The Place Where Time Began Lost Kingdoms of Central America

We live in a decade when, for the first time in human history,

more people live in cities than in the countryside.

Cities are thought of as shining beacons of progress.

As they grow, they draw people to them, in search of prosperity,

security or a better life.

21 million people live in Mexico City.

It's one of a handful of global cities that define our modern era.

It's the political, cultural and economic driver of all Mexico.

But its size is one of its biggest problems.

How do you maintain order and control so many people?

This is not a modern problem.

2,000 years ago, another metropolis dominated

this part of the Americas, what we call Mesoamerica.

And this ancient city grew to dominate its population

and influence distant empires.

That city was called Teotihuacan.

For hundreds of years, it was the biggest in the New World.

Its rulers built monumental structures

and then went on to build a vast empire.

Yet, the identity of the people

who led this civilisation remains a mystery.

My name is Jago Cooper...

I'm a specialist in the Archaeology of the Americas.

In this series, I will be exploring

the rise and fall of forgotten civilisations,

from the crystal blue seas of the Caribbean,

to the New World's most impressive pyramids,

over the smoking volcanoes of Costa Rica

and deep underground, in the caves of central Mexico.

I'll travel in the footsteps of these peoples,

to reveal their secrets, to unearth the astonishing cultures that

flourished amongst some of the most dramatic landscapes in the world.

So, what lay behind the rise to power

and influence of this great city of Teotihuacan?

What drove this city and its people was a powerful

ideology of conquest, trade and religion.

And what I want to understand is how this once-powerful civilisation

went up in flames.

This is the Basin of Mexico.

It's a high plain valley where Mexico City lies.

But just 30 miles north-east of the capital

lies the remains of Teotihuacan,

Another great capital, that grew into an enormous city state

almost 2,000 years ago.

Little is known about the origins of Teotihuacan,

but, by 100BC, its population could have been as high as 20,000 people.

It was significant and one of many settlements in the Basin of Mexico.

But within 200 years, there was a huge change.

This is the Basin of Mexico in 100AD.

Teotihuacan stands out as the largest settlement in the region.

So, why did Teotihuacan grow so rapidly

and what happened to those other settlements?

I've come almost 50 miles southwest of Teotihuacan, to see what

became of one settlement whose growth came to an abrupt halt.

This is all that remains of the city of Cuicuilco.

It's well placed in the valley for rainfall and agriculture

and this circular temple is the first piece of monumental

architecture in the Basin of Mexico.

Started around 700BC,

it's completed in 400BC and during this time

when Cuicuilco is rising to prominence, Teotihuacan is

still just a provincial village.

But Cuicuilco had one major disadvantage.

It lived in the shadow of the Xitle volcano.

And a series of eruptions forced its people to leave,

looking for somewhere safe from this danger.

By 50BC, most of the city was covered in ten metres of lava.

What happened to Cuicuilco wasn't isolated.

Elsewhere in the Basin of Mexico, there's another volcano,

the smoking mountain of Popocatepetl,

that looms menacingly over the landscape.

We know that, in the first century AD, it erupted catastrophically,

forcing the people to migrate, looking for a more secure future.

Volcanic activity was a major reason for the big population shift

in the Basin of Mexico in the first century AD.

People were moving north, where they could be free from

these violent natural forces.

But those who were moving to seek safety were following

in the footsteps of those who had moved in search of opportunity.

Together, they had uprooted, to seek out a place that might

allow them to build a better life.

And there was one likely destination,

a city growing fast in size and prestige, Teotihuacan.

At its height, around 200AD, there were over 100,000 people

living in the city and, as these migrants headed north,

they became part of an unprecedented, urban cultural

and economical explosion in the New World.

And, as they approached, they would have seen something remarkable.

This is Teotihuacan and it's enormous,

covering eight square miles.

Spatially, it's all organised around three monumental structures.

Archaeologists have discovered that these buildings were covered

in bright facades, with exquisite architectural motifs, all conveying

an extraordinary sense of opulence.

The city then sprawls out from the centre, with palaces,

temples and housing for its massive population.

The city is dissected by the Avenue of the Dead,

a street that runs for more than four kilometres north to south.

The whole site is built on an orientation of 15.5 degrees east

of astronomical north.

Possibly, in alignment with the Pleiades constellation,

one of the most visible clusters of stars in the night sky.

The city's origins could lie underground, in a cave.

Caves in Mesoamerica were thought of as places of creation

and emergence, so had religious significance.

One cave was considered so sacred by the Teotihuacano

that a structure was built on top of it, to mark its significance.

This is the Pyramid of the Sun.

It's the largest structure in Teotihuacan

and the most impressive pyramid in the New World.

Even without the wooden temple

that would have stood on its top platform,

it's still over 200-feet tall.

It faces west, towards the setting sun

and is the focal point of the entire site.

But it doesn't stand alone.

At the end of the great Avenue of the Dead is the Pyramid of the Moon.

It may have been built in honour of the god Tlaloc,

the bringer of rain and fertility,

which may also be the reason

why it mimics the shape of Cerro Gordo, the sacred mountain which

lies behind it and whose springs provided the city with its water.

It's not just the scale that's impressive, it's the order,

because you can't help but feel small, standing in its midst.

For the people living in this city, these structures

must have made them feel privileged.

But why was architecture on this scale undertaken?

Saburo Sugiyama has led numerous excavations at Teotihuacan

and has been trying to discover what lay behind this city's construction.

We don't know really very much about

the early stage of Teotihuacan formation.

This city was created as a brand city

from the very beginning.

If you look at this,

the scale of the city formations,

we have to propose...

that there is a very organised

political structure here.

Otherwise, you can't control 100,000 people.

There must have been a very strong political organisation.

So, to what extent do you think that the architectural layout here

reflects a level of rulership or level of control at the site?

I think that there is very strong rulers, perhaps,

charismatic rulers, who created, who invented, New World views

and they materialised these ideas -

how worlds consisted,

how we were created -

with God, you know, with suns,

moons and other stars.

All these movements, it's, kind of, interpretations of world views.

These new world views formed the religion that the rulers

used to control Teotihuacan.

The beliefs were incorporated into the very foundations of the city.

The measurements that dictated the dimensions of their buildings

were embedded in their religion and in their physical bodies.

Teotihuacan had a standard unit of measurement,

which was exactly 83 centimetres long.

And it is thought the origins of this unit of measurement

came from the median line of the chest

out to the edge of the fingers of your average Teotihuacano.

And what's amazing is that the entire urban plan

of Teotihuacan is planned out using this standard unit of measurement.

So, if we look at the base of the Pyramid of the Sun,

it's exactly 260 times 83.

And 260 represents the ritual calendar of Teotihuacan.

And if we look at the Pyramid of the Moon,

the base is exactly 105 times 83 centimetres.

And if we add 105 to 260, we get 365, the annual solar calendar.

The 260-day ritual calendar at Teotihuacan

was most likely based around significant religious events.

That it formed the dimensions of the Pyramid of the Sun underlined

a link between the construction of the city and its belief system.

But the belief system was not just reflected in the buildings.

Throughout the city, there is extraordinary artwork,

that aimed to bind a people together through religion.

David Carballo guided me through one of Teotihuacan's

most distinctive, and important, murals.

It seems like this is more than just

artistic representation for decoration. This is telling a story.

It seems to be a Teotihuacano projection of what might

seem like a paradise, a place of plenty and abundance.

We have to remember, of course, we are in a semi-arid environment,

with a really defined wet and dry cycle.

And so, this, sort of, watery paradise would be very appealing.

The state leaders were keeping the rains coming,

courtesy of the storm god up here, and keeping good harvests.

So, we see these flowing canals that are feeding

agricultural fields that have crops that we can possibly identify,

like maze, but also flowering plants, that are being harvested.

So, this is a scene of abundance and prosperity.

One thing to really focus on here is that the emphasis is on

the religious system and not on individuals.

So, you know, there are individuals here,

but they're subordinate to the storm god, who flanks them all around.

And then, this motif up here, which is larger, where there

is two personages, but they are virtually identical,

and they're priests. They're dutifully making offerings

to the central personage, who's debated.

Some believe that it's a female image - a goddess.

I think the state imagery seems to be conveying, over and over again,

this message of, the religious system matters, the deities matter,

attending to them matters.

Individual roles do not matter. Your social roles,

as a priest or a warrior, are what's critical to the society.

We can't pinpoint one individual ruler in this whole corpus

of mural art for the city.

It may offer no clue as to the identity of the rulers,

but the mural is preaching their religious message.

If the people followed, then all would be well in Teotihuacan.

But ensuring the wellbeing of Teotihuacan required more

than religious observation from the people.

One violent public ritual appeased the gods,

while demonstrating the power of the leaders

and the strength of their belief system - human sacrifice.

In the year 2000, archaeologists were excavating inside

the Pyramid of the Moon and what they found inside

was a series of different burials.

This little collection here comes from burial number four, which was

17 skulls found collected together, without the rest of the skeleton.

What's interesting about these skulls is that they were

all found with three cervical vertebrae, which tells us that

the human skull was fleshed when they were deposited,

cos otherwise the cervical vertebrae would have fallen off.

If you look here, you can see cut marks on the back of the vertebrae.

The way that we think these people were killed was with a hard blow

to the back of the head, but it is clear from the way this is

being cut is that they didn't get removed in one blow, but there has

been a sawing process coming through here, to help remove the head.

What's interesting, though, is, if we look at the mandible at the jaw

bone, you can see it would normally be in this position on the jaw.

But underneath the jaw, just in here, you can see a cut mark

coming in, which is coming from the other direction.

So, we are seeing heavy blow on the back, a sawing process,

as the head is cut from behind, but also a blade, a very sharp blade,

judging by these cut marks, coming from the front to remove the head.

So we are seeing a laborious process to remove these heads from the body.

And it gives us a sense that these are definitely people having

their heads removed specifically for the object of placing them

in some sort of ceremony within the heart of the Pyramid of the Moon.

These skulls could have been dedications

to the construction of the Pyramid of the Moon.

But sacrifice was also used

as a political tool, to control the city's diverse population.

But the religious compliance of the people depended upon

their everyday needs being met.

And in a city this size, that required a huge amount of resources.

Even though Teotihuacan was the dominant power

in the Basin of Mexico, by the third century AD, it needed more

than local resources to sustain its population.

It had to expand.

I need to go 50 miles north, to the State of Hidalgo,

to begin to understand how Teotihuacan expanded

into an empire, using trade.

And how, when it came to precious resources, it had one big advantage.

It had access to an abundant source of one of the most

valuable commodities of the time.

It was this - Obsidian or volcanic glass.

It can be styled into artefacts, but most importantly,

fashioned into the sharpest cutting tools in Mesoamerica.

At the time of Teotihuacan's rise, metal wasn't worked

in Mesoamerica, so obsidian was an essential item for everyday tasks.

Because of its value, the supply of obsidian was controlled

by the state, not just for its use in the city, but for trade.

I've got the opportunity to explore an ancient obsidian mine

that is still worked today.

In 2,000 years, almost nothing has changed.

You can still see old mine shafts,

as well as the burn marks from Teotihuacan torches.

They're claustrophobic and remain extremely hazardous.

Mining in this type of soil is very dangerous, cos it's very loose

and light, you can see how easily you can get collapses of tunnels.

So, erm, I'm sure that in these tunnels,

which have been blocked up, the pre-Hispanic tunnels,

there must be skeletons underneath, from cave collapses.

Alejandro Pastrana is an expert in the importance of obsidian

to Mesoamerican cultures

and has been studying the mines in this area for over 20 years.

This unique green obsidian was prized for its high quality,

but also its religious significance.

Throughout Mesoamerica, green was a colour associated

with fertility and with water.

Even though these miners worked a considerable distance

from the city, Teotihuacan's religion

transformed their manual labour into a symbolic act.

As well as the constant threat of collapse, miners only had

torchlight and stone tools to dig for the obsidian

that supplied Teotihuacan with its only cutting tools.

So, this is a block of really high-quality obsidian.

You can see on the one on the wall, the pressure of being underground

has fractured this block of obsidian.

But this one is clean and you could make

some absolutely lovely artefacts, particularly knife blades.

The obsidian blocks could weigh anything up to 500 kilos

and would have to be broken down by hand into more manageable chunks.

After being carried to the surface, the obsidian then entered

the next phase of production.

The obsidian mines are just over that hill

and here we have a Teotihuacan workshop.

This is where the obsidian blocks, being brought out of the mines,

would have been transformed into preforms and artefacts.

And what's particularly interesting about this workshop is that

it is laid out in exactly the same orientation

as the city of Teotihuacan, itself.

The obsidian was knapped, to create blades and artefacts.

Knapping is an ancient and unique skill that transformed stones

into tools and took a lifetime to perfect.

And here is the flake and you can see it, it's a beautiful,

translucent obsidian.

And you can see where the percussion has hit down,

the fracture marks come down, and create a perfect blade.

And this edge is incredibly sharp.

These razor0sharp blades were used

for more than just domestic cutting tools.

The obsidian mines and workshops were worked continuously,

to supply Teotihuacan with weapons.

Thousands of obsidian blades were needed for Teotihuacan's forces,

the largest army in Mesoamerica.

Marco Cervera is an archaeologist, who is going to show me

the Teotihuacan's weapons of choice.

That's good. That's good.

The atlatl is one half of a weapon.

The other half is an obsidian-tipped dart.

In the right hands, the atlatl

can launch the dart more than 100 metres, in all weathers.

So, we've hung up a leg of beef, in order to simulate human flesh,

cos I'm quite interested to find out how the technology

of the armaments of Teotihuacan gave them a tactical advantage in war.

So, the atlatl is a piece of technology

which has been around in Mesoamerica

for thousands of years, going right back to the Lithic age.

But what the Teotihuacan did was put it into an industrial scale.

If you can imagine an army of 10-15,000 throwing these atlatls,

it would have been like a rain of darts

landing on the opposition army.

So, where and when was this fighting force deployed?

It was used primarily to achieve the goals of the state.

And those goals were economic.

Teotihuacan had an extensive network of trade routes.

The bulk of the activity was carried out in what is called

the Teotihuacan Corridor.

Regions that were rich in resources were targeted.

I'm heading to the site of Las Pilas, in the state of Morelos.

It's over 100 miles southwest of Teotihuacan,

which is a considerable distance

when you imagine that the only form of transportation is on foot.

But Teotihuacan didn't want to just trade resources,

they wanted to control them.

This is Las Pilas.

It was a small settlement that came under the direct control

of Teotihuacan.

Their presence can be seen throughout the site.

The structures copy a Teotihuacan architectural style,

called Talud-tablero - a platform on top of a sloping base.

The layout of the buildings is similar to those

back in the capital, with a common platform linking enclosed plazas.

Below ground, archaeologists found more evidence

of Teotihuacan's influence.

Burials excavated in this central plaza contained

extensive evidence of Teotihuacan-style ceramics.

In several instances, individuals were found

with offerings of over 100 vessels of both imported and local wares.

There was a strong reason for this heavy Teotihuacan presence

at Las Pilas - to ensure large quantities of a precious crop

that couldn't be cultivated in the Basin of Mexico.

The climate around Teotihuacan wasn't hot enough

to grow this, cotton. The Teotihuacano needed a lot of cotton,

to spin it for their clothes,

their vibrant textiles, even the armour fibre they used in war.

So, they need to guarantee a reliable supply.

Because of its size,

Las Pilas would have offered little resistance to such

a powerful force.

Local people would have been used

to work the land, to ensure enough cotton was produced to satisfy

the demand back at the centre.

But the demand for raw materials was only going to increase.

As Teotihuacan got bigger, it needed more resources.

Existing trade routes were maintained,

to sustain the power and wealth of the city,

but new areas had to be explored.

They sent out people to conquer and exploit new territories.

I'm driving 300 miles southeast, to the Oaxaca Valley,

which was the stronghold of another ancient civilisation, the Zapotecs.

There had long been peaceful contact between the two civilisations,

but around 250AD, Teotihuacan's insatiable demand

for resources brought that relationship to an end.

This is Monte Alban, the hilltop citadel of the Zapotecs.

From this vantage point, the Zapotecs could rule over the valleys

around or, at least, until the forces of Teotihuacan arrived.

Hundreds of well-armed warriors had marched on Monte Alban,

prepared for conflict.

By the time they isolated the Zapotec elite,

on top of the city's north platform, the battle was already won.

70 of these were found at the top mound of the site.

They're obsidian and they are different types of obsidian,

coming from the mines of the central highlands of Mexico.

So, these come from the area of Teotihuacan.

You can't find obsidian in this part of Mexico.

Many of these points, probably uses as atlatl points,

have broken off at the tips and snapped off at the shaft,

which tells us that they have been broken on impact.

So, this tells me that the contact between Teotihuacan

and Monte Alban is a violent one.

The violence continued after the conflict.

In the wake of their victory, the new rulers of Monte Alban

sacrificed 18 children.

All this bloodshed was over a resource that the Zapotecs

didn't even use, but was coveted by the Teotihuacan...

..a fragile and delicate mineral, called Mica.

The Teotihuacan prized its translucent

and reflective qualities and used it to decorate

their religious buildings and ceramics.

I met up with Marcus Winter, to discover, what were the factors

really driving this aggression?

Do you think the Teotihuacano are expanding for practical reasons,

they need to bring in more resources to sustain their expansion?

Or is it more ideological, that they are trying to dominate

their ideological world view on the rest of Mesoamerica?

I'd say that they wanted to expand, to bring in more resources.

I think, as cities grow, all over the world, what happens

is they have too many people, they need more resources to feed

the people, they need more activities for people to participate in.

Sometimes, they have too many young men with nothing to do and a good

thing to do with them is to send them out to take over other areas.

How do you think life changed here

when the Teotihuacano were in control?

Do you see a thriving community, which is capitalising

on Teotihuacano innovations?

Or do you see a repressed population suffering from a dominating force

-overcoming them?

-I'd say it was a repressed population.

-Yeah.

Because a lot of things stopped.

There was a trajectory that had gone on for about

500 years or more, 700 years, of growth and development,

renewal of the buildings and all of that stopped.

There is still a big population. People lived and continued with their

daily lives and things, but there was not the growth and innovation

that had been here previously.

Teotihuacan's imperial adventures were at the expense of cities

like Monte Alban.

But the drive for resources and expansion didn't stop here.

This site of Monte Alban shows how the Teotihuacan were able

to control regions over 350 miles from the capital.

What's even more incredible is that their influence spread yet further.

We know that at sites like Tikal, over 600 miles in that direction,

in modern-day Guatemala, also felt the influence and power

of Teotihuacan.

The rulers of Teotihuacan had developed a strong religion

that allowed them first to govern a city and now to control an empire.

The economic benefits of trade and empire were enriching the city

but they were also strengthening the position of the rulers.

As an illustration of how this increased power was expressed,

the last monumental structure built at Teotihuacan is significant

and one building within it is of crucial importance.'

It stood at the centre of this massive, 16 hectare,

walled compound called The Ciudadela - the citadel.

The whole population would have been able to fit within its grounds.

The people of Teotihuacan would have come to this space to witness

religious rituals at the building that was now the new

ceremonial heart of the city.

The building was completed in the middle of the 3rd century

and stands remote from the Pyramids of the Sun and Moon.

Archaeologists believe that this construction was a conspicuous

display of power by a new ruler in Teotihuacan.

This is the Temple of the Feathered Serpent,

a mythical creature found throughout Mesoamerica.

It's a potent symbol of power and that is important, because this

temple provides strong evidence that an increasing power of an elite

is administrated through religion, but maintained through fear.

The whole temple structure is a sacred mountain

emerging from the sea.

These crocodile heads marked the beginning of time.

Time began when the crocodile emerged from the water

onto dry land.

The feathered serpents represent the power of the rulers.

These rulers now appeared to be directly linking their power

to the beginning of time.

But that power was not just reinforced

by the grand ornamentation of a building.

If anything pointed to a change in the nature of the leadership

of Teotihuacan, it was the escalation in human sacrifice.

260 victims were found during excavations of this temple...

..one for each day of the ritual calendar and hundreds more

than any human remains excavated from the earlier pyramids.

Sergio Gomez was involved in the excavations at

the Temple of the Feathered Serpent that unearthed human remains.

HE SPEAKS SPANISH

But what archaeologists know

is that, after the temple was completed,

perhaps around 300 AD, there was another change in Teotihuacan.

A large structure was built in front of the temple,

obscuring its opulent facade.

Could it have been a rejection of the power structure

that the temple represented?

Without knowing more about those who wielded the power in Teotihuacan,

archaeologists have struggled to understand the full significance

of the changes centred around the Temple of the Feathered Serpent.

'But Sergio Gomez may be on the verge of a major breakthrough.

'In 2003, he discovered a tunnel underneath the temple.

'It had lain undetected for over 1,000 years, because Sergio

'believes it had been filled in deliberately by the Teotihuacano.

'His initial excavation pointed to this being a very special place.'

'After years of painstaking excavation,

'Sergio and his team have reached the end of this tunnel.

'They could be on the verge of a unique discovery,

'but face a massive obstacle.'

What lies behind this rock could be a first for archaeology

at Teotihuacan - the discovery of a ruler's tomb.

This is backed up by what Sergio has found

amongst the 60,000 artefacts unearthed during the excavation.

Without a shadow of a doubt, this is one of the most amazing

excavations I have ever visited.

Hopefully, when they get into the last chamber,

what they find inside will give us the crucial clues to understanding

what happened at Teotihuacan in the third century after Christ,

because we know that there is a complete shift in power in the city.

Sealing off this tunnel and filling in

a possible ruler's tomb are symbolic acts.

It's impossible to say, outright,

if they were rejections of the existing power structure,

but what we do know is that there is no subsequent evidence

of sacrifice on the scale witnessed

at the Temple of the Feathered Serpent.

And no more monumental structures were built after its completion.

Building did continue, but it was a departure from the past.

'It was no longer structures that honoured the power of the leaders,

but something that could point to a cultural and political peak.

The state was now rebuilding the city, to house its population

in a style and scale that had never been witnessed before.

This is urban planning on an unprecedented scale.

The old adobe structures were torn down and replaced with these -

buildings of stone, concrete and plaster.

But what's really interesting is that the architectural style

is uniform throughout the city.

There was access to running water, a drainage system.

All the compounds were walled, to give the people their privacy.

Outside, there were streets and, for the first time in Mesoamerica,

the entire city was now a huge grid.

'I met up again with David Carballo, to see one of the best remaining

'examples of Teotihuacan housing.'

What makes it residential? If you look around the architecture

of the place, what is it we are actually looking at?

In a typical pattern in central Mexico

and other parts of Mesoamerica are folks inhabiting a patio.

In fact, a lot of the indigenous terms for a household

are a shared patio space and so, over here, we have a relatively large one,

with nicely-made masonry architecture and, so, one of the lines

of evidence that this is a higher-status residence

is that you have these large cut blocks.

-These are original stairs?

-Yes, these are the original stairs

that went up to some, sort of, temple complex and then,

these smaller structures around, that have a frontal portico,

leading to a back room, were likely residences

-of higher-status family members.

-How many of these are there

in Teotihuacan? And how does that population grow?

It's estimated that there is about 2,300 of these

and that each, sort of... a mean population, might have been

in the sixties, 65. You have some that could push into the hundreds

and some that seem smaller - in the dozens of people.

And because of that, because we can count apartment compounds

and estimate roughly their population, we do have some

decent estimates for the city, as a whole, during its apex

and that it was 100,000-plus inhabitants in the 4th-5th century.

Just for comparison, London didn't break 100,000 people

till 1,000 years later, in the late 16th century.

So, this was a very large place. Likely, the largest city

in the Americas for a few centuries.

To what extent do you think that the expansion is planned by an elite?

Because looking after 100,000 people in one centre

-is a pretty challenging thing to do.

-I think that there is some...

There must be some, sort of, mix of civic top-down planning

and more grass roots,

organic processes happening at the neighbourhood level.

So, you know, we do see these structures are all on a similar

orthogonal, or grid-like, system and they seem as far away

as three kilometres from here to also be hewing to that orientation,

so that seems to suggest some significant planning.

This house building may have reflected

the growing prosperity of the city,

but it could also be an example of how the style of leadership

had changed.

If there was a rejection of rule by violent coercion,

then it may have been replaced by a subtle, but still effective,

message that encouraged the population to think as one.

It's been referred to as civic pride and it was sold as a virtue.

And, if widely accepted, it's a far more effective way

of maintaining stability than the use of force.

It's not very different to how we survive

in the modern world today. So, the supply of housing

in Teotihuacan could have encouraged the sense of collective will -

all working towards the same common goal.

But how was a political message as subtle as civic pride

spread throughout the population?

A discovery in the 1990s could be evidence of how

these political messages were spread throughout the Teotihuacan.

Archaeologists had no evidence of a Teotihuacan writing system

until a series of symbols were uncovered on the floor

of a temple complex called La Ventilla.

They have been dated to the middle of the fourth century AD

and Ruben Cabrera Castro thinks they may have been part

of an education system.

The placing of the glyphs were first thought to be random,

but Ruben thinks the horizontal and vertical lines on the floor

could be ordering these symbols into the pages of a codex -

an ancient book.

These glyphs, with the lines dividing them,

that Ruben's shown me, I'm completely convinced by his argument

that they are a codex. They are very much a form of writing,

they can communicate information between generations.

If we start to think about Teotihuacan as a multi-ethnic,

multicultural city, how is it that that culture

is bound together by an ideology?

For me, writing is crucial for communicating information

through generations and binding Teotihuacan together.

A possible education and writing system are evidence of how

Teotihuacan had become a politically-complex civilisation

and it underlines why, in 500 years, it had grown from

unspectacular beginnings to the biggest city in the New World.

It had grown through strong leadership,

driven by religious ideology.

And its empire was built on trade, but secured through force.

Every society reaches a peak,

what would normally be called an apex,

something to be celebrated

as a high watermark of political and cultural achievement.

But it is also the point in which culture can rise no further.

At what point would Teotihuacan's expansion stop?

Given its size and its power, what could possibly stop it?

A city of this size was costly to maintain

and there were constant stresses in preserving its prestige.

The religious system served to support the city's power structure,

as well as subdue any possible threat to the leadership.

Teotihuacan's empire had to be protected,

to secure the resources that fed its people and enriched the elite,

but the leadership, itself, could only survive

as long as the city thrived.

It's a cruel irony that the very natural forces that gave

Teotihuacan an advantage in its rise to power

could now be responsible for triggering its demise.

In 536 AD, there was a moment of sudden environmental change

in the Basin of Mexico.

Exciting new research suggests that this could have been a key factor

in pushing the Teotihuacano to the brink of disaster.

It was the biggest volcanic eruption ever recorded in central America.

The Ilopango volcano was in modern-day El Salvador.

Even though it was almost 800 miles away, the ash and gas

it spewed out from the eruption clouded the skies over Teotihuacan.

With no heat and light from the sun, the crops failed repeatedly,

which had grave consequences for the leaders.

The stability of Teotihuacan relied on its people perceiving

that their needs were being met.

If there was a downturn in their fortunes,

the next chapter in this city would be violent.

In the middle of the 6th century AD, the people of Teotihuacan rose up.

They targeted the symbolic heart of the city and set it alight.

David Carballo was part of the team that retrieved charred remains

of the wooden temple structures that lined the Avenue of the Dead.

Burned roof structures gives this sense of drama,

of things on fire and ending in quite a dramatic way.

-Do you think that is true?

-Certainly for the central part of the city.

It seems like very selective, concerted burning, within temples

and palace complexes all around the Street of the Dead.

That whole street would have been on fire at the same time,

-you'd imagine?

-Perhaps. It seems like a planned burning event.

It was a symbolic termination of the civic architecture of the city.

What do you think brings about...

What motivates people to come and bring up arms

and take against the elite?

There must have been something in the system failing people

and so, whether that's the city became more factionalised

and there is more internal squabbling.

Also, that trade routes seemed to be co-opted by areas that were former

provinces and that were formerly under the sway of state

and so people here in the centre, perhaps weren't having

the ready access they once had to all sorts of goods around Mexico.

Do you think the decline of Teotihuacan was inevitable,

because of the way that the state rose up over time?

We have to consider Teotihuacan a successful state, in that

it survived for five to six centuries as, you know, possibly

the largest place in the Americas, one of the largest cities.

But there certainly were some challenges in the 6th century

that the governing apparatus seems to not have been able to overcome.

And so what exactly it was that they couldn't evolve to

is still a question to be answered.

In the wake of the revolt, the state system collapsed.

The religious ideology that had bound the people together

had been rejected.

First to leave were the elite, whose power structure had been attacked.

A population of 100,000 eventually dwindled to 20,000 people.

The might of Teotihuacan existed in the past.

It would take hundreds of years for the significance of this city

to be rediscovered - by another great civilisation.

When the Aztecs discovered this vast, abandoned city,

they believed it could only have been the work of giants, even gods.

When they arrived it had no name.

They called it Teotihuacan - the place where time began.

And now, almost 2,000 years since this city was built,

it's still immensely significant.

An ancient place that, for me, echoes the concerns our urban world

still struggles with today.

Teotihuacan is an extraordinary city

and revealing some of its secrets, getting to know its inhabitants

has transformed my understanding of urban life.

Architecture, people and power come together

to create a new type of culture,

a fragile urban culture that sets the New World on a pathway

that is to last thousands of years.