The UnXplained

A legendary treasure of gold

carefully hidden for 500 years.

A terrifying tower of skulls

marking the madness of a once-great empire.

And rare artifacts believed to invoke

mystical and frightening chaos.

Five centuries ago, in what is now modern-day Mexico,

the Aztecs built one of the most powerful empires

the world has ever seen.

It was a sophisticated society

focused on engineering, agriculture, education, and

a brutal religious practice that included human sacrifice.

But despite their immense power, much of the Aztecs' history

has been lost or destroyed,

leaving only fragments to piece together

a once-mighty civilization.

What mysteries are yet to be revealed

about the Aztec Empire?

Well, that is what we'll try and find out.

This expansive metropolis is the largest city

in North America.

And hidden beneath the colonial-era buildings

and modern architecture

lie the ruins of a once powerful and feared civilization.

From the 14th to 16th century,

this place was known as Tenochtitlán,

the great capital of the Aztec Empire.

It's a very modern city, Mexico City, and yet,

just below the surface,

is this great history of the civilization

that founded the city.

It's called Tenochtitlán.

And underneath the ground in Mexico City,

there's temples, there's pyramids.

Every time you go to build a road,

who knows what you'll discover?

Because somewhere around the 13th century,

the Aztecs appeared in central Mexico

and very quickly built this massive, powerful empire

that so fascinates us, but about which we know so little.

Only 500 years ago, the Aztecs

were the last great civilization in Mesoamerica.

So why is their culture such a mystery to us?

Many point to the Spanish invasion led by Hernán Cortés,

where, from 1519 to 1521,

a campaign was launched to destroy

the mighty empire.

At the time of the Spanish arrival...

the Aztecs controlled a large swath of Mesoamerica...

reaching from what is now Mexico City,

all the way down to the border of Guatemala.

It was run by Montezuma in 1519.

He's been ruling the Aztecs for 18 years or so.

He's treated as a god.

He has effective control over many millions of people.

But ultimately, the Spaniards came in,

and they would raze to the ground

and set ablaze

everything that reminded them of the Aztecs,

and that was devastating.

The Siege of Tenochtitlán

actually takes about 80 days.

They physically buried some of it.

The Spanish built buildings

over the top of existing Aztec infrastructure.

They eradicated culture and language,

and because of that, we have only fragments.

What we don't know

about the Aztec culture

is so much bigger than what we do know.

So much of what we know about the Aztec Empire

we know from the people who destroyed it,

from the conquistadors from Spain.

And so, we can't be sure

how much interpretation was being made there,

how much exaggeration,

how much work was being done

to justify the complete destruction of this culture.

While much of their history was destroyed

or rewritten by the Spanish conquest,

there are a small number

of authentic Aztec illustrated manuscripts,

called codices, that survived,

offering a glimpse of the mighty empire.

And according to scholars, before it was destroyed,

the Aztec capital of Tenochtitlán

was a true wonder to behold.

The origin of the Aztecs

is part myth and part history.

But most everyone agrees they were

a nomadic hunter and gatherer group

that settled on the banks of Lake Texcoco.

In the early 1400s,

there was a natural island there,

and then, they expanded that island

that eventually became this amazing city of Tenochtitlán.

When the Spanish first saw Tenochtitlán,

they were just amazed.

The city itself was-was clean and beautiful.

They had canals.

They had raised fields where they grew

crops very efficiently.

I don't know if anyone has figured out

how they acquired their technology.

Tenochtitlán was larger than just about anything in Europe,

perhaps except for Paris.

We estimate that it had somewhere between

150,000 to 250,000 people.

It was planned along a grid

that was connected by causeways to the mainland.

It had two flourishing marketplaces.

There was a compulsory education system.

And, very importantly for the Aztec people,

it was seen as being the literal center of the universe.

We know the Aztecs had math.

We know they had astronomy.

We know they had religion and artistry.

They had this huge agricultural system.

Fresh water, magnificent palaces and terraces.

They were a very unique culture.

But the Aztecs were a fairly short-lived empire

that still captures our imagination

in part because of some of their proclivities

for human sacrifice, for their interest in death.

But there's a lot of unanswered questions about the Aztecs.

We lack sort of those-those fundamental

texts from the Aztecs.

Utility workers digging at a site

in Mexico City's historic center

uncover a massive, circular Aztec stone

depicting a dismembered and beheaded goddess.

This astonishing discovery launches ongoing,

official excavations,

which reveal what is considered the holiest site

in Aztec society,

the great temple of Tenochtitlán...

Templo Mayor.

Templo Mayor is the largest monument

in the sacred precinct, the center of Tenochtitlán.

And, it was built in the late 1400s.

It was an impressive structure

that stood perhaps 200 feet high or so.

It was a twin temple dedicated to two of the Aztec gods.

On one side was Tlaloc, the god of rain or water.

On the other side was Huitzilopochtli,

the god of war and the Aztec principal deity.

The excavation of the great temple

is a gift that never stops giving.

Imagine that, in the last 40 years,

this excavation has continued, and every year,

sometimes every month, sometimes every week,

something sensational and significant has been found.

The Aztecs actually buried in the temple

over 200 offerings, some in boxes, some in the open.

And these were offerings to make the divinities

protect and nurture the Aztecs.

The whole Aztec world was deposited

in the floors of the great temple,

but you know, they've only excavated,

ten, 15% of what's there.

So, there's a great deal to be explained in the future.

With each excavation, clues to how the Aztecs lived

are carefully pieced together.

But there is one surprising mystery

that has been eluding academics for years.

It is the simple question:

where did the Aztecs come from?

The term "Aztec" comes from their homeland,

which is a place they called Aztlán.

And Aztlán we don't know much about.

We know that it was in the north,

and we know that it took 250 years

of wandering to get to the site of present-day Mexico City.

The location of wherever they were coming from is a guess,

but they certainly came from somewhere.

It's puzzling to think that so little

is known about the life and origin of the Aztec people.

But perhaps the truth of their mysterious homeland

can be found in a modern-day quest to find

the lost treasure of King Montezuma.

Montezuma II, the last true Aztec emperor,

welcomes Spanish conquistador Hernán Cortés into his city.

Soon after, the great king is taken captive

and Cortés turns his attention

its vast treasure.

When we meet Cortés arriving at the shores of Mexico,

He's driven by a lust for gold.

So, while Cortés and his men are in the palaces,

remarkably, one man finds this room.

It's a giant room, and it houses

the spoils and tribute and treasure of

many Aztec leaders over the centuries.

And this place is filled with gold, silver, jade.

It's amazing, and

in today's value, probably worth...

the gold alone... on the order of

three to five billion dollars.

And so, they start to pack some of this stuff away.

They're basically pillaging Montezuma's treasure room.

Obviously, the Spanish made away with some treasure.