Original subtitles

[reverent music]

- [Narrator] The world's greatest structures

push the boundaries of engineering,

all fueled by a constant desire to innovate.

- Without engineering, there would be no modern world.

- [Narrator] Gigantic buildings,

complex infrastructure,

and ingenious inventions.

- Engineering is the key that turns dreams into reality.

- [Narrator] Many of today's incredible achievements

rely on breakthrough technologies

first devised by ancient engineers.

- It's astounding how they achieved this.

- [Narrator] Early civilizations

built on an unimaginable scale

and with incredible precision.

- The raised the bar for engineering

in a way that no one thought possible.

- These are some of the finest engineers in history.

- [Narrator] Redefining the known laws of physics

and dreaming up the impossible,

they constructed engineering wonders,

from colossal stadiums to mighty waterways

and complex machines, all with the simplest of tools.

- You can not imagine the skills

people would have needed to build like this.

- [Narrator] By unearthing the mysteries

left by these ancient engineers,

we can now decode their secrets-

- That so many of their creations still survive

is testament to their engineering prowess.

- [Narrator] And ultimately reveal how their genius

laid the foundations for everything we build today.

[reverent music continuing]

Nothing has defined human existence

like our relationship with water.

- Water is essential for humanity

and always has been since ancient times.

We all need to drink it.

We can't survive without it.

- Agriculture is the most important part

of modern civilization.

Water is the key to that.

- [Narrator] And waterways have played a vital role

in the development of cultures and the growth of societies.

- You can't underestimate the importance of waterways.

They've united empires,

they facilitated the movement of people and goods

all over the world.

- These great civilizations achieved their prosperity

because of their control of great waterways and canals.

- [Narrator] Since ancient times,

engineers have invented ways to exploit water

for human benefit.

- We all owe ancient engineers huge thanks

for their work in mastering water

to allow us a way to navigate,

to transport goods, and to build economies.

- [Narrator] And where land didn't exist,

we've even created it on water.

- Nothing could stop these ancient engineers dreaming big.

- [Narrator] But water is also a destructive force

we've had to overcome.

- We've had to engineer ways to protect ourselves

and our homes from this enormous power.

- [Narrator] Pioneering structures like Palm Jumeirah

and the Burj Al Arab prove there are no limits

to our creativity in building on water.

- So, this is harnessing water

using the most advanced modern engineering.

- [Narrator] But how did ancient engineers

first unleash the potential of waterways?

[gentle music]

The first manmade waterways

were built for transporting water itself

in one of the world's earliest civilizations:

ancient Mesopotamia, a land situated on the floodplains

between two great rivers, the Tigris and Euphrates.

From the 5th millennium BC, it's southern half was home

to one of the most innovative societies in history,

the Sumerians.

- Ancient Mesopotamia has often been described

as the cradle of civilization, the Fertile Crescent.

This was an area where agriculture

was first developed in the Near East.

- [Narrator] But to reap the land's full potential,

the Sumerians faced a challenge.

- Mesopotamia, a large part of which

now falls within modern-day Iraq,

is essentially large landmass.

It's incredible fertile,

but outside the banks of the river it was very arid.

And when the water did come,

it more often came in the form of flash-flooding.

- When there was too much water,

that devastated their farms,

so they needed to be able to control

the amount of water going into their fields.

- [Narrator] As a result,

they developed ways to control these floods

and maximize the potential of the two rivers.

- The Sumerians were master engineers.

They knew how to survey their land

and they built huge levees or dikes

of earth, reed, and mud bricks,

which allowed them to build huge, strong structures

to hold back water.

- [Narrator] To divert it to where it was most needed,

the Sumerian engineers designed channels

for the purposes of irrigation.

- And it's in this process of digging

and casting up of earth that they create

a really very complex system of canals.

This is almost certainly

the first canal system in the world.

- [Narrator] But the real mastery of the Sumerians

was in how they controlled the flow

of such a precious resource.

- So, the Sumerians were incredible basket weavers.

Reeds grow naturally in the Euphrates-Tigris area.

So, reeds themselves are quite weak,

but the way that they were interwoven in a complex way

created a material of great strength.

That could be used to make dams, sluices, boats, or houses,

but especially was used in irrigation.

- [Narrator] When closed, sluice gates woven from reeds

would prevent oversaturation of the farmland.

To water crops, they would be opened,

diverting water either to reservoirs

where it could be stored

or into smaller canals which led to the fields.

- In many ways, what they did is turn a very arid

and inhospitable land into what is,

in essence, an earthly paradise.

- [Narrator] And to ensure these gates and canals

did not leak, the Sumerians used an ingredient

that was in plentiful supply...

Bitumen,

a form of crude oil which occurs naturally in the ground

across many parts of Mesopotamia.

And the Sumerians made good use

of one of its valuable properties.

- Bitumen is a thick, oozy, sticky, viscous oil

and the fantastic thing about this is it's hydrophobic,

so it literally repels water.

It doesn't mix with water.

This meant they could waterproof their irrigation systems,

everything from their sluices, their weirs,

their canals, and their dikes.

- If you could waterproof your canal,

you're in much greater control of the entire process

and you reduce the amount of water that you lose,

something that was crucial in the middle of the desert.

- [Narrator] The ability to harness water

not only secured a food supply through irrigation

but also allowed the movement of goods and trade

and expanded the Sumerian economy,

which in turn allowed a rich culture to blossom.

- Alongside these irrigated fields,

the Sumerians built great cities,

huge, large structures.

A central feature would often be the central temple,

the ziggurat.

These complexes would have

tens of thousands of people living within them.

And with this form of complex urban life,

we see a beginning of writing

in order to keep a bureaucracy going.

They really were the first cities on Earth.

- [Narrator] Through their expert control of water,

the cities of Sumeria grew to become

the most sophisticated of their time.

But the rise of another great ancient civilization

was powered through its ability

to better navigate its most important river.

[pensive music]

The Nile,

ancient Egypt's central artery.

- I think it's very difficult to overstate

just how important the Nile was to ancient Egypt.

Without it, there would be no Egypt at all.

- [Narrator] The only source of fertility

in this desert land,

which the Egyptians also controlled through irrigation.

- So, Egypt becomes this sort of machine

for producing food

and it allows for a population to flourish.

- [Narrator] But the Nile had a problem.

While it was mostly navigable,

in several places to the south,

it was partially blocked by massive patches of rock,

known as cataracts.

- Cataracts are stretches of the Nile

which are really treacherous to navigate through

on account of the fact that the riverbed

is so close to the surface

and you've got lots of exposed rocks.

- One way of navigating the cataracts

was to take boats onto the land

and then haul them along and then drop them back down again

at a point at which the river is navigable.

- This wasn't just a few feet,

it was often several hundred feet,

and the cargo was often very heavy

and the boats themselves very cumbersome.

So, for their society, their prosperity, their culture,

it was essential that they found a solution.

- [Narrator] There's evidence that in the 3rd millennium BC,

Pharaoh Merenre I had cut a canal

through the rock and stone of a cataract

using sheer manpower and determination.

Using their skills as master stonemasons,

succeeding pharoahs used these same methods

to carve more canals.

- They were prepared to go to what seemed to us

to be extraordinary lengths to achieve their ends,

and that meant even manipulating the course

of one of the world's great rivers.

- [Narrator] A few hundred years later in around 1870 BC,

Pharaoh Senusret III invented a faster

and less labor-intensive method.

He would build a slipway on the riverbank

and slide his boats around the cataracts.

- Imagine railway sleepers with wooden tracks laid on top

greased with river mud,

which allowed those boats to be dragged.

- [Narrator] Around 10 feet wide and over a mile long,

the slipway was supported on both sides by bricks.

Wooden beams then ran across it like railway sleepers,

while river mud created a gently-sloped base

to support the boat and help it slide along more easily.

- This was still no easy task.

It would have taken an immense amount of muscle power

to move those boats just a few feet.

But now the Egyptians could transport materials

in either direction.

- And that allows them to band their territory,

expand their control of the region

and its resources even further.

- [Narrator] By finding ways around the cataracts,

Egypt's pharoahs were able to extend

their country's southern border,

opening up trade with the rest of Africa

and making the Nile one of the most efficient

inland waterway systems in the world.

- I think it's a testimony to just how powerful,

capable, and determined the Egyptians were.

- [Narrator] In the modern era,

manmade waterways have revolutionized global trade,

exemplified by the Suez Canal.

An engineering triumph completed in 1869,

it cut through a 75-mile-long stretch of land

connecting the Mediterranean with the Red Sea

and halved the journey time

for vessels traveling between Europe and the East.

- The Suez Canal was the first canal

to be built right through a country

and to be accessible to all nations.

- [Narrator] Today, nearly 20,000 vessels a year

travel along the Suez Canal,

transporting cargo worth in excess of $3 trillion.

Just how important this canal is became clear in March 2021,

when a cargo ship called the Ever Given

was blown off-course by sideways,

completely blocking the route for other traffic.

- And although it was only blocked for six days,

this had a ripple effect across the world,

affecting global trade for months.

- [Narrator] Proving just how vital

manmade waterways still are to our modern world.

But in ancient times, canals were so important

they could even become the means of uniting a vast empire.

China, one of the world's oldest civilizations.

Like many other cultures,

China's earliest settlements grew up alongside its rivers.

- The northern frontier of China

was against the great grasslands of Central Asia.

This presented a continual military threat

to the settled agricultural communities

of southern and central China.

So, how do you get the grain

needed to feed the army to the north

when it's all being grown in the south?

- [Narrator] Most of China's great rivers flow west to east,

with very few natural connections joining them.

This meant there was no easy way to travel between them.

To overcome this, the Chinese built a series of canals

traveling north to south across China's five major rivers

that would become known as the Grand Canal.

Built in sections over hundreds of years,

it would eventually span a length of over 1,000 miles.

- The first canal, built in the 5th century BC

between the Yangtze and the Hubei Rivers,

was constructed by the Wu kingdom

in order to transport troops

and particularly food and supplies

to protect their northern frontier.

- [Narrator] And as subsequent dynasties

expanded their territories,

they also built sections of the canal.

A thousand years later, in the 7th century AD,

leaders of the Su Wei dynasty

had the visionary idea to connect all these smaller canals,

creating one vast waterway.

Built entirely by manual labor,

this monumental task required a workforce

only an emperor could enlist,

but this would come at a huge cost.

- The emperor, Yang, mobilized about five million people

it's been estimated would have been employed

in constructing this grand canal.

- By any standards, this is an amazing engineering feat,

but I don't think we should underestimate

the cost to human life because the conditions

within which these people were working were truly grim.

- [Narrator] Over the following centuries,

the route of the canal was further developed

to resemble its current course today.

Extending for over 1,100 miles,

the Grand Canal was the longest manmade waterway

in the world.

It stretched from the city of Beijing in the north

to Hangzhou in the south,

the equivalent of the distance between New York and Miami.

- This was the world's largest,

most extensive civil engineering project

that anyone had ever seen

prior to the Industrial Revolution.

- [Narrator] But as the canal grew in length,

this disparity between the water levels

of the connecting rivers became a major obstacle.

- One of the big engineering challenges here

is that this enormous water network

is joining together five different rivers

that were all at different heights.

So, how do you join waterways

when you've got a height difference to contend with?

- So, what the canal needed

was a system of moving uphill and downhill,

and this came in the form of really quite rudimentary locks.

- The earliest invention of the lock

was called a flash lock.

It was created with just one gate

and it would hold back the water, keeping a vessel high.

It would open, and then the vessel would travel downstream

with that flow of water.

- [Narrator] A flash lock was a narrowed section of canal

with a navigable gap.

Planks were slotted into grooves

in two vertical wooden poles

embedded on either side of the riverbank

to form a wall that could dam water behind them.

Once a sufficient level of water had built up,

the planks were quickly removed, releasing a surge of water,

carrying the vessels downstream and over any rapids.

But going upstream still required manpower

to physically winch or drag vessels to higher levels.

This was fine for smaller boats,

but larger vessels were prone to getting stuck,

especially on steeper gradients.

So, some expert believe that Chinese engineers

of the Song dynasty in the 10th century

found a revolutionary solution.

- So, the big innovation came

with the invention of the pound lock.

- [Narrator] To raise or lower a vessel

between stretches of water at different levels

required an enclosure to be created

by the introduction of an upper and lower gate.

Wooden planks are set into grooves in the upper gate

to dam the water ahead at the higher level.

As a boat enters, the lower gate is closed behind it,

creating an enclosed pound or chamber.

Water is allowed to flow into this chamber,

raising the level along with the boats.

Once the water level is the same as the river ahead,

the remaining upper gate planks

are removed by a rope and pulleys

and the boats carry on their journey.

- This meant that not only could it better deal

with different heights of water

but you could also travel in either direction.

- [Narrator] The pound lock was a revolutionary invention

and one that's still in use today all over the world.

This was the piece of the puzzle that allowed boats

to be able to reach any part of the system.

The Grand Canal became the backbone of the empire,

not only facilitating the easier transporting

of goods and troops

but also providing a link between the peasants

and the Imperial Court,

helping them to create and control a more unified China.

- This engineering endeavor,

I think it's one of those feats that almost defines a people

and defines a culture.

- [Narrator] Although sections of the Grand Canal

have fallen into disuse,

China still has the most extensive waterway system

anywhere in the world,

with over 75,000 miles of navigable rivers and canals.

[pensive music]

Where the Grand Canal's locks

allowed rivers of varying heights to be connected,

in recent centuries canal locks have also been used

to connect oceans, each with differing tidal ranges.

- The Panama Canal is a 51-mile-long waterway

connecting the Pacific and the Atlantic.

It's huge in its scale.

And when it opened in 1914, it was innovation

at the very peak of what humanity

thought they could achieve at the time.

- [Narrator] It was one of the world's

greatest-ever engineering works, saving ships the lengthy

and often treacherous trip around Cape Horn,

the southernmost tip of South America,

opening coastal shipping routes to the American West Coast.

- The challenge with the terrain for the Panama Canal

was that it wasn't flat; it was mountainous.

Instead of building a trench,

which would have been incredibly time and labor intensive

and cost a huge amount of money,

they lifted the vessels up by creating locks

that would allow the vessels

to go over the mountainous terrain.

- [Narrator] Connecting these two vast bodies of water

presented one of the greatest

engineering challenges of the time:

how to bridge the 85-foot height difference

between the two oceans and Lake Gatun,

the body of water dividing them.

The only way to raise the ships high enough

to reach the lake was to create a series of massive locks.

- The engineers built a series of three locks

on the Atlantic side that would lift the vessels up

the 85 feet to the Gatun Lake.

At the highest point, they did channel through

the most mountainous section.

And then on the Pacific side, they had another three locks

that would lower the vessel down.

- [Narrator] And to ensure the canal was protected

from the Pacific's strong currents,

the engineers devised another solution.

- They designed these V-shape locks,

entirely different from the Atlantic side.

These V-shape locks were huge, massive steel gates,

730 tons, and they were vastly deep,

but this allowed them to mitigate

against the volumes of water and the changing tidal range.

[water rushing]

- [Narrator] The lock gates close in a V shape

with the tip upstream

so the force of the water from the upper side secures them,

keeping the mighty Pacific Ocean at bay.

- The fact that they were V-shaped was very important

because that V resisting the hydrodynamic pressure here

actually, in a way, wants to close the gates even more,

so they fail safe.

- [Narrator] Thousands of workers were employed

over the course of a decade to carve these locks

out of the difficult mountainous terrain

using heavy machinery and dynamite,

with the tropical climate

significantly adding to this challenging work site.

Today, the canal sees around 13,000 to 14,000 ships

pass through it every year

and hundreds of millions of tons of cargo,

providing a reliable passageway between opposing oceans.

[reverent music]

Both natural rivers and manmade canals can be vital conduits

to the growth of cities and civilizations,

but harbors have also played an important role.

- Since ancient times,

sea harbors have been crucial waterways

because they enable different cultures

to interact through trade

and also provide a base for navies to protect that trade.

- [Narrator] Through the ages,

many port cities have prospered

and grown around existing harbors.

- The preferred choice for early sailors

was a natural harbor where they would find an inlet

which was protected by land from the sea, from the wind,

and therefore they could set up their harbor

and then they could carry out their trade all very safely.

- [Narrator] But not every port city has a natural harbor.

Some had to be created artificially.

[reverent music]

Caesarea, on the coast of Judea

in what is now modern-day Israel.

At the end of the 1st century BC,

it was ruled by King Herod of biblical fame,

an independent kingdom under the ultimate control

of the Roman Empire.

To rival the architectural marvels of Rome

and tap into its lucrative maritime trade,

Herod embarked on an ambitious plan

to found a new port city and give it a grand manmade harbor.

- Herod's objective here was to replicate

the success of Alexandria in Egypt,

which was famed across the Roman world.

So, he wanted to build a harbor

where he could export good from Judea,

things like olive oil,

but also provide safe harborage for Roman merchants,

which would then enable all the wealth

from the Roman Empire to come to Judea.

- [Narrator] But this visionary idea

was never going to be straightforward.

Judea lay on a particularly long, flat stretch of coastline

in the Mediterranean with no natural features

that could be used as a harbor.

- Virtually devoid of bays and inlets.

If you wanted a harbor here, you'd have to build one.

- [Narrator] But this coastline came with another issue.

- It's also susceptible to very strong longshore currents,

which causes shifting sands and creates an unstable base,

very difficult for building strong foundations.

- [Narrator] To help realize Herod's maritime ambition,

his engineers looked to one of Rome's

greatest engineering innovations...

Concrete.

- It was strong, durable and, crucially, easy to make,

and the Romans used it everywhere.

So, they used it for the amphitheaters, for their theaters,

they used it in their aqueducts.

- [Narrator] But to build the harbor foundations underwater

and to ensure it would withstand the most turbulent seas,

the concrete needed to be even more durable.

- Roman concrete famously had many ingredients,

but one of the most famous was pumice porcelain,

which is a volcanic ash which comes from the Bay of Naples.

And this has a very unique quality

because it enables Roman concrete to set underwater.

- [Narrator] Herod imported this crucial ingredient

all the way from the volcanic basin of Vesuvius in Italy,

1,000 miles away.

- The Romans had very advanced maritime transport networks,

which made it very easy to transport the pumice porcelain

from Italy all the way through to Judea.

- [Narrator] To create the massive breakwaters,

the Romans floated large wooden boxes,

called caissons, out to sea.

These were half-filled with marine concrete

to stabilize them in rough seas.

From boats, the caissons were then

fully filled with concrete until they sank to the seabed.

This process was repeated

until the breakwater foundations were complete.

Over time, the concrete would slowly harden

and the wood would rot away.

This breakthrough allowed them in just 10 years

to construct the largest harbor ever built in the open sea.

- Herod called his new harbor Caesarea Maritima

and it became one of the most famous harbors

in the ancient world.

- [Narrator] Enclosed by its two massive breakwaters,

it covered an area of 25 acres,

the equivalent of 20 football fields.

- The Roman engineers used

1.25 million cubic feet of concrete

and all mixed without the use of modern machines.

- Herod's ambition was big,

but his achievement was incredible.

By the time it was finished,

the harbor was one of the jewels of the Mediterranean.

- [Narrator] And it stood the test of time.

Even 2,000 years later,

the harbor's concrete breakwaters are still visible

due to a remarkable chemical reaction.

- This reaction makes a binder or glue

which bonds the volcanic particles to the mortar

to make a very durable concrete.

- [Narrator] Volcanic glass and crystals

inside the pumice dissolve

and react with alkaline fluids in the concrete.

Over time, this creates a mineral

called aluminous tobermorite, which forms within voids

and prevents cracks from traversing through the concrete,

increasing its resilience.

- The Romans used their marine concrete

across their entire empire.

It was absolutely astonishing.

- [Narrator] But it's not only defense against the sea

that ancient harbor-builders have had to overcome.

[gentle music]

The Bosphorus, Istanbul.

Located in northwest Turkey,

it is the world's narrowest strait

used for international navigation

and one many have tried to control throughout history.

[pensive music]

Back in the 8th century AD,

when the city was known as Constantinople,

it was the center of the Byzantine Empire.

- Founded by Constantine the Great,

who was the first Christian emperor.

Constantinople was built in a key strategic location

because it's on the Bosphorus,

which links the Black Sea with the Mediterranean.

- [Narrator] Due to its geographical position,

Constantinople found itself

under constant threat of invasion,

including, at this time in the 8th century,

from the Arabic Umayyads in Syria.

- Constantinople was famous because of its walls.

It's a triple series of walls,

each one bigger than the other,

each one with a huge ditch in front of it

and then a flat area of land,

which gives you a killing ground.

So, it's almost impenetrable.

- [Narrator] While the city's fortifications

protected it on land,

there was still one major weak point:

the Golden Horn, a great natural bay

that flowed through the heart of the city,

leaving Constantinople vulnerable to attack from the sea.

- In the first few centuries of Constantinople's history,

the Byzantine fleet was one of its greatest strengths.

But during the 8th century,

there were a series of naval defeats,

meaning that their navy was severely weakened.

So, to protect their city, they had to think again.

- [Narrator] Finding a way to effectively defend themselves

would require a design that drew on skills

acquired across a range of disciplines.

- The Byzantines were great engineers,

from siege weapons, grenades, the pendentive dome.

- And they even invented a mechanical throne

which allowed the Byzantine emperor

to disappear below the floor.

- [Narrator] The solution came

in the shape of an immense mechanical chain

at the entrance to the Golden Horn,

a defense system the likes of which

had never been seen before.

- And this involved an enormous metal chain

which went all the way along the entrance to the Golden Horn

and was set in place with two towers at either side.

The real genius here was that the chain

could be raised or lowered at will

using very advanced hydraulic technology.

- [Narrator] An enormous cast-iron chain

almost half a mile in length

was strung between two stone towers,

one on each side of the entrance to the Golden Horn,

and held in place by eight wooden floating pontoons.

To allow friendly ships to pass over it,

the chain was lowered between one set of pontoons

on each side of the river.

But when an invading fleet approached,

the chain could be pulled taut, raising it,

effectively closing any safe passage through.

Despite its enormous size and weight,

the chain could be maneuvered surprisingly quickly

when an alert was given.

- Soon as the enemy is seen coming over the horizon,

you can give the warning.

In as little as 20 seconds, that chain could be raised.

- [Narrator] But its real genius lay inside the two towers.

They contained a complex hydraulic operating system.

Seawater from the Golden Horn was channeled into the towers

and then pumped into a holding tank above.

When enemy ships approached,

this water could be quickly released

into large counterweight containers

attached to each end of the chain via a massive pulley.

[water sloshing] As they filled with water,

the counterweights lowered, raising the chain,

closing the gaps on each side within a matter of moments

and preventing enemy invaders from entering the Golden Horn.

- [Sam] Operating the chain required exact coordination.

A lookout on each tower would communicate with light signals

so they would know the exact moment

to let water into the counterweight to operate the chain.

- And the reason why the chain

was so good at sealing the harbor off was the enormous size,

the enormous weight of these cast-iron links

in the chain itself.

- [Narrator] With each link weighing over half a ton,

this colossal chain defended the city of Constantinople

from its enemies on many occasions.

- The enemy ships trapped at the entrance to the Golden Horn

could then be attacked on two fronts

by military on the land and also by Byzantine ships at sea,

perhaps using Greek fire,

a combustible compound released with a flamethrower.

- [Narrator] When the threat had passed,

water was released from the counterweight

by unscrewing a plug in the bottom,

forcing it to rise as the weight decreased

and lowering the chain.

When the city finally fell to the Ottomans,

it wasn't because of the chains' ineffectiveness.

During the last siege of Constantinople,

the chain, together with a handful of Byzantine ships,

repelled the attempts of Sultan Mehmed II's great navy,

forcing them to find another route.

- When the Ottoman Turks finally captured the city,

they had to drag their warships over land

to get into the Golden Horn.

- [Narrator] The fall of Constantinople

also marked the end of the great chain.

Abandoned by the victorious Ottomans,

it fell into obscurity on the seabed.

Throughout history,

people have gravitated to living near waterways.

And in ancient times,

entire cities were even constructed on them.

- Venice is the most famous medieval city built on water.

It was inhabited by people

who fled to the swamps to survive,

and there they built the most magnificent city,

all through the wealth generated by maritime trade.

- [Narrator] Although people have lived for centuries

in the Venice Lagoon, the city is thought to have been built

from the 5th century AD onwards.

- Towards the end of the Roman Empire in the west,

many Germans and Goths started invading northern Italy

and the people living locally needed a place of safety.

And so, they chose a series of islands

surrounded by lagoons and marshes,

which provide a natural defense against the invaders.

- [Narrator] But building on this soft, saturated marshland

wasn't easy.

- Given that this was just a vast swamp with no firm ground,

it's an incredibly challenging environment

to take on building something of Venice's scale.

- This is the last place on Earth

you'd dream of building a city.

Not only did they have to build houses in the lagoon

but everything they used to build those houses,

the raw materials, had to come by sea.

- A key component in building is the ground material.

There's a lotta sediment brought in by sea

and a lot of soft sediment, very difficult to build upon.

And of course, with that tidal range,

you're facing designing against flooding.

- [Narrator] The engineers of Venice

needed an innovative way

to be able to support their buildings

in such a waterlogged environment.

- To create solid foundations,

the engineers had to drive 60-feet-long wooden poles

made of oak into the lagoon bed.

[reverent music]

- [Narrator] These piles were driven into

the hard clay layer sitting beneath the soft silt

and were tightly packed together to keep them stable.

Voids between them were filled with rubble

and capped by a platform of wood and stone,

which extended above the waterline.

Finally, a layer of white limestone

was used as a damp course

to help stop the infusion of water into the buildings above.

- If you could lift up Venice,

you would find a forest of wooden piles, 10 million of them,

all supporting an entire city.

- [Narrator] This method of building was so effective

that it changed little over the centuries.

Today, the historical center of Venice

has around 50,000 residents,

as well as millions of tourists annually.

- Considering the foundations of these buildings

is just in mud and clay and sand,

it's incredible that they've stayed

as stable and secure as they are.

I mean, some have subsided slightly,

but essentially the city is still standing.

- [Narrator] And the fact they have remained so solid

in the water is down to clever engineering

and the unique environment of the Venice Lagoon.

- The thing with the log piles

is that they have been preserved because they're sitting in

what we call anaerobic conditions.

They're underwater, they've been starved of oxygen,

so they can survive for hundreds, thousands of years

as long as they're kept wet.

- [Narrator] But in recent decades,

the delicate balance of Venice's original building work

has come under threat; ironically, by modern engineers.

- In the 1950s and 1960s,

they were digging wells down through the city's substructure

to get hold of fresh water for their growing population.

The problem was that once you start

sucking the moisture out of the clay

within which the city is built

is it starts contracting and you start getting voids

and the very posts upon which the buildings are founded

start to sink.

- [Narrator] Together with rising sea levels,

Venice faced potential catastrophe.

[water sloshing]

[pensive music]

Venice's seasonal flooding, known as acqua alta,

has increased in intensity over the last few decades...

But modern engineers may finally have found a solution:

the MOSE project,

a system of electronically-operated flood barriers

which can close off the three main entrances

into the lagoon.

- What happens is when a flood warning is given,

at a touch of a button,

the gates are initiated and they rise up,

providing a barrier,

protecting the city to flooding up to three meters.

- [Narrator] But while their design is highly complex,

some simple engineering underpins their operation.

78 giant steel gates

straddle the three entry points from the sea.

When not in use, these water-filled gates lie flat

within concrete bases anchored to the seabed.

At times of increased flood risk,

the water is expelled by being replaced with compressed air,

causing them to rise, closing off the lagoon from the sea.

For many years, the Venetians doubted their effectiveness.

But in 2020, during an exceptionally high tide

which normally would have put half of Venice underwater,

the barriers were activated

and successfully kept the city dry.

- Similar barriers could be the saving grace

for various cities across the world

with the rising sea levels due to climate change.

- [Narrator] But Venice is not the only city

which historically has had to protect itself from flooding.

[reverent music]

Mesoamerica.

In the 14th century, in the verdant valley of Mexico,

the Aztec people were on a mission.

They were in search of a promised land

where they would build a great new city.

- Their tribal god, Huitzilopochtli, told them,

"When you find an eagle perched on a cactus,

"you would find the place

"where you are to build a huge empire."

- [Narrator] Unfortunately for the Aztecs,

the place they first saw such a spectacle

was on a small marshy island in the middle of Lake Mexico,

so that's exactly where they built their city.

They called it Tenochtitlan,

likely meaning "place of the prickly pear cactus."

- Tenochtitlan was a marvel of the ancient world,

the beating heart of Aztec power in what is now Mexico.

- It became so large

that it covered an extension of five square miles,

which is really considerable

when we think that they arrived in a marshy island.

- [Narrator] As the city grew,

the Aztecs expanded their farming land

by building artificial garden islands known as chinampas.

- Chinampas were key to the success of Tenochtitlan.

Chinampas are arable plots of land

that consist of earth from the bottom of the lake

that is full of good nutrients, full of organic components,

and it made for very rich agricultural lands.

- [Narrator] As a result,

farming became increasingly productive,

both in terms of feeding the population

and in growing trade.

- The city grew in size and importance,

and this was possible thanks to the skill

of the engineers that were working in Tenochtitlan.

- It was replete with squares, temples,

storehouses, settlements,

and a network of paths and canals to service all of that.

In many ways, this was the Venice of Mesoamerica.

- [Narrator] However, the Aztecs still had

a significant challenge to contend with.

In the dry season,

Lake Mexico was one of a series of six lakes.

[thunder cracking]

But in the rainy season,

these would join together, creating one huge body of water,

not only flooding the city

but contaminating the lagoon's freshwater

with the brackish water of Lake Texcoco.

To overcome this, the Aztecs built a vast flood defense

known as the Nezahualcoyotl dike,

separating the lakes once and for all.

It was an absolutely enormous undertaking

on a scale never seen before:

a double wooden fence roughly 10 miles long,

nearly 30 feet high, and 12 feet wide.

- That enabled the people there at Tenochtitlan

to grow crops and to create

this sort of agricultural wonderland

that was absolutely central

to the success of their settlement.

- [Narrator] Aztec engineers

chose the dry season to build the dike,

when the water level was low.

A wooden barrier formed from curved timber panels

was secured to the lake bed

by driving in huge wooden piles at regular intervals.

Gaps in the barrier were then filled

with a mix of dense stone and clay

to help waterproof the structure.

Wooden gates set into the dike

allowed the Aztecs to regulate the water level

so when Lake Mexico rose too high,

water could be released into Lake Texcoco.

Through the Aztecs' mastery of controlling water,

Tenochtitlan eventually grew into

one of the largest cities in the world at that time.

- Tenochtitlan was built on water

and what really is extraordinary,

how they were able to make construction so solid.

These very sophisticated buildings

were extraordinary feats of engineering.

- It was from this base

that the Aztecs went on to conquer much of Mesoamerica,

an empire that stretched from the Atlantic to the Pacific.

- [Narrator] But everything changed in the year 1521.

Spanish conquistadors led by Hernan Cortes,

who had invaded Mexico two years previously,

came across the city.

- So, when Cortes first clapped eyes on this place,

he would have been taken aback

by this amazingly sophisticated city.

I mean, here you had all sorts of buildings,

you had tree-lined avenues, a complex of waterways,

lush gardens, all sitting on an island

in the middle of this shimmering lake.

- [Narrator] What happened next seems unthinkable today.

- The invaders destroyed it.

They destroyed this amazing feat of engineering,

this incredibly sophisticated city

at the heart of this civilization,

and they destroyed the Nezahualcoyotl dam as well.

I mean, this is in essence wiping a civilization

off the face of the Earth.

It's an absolute tragedy

and it's something that I think we'll regret forever.

- [Narrator] Today, the entire valley basin has been drained

and is now occupied by Mexico City,

leaving few traces of the lakes,

the city, or its great dike.

[gears turning]

Today, modern engineers continue to challenge

the notion of what can be built on water.

Palm Jumeirah: an archipelago of artificial islands in Dubai

created through land reclamation

which extends into the Persian Gulf.

First opened to residents in 2007,

it's one of the world's most unique residential projects

that, together with other linked developments,

increases Dubai's shoreline by a total of 320 miles.

To get the palm-shaped design right,

engineers used a cutting-edge technology

called differential global positioning systems

to plot each island within 0.39 of an inch of accuracy.

- These unique artificial islands were built

using 3.2 billion cubic feet of sand dredged from the seabed

and brought all the way from the Persian Gulf.

So, even though Dubai is surrounded by sand,

it was the wrong type of sand.

It's too fine to be able to build these islands.

- [Narrator] To ensure the islands did not collapse,

state-of-the-art vibrocompactors

were used to help solidify the sand.

Palm Jumeirah now houses as many as 80,000 people.

And with its villas only 10 feet above sea level,

engineers also found a solution

to protect it from the tides,

constructing a massive crescent-shaped breakwater.

[reverent music]

- It's one of the most impressive manmade islands.

You can even see it from space.

- [Narrator] Such developments

in water-based construction technology

may now be more the preserve of the rich,

but some of these engineering innovations

may become vital to the future of entire populations.

- This is something that will have to become

more common around the world

as climate change causes water levels to rise

and we literally have less land to live on.

- [Narrator] And more extreme weather

may also mean our flood defenses and waterways

will need to become even more effective.

- With climate change exacerbating extreme weather

and rising sea levels,

we're going to have to use new innovations and old ones too,

whatever we can do to protect our communities

from the adverse consequences of a rising sea level.

- [Narrator] Modern engineers may need to turn once again

to proven technologies from the ancient world.

- I just hope that we can take some inspiration

from the ways in which ancient engineers

have fashioned watercourses, dams and irrigation systems

and learn something of that to inform

the way in which we sustainably manage water in the future.

[reverent music]

- [Narrator] The many innovative ways

in which ancient engineers harnessed the power

and life-giving properties of water became the catalyst

for the growth of all great civilizations around the world.

From the Sumerians' and Aztecs' complex irrigation systems

to China's empire-unifying canal,

Herod's showstopping harbor embedded in Roman concrete,

and the Venetians' far-reaching vision

to construct an entire city on water,

today's engineers remain astounded

by the achievements of the past

and the potential of the future.

[reverent music continuing]