.

But of course, most of what Hiram Bingham established in his conclusions

has since been reinterpreted.

-A bit too imaginative, was he?

-Exactly.

We now think that the stone is more of a focal point.

It seems to be a replication of Huaynu Pichhu mountain

and because it's the highest point in Machu Picchu,

it represents Huaynu Pichhu as the protective deity of Machu Picchu, the city.

Yeah. It looks obvious, but also its just a beautiful rock, isn't it?

It's exquisitely carved. It's like a piece of art.

It certainly is. It's Inca art.

Inca art, yeah. Geology and art.

Fabulous.

For me, it doesn't get any better.

I've learned that the Inca story is inextricably woven into the rocks.

These rugged lands, the climate, the gold - all were products

of geological forces at work beneath the feet of these great people.

That they built such a successful empire

was down to their deep understanding of this unique natural landscape.

The secrets of Machu Picchu that have recently been revealed

show that the Incas not only understood these magnificent mountains,

they worshipped them in a profound, and to me, even moving way.

And what I've discovered from my journey through modern Peru

is that this respect and veneration for the Andes

is still a vital part of life here today.

Subtitles by Red Bee Media Ltd

Email [email protected]