The Assassin (Sha ren zhe Tang Zhan)

Today, on "Impossible engineering,"

the Shard, a record-breaking super tower...

Construction never been attempted before on a building

this tall in central London.

Taking structural design to new heights.

It's amazing to think the first 21 stories

of this concrete structure

went up before they'd even finished the foundations below.

It took revolutionary engineering...

Look at the crazy amounts of glass

that this building uses: 11,000 separate pieces.

What a stunning sight.

To make the impossible possible.

captions paid for by discovery communications

Some of the world's most iconic buildings

dominate its skyline.

And, in 2012, a modern marvel

was constructed, altering the city's look forever.

Meet the Shard.

At 1,016 feet tall,

this futuristic skyscraper is the tallest in London.

Wow. Look at that.

It looks like a piece of crystal,

a splinter coming up through the earth.

It's so tall.

And it's such a prominent part of the skyline

that you can't help but notice it.

That's an incredibly audacious piece of architecture

and some very impressive engineering.

The Shard rises up from the heart of downtown London.

This jaw-dropping tower is over three times

the height of the statue of Liberty.

Its facade is made

out of a staggering 11,000 glass panels.

That's enough glass to cover 130 basketball courts.

Beneath the 196-foot spire lies the spine of the building.

The colossal concrete core supports 72 levels,

totaling over a million square feet

of floor space.

Building these buildings is always exciting.

You're building up taller than anybody's gone in Europe.

But this one was particularly difficult.

The population in London is surging.

It's estimated that the city

could reach 10 million inhabitants by 2030.

With limited room to grow,

designers of any new buildings are looking to the skies.

Looking across the London skyline

as we approach from the south,

you can really see, it's just crowded.

There's buildings everywhere.

Building in that tight space is a very, very difficult task.

Finding enough open space

to build a mega tower in this bustling city

is a seemingly impossible challenge.

It's in this congested site

just in the corner of a railway station,

things going on all the way around it.

We had no spare site in which to build it.

It's smack in the center of London

with London bridge station on one side,

guy's hospital tower on the other,

the jubilee line of the two passing very close underneath.

And to add to all of that complexity,

there was a building already on this site

that needed to be removed before construction could begin.

The 25,000-ton weight of the old tower

is problematic for the engineering team.

When you put a building on the ground,

the very heavy weight compresses the soil.

It's like a sponge.

It creates a divot where the building is.

And when you take that weight off,

the ground wants to heave,

wants to respond and go back to where it was originally.

And time is of the essence.

You want to put the weight back onto that ground

as quickly as you can.

Building the Shard on this site

would be impossible without help from a great innovator

from the past.

When it comes to building up... - There we go.

Man has always defied the odds.

Well done. Well done.

The builders of ancient Petra carved vast structures

out of sandstone cliffs

in the Jordan desert... - Very good.

Safely perched on a series of giant stone steps.

Oops.

Sorry!

In 12th-century Italy,

warring noble families in bologna

erected over 100 towers.

Bigger was definitely better.

Mama Mia, what a whopper!

Perhaps the first skyscraper city

was 16th-century Shibam in Yemen.

Adopting ancient techniques, its engineers used mud bricks

to build high-rises that were so impressive,

it's known as the Manhattan of the desert.

In the 1950s, the bustling Italian city of Milan

wanted to build a subway.

But tearing up the city streets simply wasn't an option.

Engineers needed to figure out a way

to build without disrupting city life.

We've come here to a site

where the Metro network is being expanded.

Being here and seeing the scale of this site,

you can imagine the tremendous disruption it would be causing

if you tried to do this in the center of the city...

Really an enormous challenge.

Poor soil conditions made tunneling

beneath the city streets nearly impossible.

This is actually a really good illustration

of one of the key problems here in Milan.

We're down here at the bottom of the excavation.

And you can see this is an area

where they're just getting ready to start to dig a tunnel.

And the engineers have gone to really heroic efforts there.

Around that wall, you can see they've put some rubber gasket.

And they've really tried to keep the water

out of the site.

But, in spite of that, you can see

how much water is still flowing in.

So these are about the worst conditions

you could hope to be digging tunnels in.

Engineer Dr. Christian Veder

came up with a brilliant solution.

Before Veder, a technique

called cut-and-cover was the go-to option for tunnelers.

Let's imagine that I want to dig a trench

down between these buildings.

And you can see what happens

when I do that in this Sandy soil.

Initially, there's no problem.

But if I push just a little bit too far,

you can see that, eventually, I destabilize the soil

and my structures will fall into the trench.

And, obviously, that's completely unacceptable

on a site in a congested urban center.

Veder turned the cut-and-cover concept on its head.

Instead of building one big trench initially,

he built two little trenches down the sides of the buildings.

And into those trenches,

he inserted reinforced concrete walls.

And these then became known as diaphragm walls.

After dropping in diaphragm walls,

Veder built a roof over the tunnel,

allowing city traffic to resume above.

Digging, tunneling and construction

could take place without disrupting life in Milan.

Instead of cut-and-cover,

Veder's technique covered, then cut.

It's now known as top-down.

This method was adapted around the world as a way

of completing major subterranean projects with little

to no interruption to the world above.

So what Veder achieved here in Milan

was to pioneer a really innovative construction method.

And it's fair to say that, standing down here

in one of the new tunnels of the Milan Metro,

it simply wouldn't exist without that construction technique.

In 2009, engineers at the Shard

take Veder's top-down method and turn it on its head.

Normally, a building like this would be built

by building a basement first and digging a big hole

down to the bottom level of the basement.

And then starting the... The core from that lowest level

and building upwards.

Now, we built the core on stilts effectively

but held up the core

while we were building it upwards.

And then we were, at the same time,

digging underneath it and going downwards.

That had never been done before.

It was an innovation for the Shard.

Just 23 piles support the Shard's concrete core

as it rises from a void in basement level two.

As excavations of the underground floors

continue around the pre-sunk columns,

the core rises as if balanced on a tabletop above.

This allowed the engineers to shave, literally,

months off this project.

And the Shard was built much more quickly

and much more cheaply than it could otherwise have been.

This really is an engineering marvel.

Pouring the Shard's 800-foot concrete core

requires an epic engineering solution.

It's amazing to think the first 21 stories

of this concrete structure

went up before they'd even finished the foundations below.

At 1,016 feet, the Shard in London

is taller than the Eiffel Tower.

It has 72 habitable floors,

44 elevators and 306 flights of stairs.

Building this mega tower

required almost 2 million cubic feet of concrete.

The Shard is an incredible building.

It just stands high, proud above the London skyline.

And this incredible bit of architecture really