The Assassin (Sha ren zhe Tang Zhan)

Today on "Impossible engineering,"

the world's toughest structures.

The largest hydroelectric power station in the world.

And a football stadium of record-breaking proportions.

It's the longest continuous single arch span in the world

and, really, an incredible engineering feat.

It took revolutionary engineering

to make the impossible possible.

Captions paid for by Discovery communications

China, the world's most populous country.

At 1.3 billion people and rising,

the country's infrastructure is under immense pressure.

Living in downtown Shanghai, it's so easy to see

how much energy is consumed on a daily basis.

In this city alone, population has grown to 24 million people.

To sustain this many people,

the country consumes almost as much coal

as the rest of the world.

But China needs a more sustainable way

to keep the lights on.

Their solution... the three gorges dam,

the largest hydroelectric power station

in the world.

It's over 7,500 feet long,

that's 21 football fields,

and holds back a 400-square-mile reservoir.

Construction began on this incredibly tough structure 1994.

Getting the build right is a matter of life and death

for the millions of people

living further along the Yangtze river.

With millions of tons of water pushing against the wall,

the residents downstream

are really depending on this wall to stay up.

Any imperfections

and the consequences could be catastrophic.

A concrete structure of this magnitude

would be impossible without one of America's

greatest engineering achievements.

Wow.

This is the hoover dam.

At the time of its construction,

this was the world's largest concrete structure

that had ever been built.

Even today, roughly 80 years later,

it takes your breath away.

Weighing in at 6.6 million tons of concrete,

this was an unparalleled engineering marvel.

The dam harnesses the power

locked within the mighty Colorado river.

The dam stands 700 feet tall

and has a base thickness of 660 feet.

The biggest problem

and the biggest challenge was one of sheer scale.

But the severe southwest heat

makes building a structure as big as the hoover dam

extremely difficult.

If engineers poured all of the hoover dam's concrete

in one go, it would take 125 years

for it to cure and cool, meaning uneven setting

and potentially catastrophic cracking.

Hoover dam project supervisor

frank Crowe came up with an ingenious solution,

one that can still be seen deep within

the old inspection tunnels running through the dam.

The solution was to pass extremely cold water

through one-inch pipes.

The amazing thing is we can actually still see evidence

of those pipes here.

In 1931, thousands of workers began building the hoover dam

using gigantic blocks, cooling the concrete

with ice water produced by a refrigeration plant.

As an engineer, this is an incredible sight to see.

Over 80 years later, the hoover dam

still provides electricity to three states.

The three gorges dam is five times bigger

and generates an incredible 11 times more power

than the hoover dam.

Building it requires

almost a billion cubic feet of concrete.

In 1998, pouring begins.

To accelerate the curing process

and reduce the risks of cracking,

engineers take techniques pioneered at the hoover dam

to a whole new level.

The ingredients are air cooled before they're mixed.

High-speed conveyors take the concrete from mixing zone

to site in just 15 minutes.

Workers pour an average of 700,000 cubic feet every day.

Water cooling is supplemented

with a mist sprayed over the working area

to reduce the effects of the hot summer weather.

It takes eight years to pour all the concrete.

This massive structure harnesses

the clean hydroelectricity potential

of China's largest river, the Yangtze.

The Yangtze river is the third largest

and longest in the world.

And the river is now the lifeblood

for the thousands of people who live along it.

But blocking ship traffic on this busy waterway

with a giant dam is simply not an option.

To get the ships through,

the three gorges team are employing a technique

that dates back to medieval times.

The lock is almost a mile long.

It raises and lowers river traffic 370 feet

through five giant steps.

But there's a catch.

Taking four hours to pass through the locks

is simply too slow for most ships traveling down the river.

So Mr. Ding and his fellow engineers

must come up with another solution, and fast.

Tough enough to hold back the mighty Yangtze river,

the three gorges dam

is the largest hydroelectric power plant in the world.

But getting vessels through the dam's

massive ship locks quickly would have been impossible

without the pioneering engineers of the past.

In 1870, designer Edwin Clark was asked to solve

a particularly tricky problem

facing the small village of Anderton

in the north of England.

He was asked to link the busy Trent and Mersey canal

with the adjacent Weaver river to speed up journey times

for barges carrying valuable commercial cargos of salt.

This was a pretty stiff challenge for Clark

in the late 1800s because the height between the canal

and the river was about 50 feet.

So Clark developed an ingenious solution.

Known as the iron spider,

the Anderton is the oldest operating boat lift

in the world.

It's extraordinary.

It's a three-story-high marvel of Victorian engineering.

Clark's creation scoops up barges

and the water they're floating in,

transferring them in one smooth action.

At the time, this was revolutionary,

but the iron spider is based

on a fundamental principle of water pressure.

If you apply a pressure on a liquid in a closed system,

then that pressure is distributed

throughout the whole liquid in the system.

And Edwin Clark used this to great effect

with his boat lift at Anderton.

He started off by making two watertight caissons.

These are the tanks which held the boats

and the water in which they floated.

And he supported those caissons on top of two hydraulic rams,

and there was a liquid in those rams

and a pipe joining them both together.

So as I exert some additional force

to this hydraulic ram,

I can start to see

that the pressure is being pushed through

the adjoining pipe and lifting up my other hydraulic ram.

It's brilliant.

At the three gorges dam, designers are building

a ship lift similar to Edwin Clark's,

but on an epic scale.

It feels like I'm in a big, massive swimming pool,

and I'm being pulled up.

It's an engineering feat, a beauty in so many ways.

And super impressive.

The three gorges ship lift

can carry a 3,000-ton passenger liner.

Reinforced concrete towers

support the lift's 433-foot steel pool.

Instead of hydraulics,

the lift uses massive counterweights and pulleys

to raise the pool and vessels floating in it

a vertical distance of 370 feet.

This world record-breaking lift allows ships

to pass through the dam quickly and easily.

Stretching nearly a mile and a half

across the Yangtze river,

it holds back an almost 400-square-mile reservoir.

But excessive water can produce serious consequences.

Now, if we imagine a flood situation

where we have lots and lots and lots of water

flowing over this dam,

we've got all that water hitting the bottom of the dam.

As we can see here, the integrity of the dam

becomes very questionable and it starts to collapse.

Oh, there it goes.

To prevent this,

engineers had to look to the past for inspiration.

The Marèges dam in central France

is almost 300 feet tall.

Its builders feared overflowing water

would destroy the foundations,

eventually leading to its collapse.

To prevent this, engineer André Coyne

developed something novel.

This is a ski jump spillway,

so called because it has a lip at the bottom,

just like a ski jump.

And the ski jump prevents that water carrying

all the way down to the base of the dam,

where it can seriously erode the foundations.