Unearthed

The great gothic cathedrals of Europe,

soaring medieval skyscrapers that hold age-old secrets.

How did their creators build so high

using only simple tools?

What beliefs inspired them to reach for the sky?

It is an incredible building.

The height, the footprint, the presence...

It's really spectacular.

And can new technology unlock the secret science

that protects these stone giants from disaster?

They had to be afraid

that the earth pressure would bring the pits to collapse.

The only way to solve these mysteries

is to blow apart these mighty, medieval megastructures

stone by stone.

Diving deep through their walls, vaults, and foundations

will help us unearth the astonishing engineering secrets

at the heart of these age-old wonders.

captions paid for by discovery communications

the German city of Cologne

on the banks of the mighty rhine river...

This is the home of the world's most ambitious megastructure.

Cologne cathedral...

a mysterious monument that began life in the middle ages.

750 years ago, the people of this city

embarked on a colossal challenge

to build the tallest cathedral of the day...

20 times higher than the wooden buildings in town.

The sheer scale of this church

astounds modern experts who try to unlock its secrets.

The amazing thing about the Cologne cathedral is,

if you go back in time, someone actually had the vision

to design and conceive a building of this size,

and it's spectacular.

Cologne cathedral is a 130,000-ton stone giant.

There's enough glass in the walls

to clad a 30-story skyscraper.

Huge stone ribs prop up the ceiling...

as well as the 600-ton lead roof.

Stretching nearly 525 feet into the sky,

two enormous towers once made this church

the tallest building on earth.

So how did they pull off this medieval marvel?

Historians know surprisingly little

about what actually went on

during the early days of Cologne's construction.

Few records have survived.

Now architect Doug Pritchard

is rediscovering how the cathedral was built

by stripping it apart stone by stone.

Doug's got the perfect tool to do this...

A digital laser scanner.

This machine will allow him to reveal the building

in unprecedented forensic detail.

The scanner is really quite incredible.

It can generate

approximately 1 million dimension points per second.

The way that the scanner works

is that it does a 360-degree sweep.

It will show things like the depth of the vaults,

the size of the columns, in a very precise way.

We take all that data together,

and the result is you have a 3D map of the cathedral.

Today, Doug's adding

another important piece to his puzzle.

Guten tag.

Hi.

He's on his way to the central balcony.

If he can use scanning

to expose the cathedral's underlying structure,

he can start to dissect how it works.

One of the great things about laser scanning

is that you could pull the data apart.

Let's use the scanned data to explain the architecture...

How the building stands up, how the engineering works.

Doug's scanner fires an invisible laser beam

as far away as 614 feet.

It records each time the beam hits something solid

and bounces back...

and then turns this data into incredibly detailed images

so that Doug can study the structure from every angle.

In total, right now, we're at about 660 scans.

Probably, by the end of this week, add in another 20 or 30.

Doug now needs to crunch a huge volume of data.

But he can already start to see the master plan

of the people who built this incredible monument.

The two tall towers are visible for miles.

At the top, the spires are open to cut down wind resistance.

And at the bottom, the walls are rock-solid

to bear the enormous weight of the towers.

But the church behind is completely different.

Under the skin sits a skeleton of slender stone arches.

Incredibly, it looks like the core of a modern skyscraper,

yet this is the brainchild of medieval master builders...

Men who lived 700 years ago.

So how did Cologne citizens even begin to imagine

that such a tall building could be possible?

200 miles away, in northern France,

another team hunts for clues at an even older cathedral.

This is Amiens... The blueprint for Cologne.

This soaring church has dominated the city's skyline

for nearly 800 years.

It's the tallest complete cathedral in France.

Investigator Guillaume Caron uses laser technology

to unlock how Cologne's predecessor was built.

I was born, actually, in the city,

so I know the symbolic aspect of this cathedral.

Guillaume is hoping to find tiny clues

that might help reconstruct Amiens' lost original design.

Currently, there is not any actual map of the cathedral,

so what we are doing with this kind of device

is to measure it very precisely

in order to have the first real, actual map of the cathedral.

Amiens' construction began in 1220.

It's one of a handful of cathedrals close to Paris

whose architects dreamed of building closer to god.

They wanted to build the highest and the widest cathedral

because at that time in the middle ages,

it was like a race between the different cities in France

to build the most important cathedral.

Guillaume's scans reveal

that the central arch is 131 feet high,

but the columns are surprisingly thin.

From this scan preview,

we can look at the details that were acquired by the device.

For instance, the columns here in the transept.

But until the 12th century,

columns this tall and slender would have been too weak

to support such an enormous building.

So how did French architects solve a puzzle

that has baffled even the great builders of ancient Rome?

The key is in the shape of the arches.

In a classical Roman arch,

the stones at the top push sideways against the columns.

As you build higher, they start to buckle...

And the ceiling will collapse.

But French engineers realized

that if you make the arch pointed...

The forces now flow down towards the strong base of the column.

Now the same columns can take more weight.

The arch can be much taller and the church much bigger.

Amiens, with its pointed arches,

showed Cologne citizens a glimpse of what was possible.

But what drove the Germans to build even higher?

The secret lies at the heart of Cologne cathedral,

inside this golden shrine.

Some believe it holds the bones of the three wise men.

Peter Fussenich

is the cathedral's current chief architect.

He oversees an army of more than 70 people

who keep this building running.

Today, the church attracts 6 million visitors a year.

But Peter thinks that in the middle ages,

it was also this tomb that was the star attraction.

The shrine is, of course, one of the reasons

why they built Cologne cathedral in the first place.

It's less the shrine... That was built later.

It's more what's inside it.

I'm talking about the bones of the three wise men.

This holy relic arrived in Cologne in 1164

and immediately triggered a huge stream of pilgrims to the city.

That meant that very quickly, the old church became too small,

which is why they had to build a bigger cathedral.

This mysterious holy shrine

inspired Cologne's founders to start work on a cathedral so big

that they knew they would never see it finished

in their lifetimes.

This is something

that's still immensely fascinating for us today.

Cologne cathedral

is one of humanity's first-ever multigenerational projects...

Something you have to hand down to the next generation

and have faith that they will actually carry on building it.

These men hoped that their children and grandchildren

could overcome the next huge obstacle...

how would they build a church with enough room

for 20,000 worshippers made from fragile walls of glass?

Cologne cathedral is a mysterious medieval skyscraper.

It covers 86,000 square feet.

How did her creators illuminate a church this big

with heavenly light...

Hundreds of years before electricity?

The answers are hidden deep

within the fabric of the cathedral.

The towers rest on thick, stone pillars

to stop them from collapsing.

But the main hall of the church is much more open.

There's enough glass here to cover a football field.

The upper walls in the middle are almost entirely glass,

just inches thick.

And they're crowned by a massive lead roof.

So how did medieval engineers construct windows so big,

so fragile, yet so strong?

In the middle ages,

craftsmen had no way to make huge, flat sheets of glass.

Ulrike brinkmann studies the ingenious solution

that allowed them to push the boundaries of technology.

Medieval glassmakers divided up each window into panels.

They made them from tiny stained-glass fragments

held together by lead.