Original subtitles

[tense music] [birds chirping]

- [Narrator] What would happen

if every human being on Earth disappeared?

No matter how we might vanish,

[tiger growls]

this is the story of what could happen

to the haunting world we leave behind,

now on "Life After People."

[static crackles]

Some of mankind's greatest feats of engineering

were built to be indestructible,

but without us, will they stand the test of time?

[building crashing]

Or will the elements find a fatal flaw?

New York City's invincible Freedom Tower

outlasts the city it was built in,

but nature doesn't give up.

- It's designed to withstand things like weather

and natural disasters, even attack.

The question is: will it?

- [Narrator] An impenetrable stronghold

guards all of humanity's greatest treasures

while the elements try to break in.

- The door to the vault alone weighs 20 tons,

so that's the equivalent of three or four elephants.

- [Narrator] And the Torre Mayor

faces off against the earthquake capital of Mexico.

[rubble crashing]

- Built in the Ring of Fire,

this tower was designed to take all kinds of abuse.

- [Narrator] Humanity's unbreakable fortresses

go toe-to-toe with the fury of a world without people

to find out what falls

and what will be the last structure standing.

Welcome to Earth, population: zero.

[tense music]

50 years after people,

the world is unrecognizable.

In Sydney, Australia, the famous Opera House

is crumbling into the Pacific.

In New York City, the Flatiron Building is in ruins.

Grand Central Station's roof has long since collapsed.

But among the ruins, one iconic landmark still stands,

a symbol of America's freedom and strength.

It was built to be nearly indestructible:

One World Trade Center.

[dramatic music]

- After 9/11, whatever was going to take the place

of the World Trade Center had to be significant.

It couldn't just be a feat of architecture or engineering.

It had to be something that would restore faith,

rebuild confidence in the American spirit.

- When it was built,

the designer specifically chose a height of 1,776 feet

to honor the year that the United States

declared independence.

- The perimeter is framed with about 45,000 tons

of high-strength structural steel.

It's fitted with 13,000 glass panels,

each of which weighs about 6,000 pounds.

- [Narrator] For 50 years, the Freedom Tower has stood tall

while buildings all over the city

have succumbed to the elements,

thanks to a near-unbreakable concrete core

rising from her foundations to her uppermost floors.

- The core is four feet thick

and reinforced with steel barriers,

so you can think of that as the building's spine

that's holding everything together.

But the concrete is no ordinary mix.

It's rated at 14,000 PSI.

That is strong enough that just one square inch of it

could handle the weight of four cars

stacked on top of one another.

- It's the most fortified part of the whole structure.

It's a powerhouse of design, built to last,

even when everything around it is falling apart.

[building crashing]

- [Narrator] For decades,

One World Trade has weathered storms, floods, lightning,

and wind, a towering beacon of strength and engineering.

But all around her, other towers aren't as lucky.

- Surrounding buildings like 200 Vesey and 225 Liberty

were built in the 1980s,

but neither tower is nearly as robust as the Freedom Tower.

- [Narrator] In the time of people,

the buildings' steel skeletons and concrete foundations

received regular inspection and repair.

But after half a century without maintenance,

200 Vesey shudders,

[building crashing]

and chunks of her facade give way.

- The building is less than half the height

of the Freedom Tower, but chunks of concrete and glass

would crash into the lower third of the building

and slam into its base.

[building crashing]

- The Freedom Tower's core was built to withstand a pressure

of 14,000 pounds per square inch,

but a 10-foot chunk of concrete falling 300 feet

could generate double that,

depending on how it falls and how it lands.

- [Narrator] The damage isn't fatal,

but it leaves the tower vulnerable,

and an ancient enemy has been lurking beneath the tower

all along, waiting to exploit any weakness.

- Turns out, this part of the city

was never meant to exist in the first place.

Centuries ago, Lower Manhattan was all wetlands and water.

- When construction teams dug out the foundations

for the original One World Trade Center,

they actually found the timbers of an 18th-century boat,

because this area was used to dock boats

on the Hudson River.

- Engineers came up with a pretty brilliant solution:

a massive underground wall.

They dug 90-foot trenches

and filled them with a special slurry

made of bentonite clay.

- Concrete was then pumped in, forcing the slurry out,

leaving behind concrete walls that provided a barrier

between the buildings and the seawater.

- [Narrator] The wall wrapped around the entire

16-acre site, a sunken ring of concrete

holding back the Hudson like an underground dam.

For over a century,

this massive concrete barrier held the line.

Without people, the engineering that once defied nature

is showing its age.

Each falling building has been an assault,

and it's riddled with cracks and fissures.

Finally, the wall gives way.

[water crashing]

- The river surges in,

filling the lower levels of One World Trade Center

with 50 feet of water.

[tense music]

- Now, typically, operators would man the pumping systems

to keep the building safe,

but 50 years after people, those systems are long dead.

- The water has nowhere to go, and so the salt starts to

corrode the steel reinforcements.

- [Narrator] Lower Manhattan has returned

to its watery roots,

its foundations drowned, its streets mostly sunk.

As moisture seeps into the cracks

in the Freedom Tower's core, how much longer can she stand?

Across the Atlantic, behind concrete walls and iron gates,

lies a citadel of secrets, a fortress built to guard

mankind's greatest treasures: the Geneva Freeport.

Priceless art and treasures aren't displayed;

they're entombed, stored in vaults

people were never meant to see,

preserved for a future that will never come.

- A free port is basically a designated economic zone

established in a given country

where the normal rules don't apply.

The duties and taxes are low or maybe even nonexistent.

Import and export laws are different.

So this one might be in Geneva,

but it's basically like its own tiny country

that's in the middle of the city.

- Very few people are allowed in or out.

Records are hard to find.

The whole idea of the place

is that the outside doesn't know what's going on inside.

It sounds like a James Bond movie,

but this is actually real.

- Experts believe it holds about 1.2 million pieces of art,

but masterworks by artists like Picasso, da Vinci, Andy Warhol,

nearly $100 billion worth of art.

- [Narrator] Here, the relics of humanity are sealed away

by systems purpose-built to outlast anything,

including the end of mankind.

- These aren't just rooms,

they're vaults designed to withstand really anything

nature can throw at them.

Inside, the temperature is a crisp 69 degrees

with about 55% humidity.

These are perfect conditions for maintaining masterpieces.

- [Narrator] Long after mankind disappeared,

the free port still does the work it was meant to do.

- The building was designed with thick, concrete walls

and no windows, so even though climate controls

have been dead for years, the treasures inside

aren't subject to the heating

and cooling cycles happening on the outside.

- There will be some fluctuations in temperature,

but nothing like what would be happening in museums

and private collections.

Those places were made to display art.

The free port is designed to protect it at any cost.

- [Narrator] Beneath the free port's concrete skin,

a critical defense is starting to give way.

For decades, it held the line,

keeping secret treasure sealed and safe.

But now, its time is running out.

- There's a protective rubber membrane

on the roof of the free port that's actually flexible enough

to expand in the summer heat

but contract in the winter cold without cracking.

- The life expectancy of this kind of roof

is 30 to 40 years.

There's an outside chance it could last up to 50 years,

but that's the maximum it could possibly hold out.

- [Narrator] 50 years after people,

the protective membrane is failing,

and the roof is expanding and cracking in the heat.

For the first time, the free port's defenses

have been breached,

and the question isn't if something will be destroyed;

it's what goes first.

- Free port was designed to be airtight, watertight,

and nearly impervious.

Now, even a small crack or leak can change everything.

Moisture and humidity seep in.

Mold colonies explode.

Nothing would be safe from decay.

- [Narrator] In just 24 hours, mold begins to grow.

Soon, spores and fungus

crawl through the free port unchecked.

But there's another intruder as well, one faster, hungrier,

and drawn straight to what's most valuable: silverfish.

- Silverfish aren't fish, and they aren't made of silver.

They're tiny, wingless insects

notorious for sneaking into buildings

through any available crack.

- Silverfish have a unique ability to digest cellulose,

that's the starchy part of a cell wall.

So they eat things that other bugs don't.

Things like canvas, or paper, or glue.

- That unique digestive ability

means bad news for any collection of manuscripts,

or paper money, or paintings,

'cause those all would be delicious meals for silverfish.

- [Narrator] The most expensive painting ever sold,

a 500-year-old masterpiece by Leonardo da Vinci,

is now under siege, as countless tiny mouths

tear into the canvas, erasing it from history.

The vault, built to be impregnable,

has become anything but.

Now that its defenses have been breached,

how long can its treasures survive?

[tense music]

- [Narrator] Five decades after people,

once one of the most densely populated cities in the world,

Mexico City, is now a wasteland.

Chapultepec Castle,

once the only royal palace in North America,

now lies in ruins.

Only the twisted husk of Mexico's largest arena,

Banorte Stadium, remains,

its walls and stands long since shaken apart

by an endless string of earthquakes.

- Each year, Mexico City and the surrounding area

are hit by as many as 100 earthquakes

measuring at least four on the Richter scale.

That magnitude means you can feel the quake 60 miles away,

and it can do at least some damage.

- [Narrator] Amid the wreckage of the fallen city,

one structure refuses to fall.

It was designed to survive in one of the most

seismically active places on Earth:

the Torre Mayor.

- It's 738 feet tall,

about four times the height of Niagara Falls.

It's pretty rare, though, to have buildings this size

in this part of Mexico because of all the seismic activity.

[tense music]

Unfortunately for Mexico City,

it was initially built on top of a lakebed of soft clay.

Here's the problem: unlike solid rock,

where the shockwaves would just

cause a bit of shaking and dissipate,

this mushy earth traps and amplifies them.

So it can make a slight rumble

turn into a serious shakedown.

[building crashing]

- [Narrator] The tower was designed to withstand

an 8.5 magnitude earthquake,

even though the ground beneath it

shouldn't be able to support its weight.

- The Torre Mayor was built on reinforced concrete caissons

driven more than half a football field into the ground.

- Its architects knew that if the Torre Mayor

was going to last, they needed to pull out all the stops,

and so they used what's known as viscous dampers.

- A viscous damper is essentially a shock absorber,

but a really big one, and it's filled with a fluid.

- 98 of these are spread throughout the frame

of the Torre Mayor.

- These viscous dampers don't actually

stop the shaking completely, it just slows it down

so that the building can easily handle it.

- Back in 2017, a 7.1 magnitude earthquake struck Mexico.

[tense music]

Nearly 200,000 buildings were destroyed in that quake,

but Torre Mayor was not one of them.

- [Narrator] The ground rumbles,

and an earthquake turns the Museo Soumaya,

once the most visited museum in the entire country,

into rubble.

The Torre Mayor brushes away the danger,

but another threat is rising.

Popocatepetl, a towering volcano just 43 miles away,

growls beneath its ice-capped peak.

What will happen to one of Earth's strongest structures

when the sleeping beast erupts?

[volcano rumbles]

[tense music]

In Switzerland, 75 years after people,

the free port that was meant to be an impenetrable vault

has cracked open.

Rot and insects have snuck in through the cracks,

wreaking havoc on the sections

they've managed to worm their way into.

- Those million pieces of art, Picassos, Warhols, da Vincis.

Some of humanity's greatest masterpieces

are now just decomposing.

- Ancient artifacts once perfectly preserved

are now covered in dirt and dust.

- [Narrator] Deep within the free port,

even 75 years after people,

some of the remarkable fortress's rooms

are still sealed,

but the climate controls that once kept them cool

have long since failed,

and some precious artifacts hide an incendiary secret.

- Tucked away in one of these vaults

are priceless film archives.

But there's a problem.

These early films were made from cellulose nitrate,

essentially gun cotton that was turned into film stock.

- Geneva may not be known for heat,

but summer temperatures here can hit the 90s.

And now, with the ventilation systems long dead,

heat seeps in through the cracked roof

and stays trapped inside, slowly roasting everything within.

- The free port's thick concrete walls,

originally designed to keep the temperature stable inside,

are now working against it,

trapping the heat, turning it into a giant oven.

- And here's the scary part.

As nitrate film ages, it releases gases:

nitric oxide and nitrogen dioxide.

If these gases build up inside of a sealed film canister,

it can cause them to explode

if the temperature hits around 100 degrees.

Guess what?

It's already way hotter than that inside.

- [Narrator] With a crackle no one's left to hear,

one wheel erupts in flame.

[fire roars] Then another, and another.

- The free port has an advanced fire suppression system

that essentially sucks the oxygen out of the room

and replaces it with other gases, suffocating the fire.

It's designed to put out fires without water

or chemicals that could damage priceless artifacts.

- The fire suppression system itself

doesn't require electricity,

so that would work perfectly even decades after people.

But the problem is, the systems that detect a fire

are electrical.

So all those extinguishers

would be sitting there, ready to go,

not knowing that they're supposed to be turned on.

- [Narrator] All it would take is a small current,

the twist of a knob.

But 75 years after people,

there's no one watching, no one to sound the alarm.

- Once the nitrate film catches fire, it's like a firestorm.

It gets over 4,000 degrees Fahrenheit.

For reference, steel melts at 3,000.

Concrete starts to break down at 600.

So, 4,000 degrees, the whole vault is in huge trouble.

[intense music] [fire crackles]

- [Narrator] The building was designed

to protect mankind's treasures forever.

But this fortified vault is under assault from the outside

and from within.

How long until what was once unbreakable is finally broken?

In New York City,

switchgrass rustles where tourists once swarmed.

Turtles bask on the steps of the New York Stock Exchange.

And in Times Square,

wild pigs root through moss and shattered neon.

- Now, there's no native population

of feral hogs in New York City today,

but they are widespread across

several parts of the United States.

They're very adaptable, and they can basically eat their way

into any place they want to go.

They're excellent colonizers.

These pigs would have decades to migrate.

They've got no people to keep them in check,

so they're gonna spread very easily from the South

and from the Midwest all across the country.

And they could very easily travel the bridges

and tunnels from New Jersey to Manhattan,

just like commuters do today.

- [Narrator] The skyline is completely transformed,

tall towers all but disappeared,

but the Freedom Tower still stands,

even as the saltwater submerging Lower Manhattan

has reclaimed everything around her.

- It's almost unthinkable that it's still upright.

That base has been wading in brackish,

salty water for years.

It's really a testament to what we built.

- [Narrator] The tower is unbroken, but not unscarred.

And 75 years after people,

the weight of her own spire is becoming too much to bear.

- One of its most iconic features is the spire,

a 408-foot-tall steel structure

that once served as a broadcast antenna.

- The spire was made up of 18 different pieces of steel,

each of which was so heavy

that many of them couldn't travel by road.

They had to come on a one-month voyage by barge

down to New York City from Quebec.

- The steel was hot-dipped galvanized in molten zinc

to keep rust at bay for up to 75 years.

But now, that protective layer has given out.

After 12 decades without maintenance,

corrosion has set in.

- [Narrator] With a gust of wind, the spire buckles,

and One World Trade Center's own body now turns against her.

- That spire doesn't just fall,

it punches through the tower like a steel battering ram.

Floors 90 to 60, gone in seconds.

It's like watching a sword

slice through a stack of pancakes.

Only the pancakes are 30 floors of crumbling concrete

and rusted steel.

- [Narrator] Concrete splinters.

Steel snaps like bones.

The powerful core that sustains her

is now fractured and weak.

How much longer can the building designed to stand forever

actually last?

[tense music]

- [Narrator] A century after people,

earthquakes have turned most of Mexico City into rubble.

[birds chirping]

Hawks and ospreys now nest where sightseers once strolled,

and bobcats hunt mice and red-bellied squirrels

in the remains of the oldest cathedral

in all of Latin America.

Through it all, the nearly invincible Torre Mayor

has held fast.

But now, there's a new rumble piercing the eerie silence.

- Popocatepetl is a large stratovolcano

located very close to Mexico City.

It's quite an active volcano.

- When Popocatepetl erupted in 800 AD,

it destroyed nearly everything in its wake.

Nearby settlements were wiped out by hot ash flows

and mudslides.

There were fast-moving flows of volcanic mud, water, ash,

and debris that spread for miles.

It essentially destroyed everything

within a 21-mile radius.

- Devastating eruptions

only happen roughly every one to 2,000 years.

But unfortunately for what remains of Mexico City,

it's about that time.

- Though there were minor rumblings

here and there over the years,

the volcano was actually dormant

for much of the 20th century, until the 1990s,

when it became active again,

occasionally shooting out ash or spilling some lava,

reminding us that the threat is still very much there.

- [Narrator] With a roar,

the volcano erupts

and a superheated flow of gas, rock, and ash

wipe out all life within 20 miles.

- The Torre Mayor is outta reach of the lava,

but this kind of eruption sends hundreds of thousands

of metric tons of volcanic ash up into the air.

That can spread for hundreds of miles.

- At large enough quantities,

this amount of ash can actually blot out the sun.

In 1815, Mount Tambora erupted in Indonesia,

spewing ash that was so great in quantity,

it cooled the average global temperature

by five degrees Fahrenheit for three years.

- [Narrator] What's left of most buildings

collapse in the aftershocks,

but the unshakeable tower still stands strong.

In the aftermath of the eruption,

a thick layer of ash settles over it.

It looks harmless,

but soon, storm clouds swirl overhead,

and the combination could be the key

to toppling the mighty Torre Mayor.

- Mexico City receives more rain on average every year

than even London,

which is the city known for its wet weather.

- Volcanic ash absorbs rain like a sponge,

and then it dries to form this cement-like sludge

that is even heavier.

- So if you have even five inches of ash on a rooftop,

just a little bit of water that's added to that ash,

and almost every roof will collapse from that weight.

[water rushing]

- After 100 years without maintenance,

all it takes is a few days of crushing, uneven weight.

[debris crashing]

Now her bones are exposed,

and the pouring rain isn't just relentless, it's mutating.

- Volcanic ash can turn your run-of-the-mill rain

into acid rain.

So Torre Mayor isn't just being smothered in ash,

it's being corroded by acid rain.

- Once the roof is breached,

any additional corrosive element

is gonna have a much easier time

getting to the interior of the building

and to those viscous dampers

that are holding everything together.

- [Narrator] It's taken a century to expose a weakness

in this marvel of human engineering,

but will it be enough to bring it down?

[tense music]

For nearly 100 years, the Geneva Free Port protected

the rarest and most precious treasures humanity left behind.

Now it's decaying from the inside out,

a once mighty fortress scarred by fire.

- It wasn't just a normal fire, it was a superheated fire,

topping out at over 1,000 degrees.

Steel loses about 50% of its structural strength

at around 1,100 degrees.

So you can imagine how weak it would be right about now.

- [Narrator] Despite all it's faced,

the free port still stands, but now winter descends,

bringing freezing temperatures and swirling snow.

- Wet, slushy snow can weigh anywhere from 25 to 50 pounds

per cubic foot, and ice is even heavier,

weighing just shy of 60 pounds per cubic foot.

That's a lot of extra weight and stress on a building

that's already been gutted and weakened by fire.

- [Narrator] With a shudder,

steel snaps and cement crumbles.

- All the supports and infrastructures

would be so weakened

that it crumbles beneath the fallen roof.

The free port collapses like a house of cards,

and just like that, a lot of mankind's

most cherished treasures are gone.

- [Narrator] It's now reduced to rubble.

But what relic of mankind still survives in the ruins?

[tense music]

- [Narrator] For 150 years,

Mother Nature has worked tirelessly

to undo all that mankind had built.

The famous St. Louis Arch has collapsed,

and in Washington, D.C.,

all that's left of the Lincoln Memorial is broken rocks.

But in Kentucky, there's a secure compound

hiding beneath what remains of a sprawling army base,

and it's designed to survive any apocalypse:

Fort Knox.

- During the Great Depression,

Americans had to turn in whatever gold they had

in exchange for paper currency.

When FDR took us off the gold standard,

all of that gold had to go somewhere,

and it had to be protected.

- Fort Knox was basically America's bank.

What was inside was the foundation

of our entire monetary system.

At one time, it was said to hold more

than 4,000 tons of gold bullion,

worth as much as $400 million in today's currency.

- [Narrator] To protect the nation's fortune,

Fort Knox was built to stand up

to any attempt to breach its vault,

including a direct nuclear strike.

- The vault's walls are made out of four-foot-thick granite,

which are lined with steel, cement, and fireproofing.

The door alone weighs 20 tons.

That's the equivalent of three or four elephants,

and the entire thing is buried underground.

- Fort Knox was impenetrable when it was built in 1937.

They used the strongest materials

available at that time.

- [Narrator] As technology evolved, Fort Knox adapted,

adding layers of security like an outer perimeter

with motion sensors, a 10-foot-high electrical fence,

virtual trip wires, and landmines.

- It's got biometric authentication.

It's got high-res infrared vision cameras.

It even has seismic sensors.

- In 2009, the area was hit with a brutal ice storm

that caused Fort Knox to go without power

for an entire week.

Officials refused to continue being dependent

on outside power after that.

- Fort Knox has 20 methane wells

found deep underground near the base

to power all of its operations.

It becomes the first energy-independent

U.S. Army base in America.

- [Narrator] A tangled web of pipes and valves

once moved fuel from nearby tanks to the fort,

like veins to a heart, powering its independent grid.

In the time of people, the system was closely guarded

and carefully maintained.

But now, it has long since failed,

and its arteries have been eroding for 150 years.

- Without maintenance, the systems that power this place

are gonna corrode, and that's gonna happen

because of the presence of hydrogen sulfide,

which is in the natural gas.

- Hydrogen sulfide is highly corrosive.

It can degrade metal, making it brittle

and ultimately eating it away entirely.

Some metals are, of course, stronger than others,

but over decades or centuries,

even the most robust metals are going to fail.

[gas hissing]

- [Narrator] Toxic gas and leaking fuel

have been creeping through cracks in the pipes,

coming up through the ground, and saturating the area.

With just one spark, one jolt,

and Fort Knox's impenetrable defenses

will be put to the test.

[tense music]

In New York City,

Lower Manhattan is no longer a financial hub.

It's a watery maze of streams and rivers,

overgrown and teeming with animals.

- The wetlands of Manhattan have reclaimed their territory

just like they were thousands of years ago.

- Now this once-buzzing financial district

is literally underwater.

What was once marshland is marshland again,

with only some remnants of man's achievements

poking up out of the muck.

- [Narrator] Once filled with iconic skyscrapers,

the city's skyline is nearly empty.

But even after every tower has succumbed to the elements,

One World Trade Center still stands above the wilderness.

It bears the scars of 250 years,

but it endures.

- After centuries of exposure,

the steel beams and columns are badly weakened.

And even that concrete core,

the spine of the whole structure,

has been quietly cracking for decades.

It's been losing the fight with Mother Nature inch by inch.

- [Narrator] The collapse of other towers

and even her own spire

have wounded One World Trade too deeply.

And 2.5 centuries after people,

with no maintenance or upkeep,

her mighty spine finally cracks.

- After hundreds of years of high winds

and relentless decay, even this giant can't take it anymore.

The top of One World Trade Center

shears off at the 21st floor.

[building crashing] [dramatic music]

- [Narrator] The tower has fallen.

But will any part of it be spared?

Or will it all come crashing down?

[tense music]

- [Narrator] In Mexico City,

the strongest tower in the country still stands,

while everything around it has crumbled.

But acid rain has been pouring through

its shattered roof for years, eating away at its defenses,

including the viscous dampers

that have made it earthquake-proof.

- The dampers are still functioning

because they don't need power,

but the steel they're attached to has become corroded.

- As steel corrodes, it expands,

cracking the welds, loosening the bolts,

pulling the damper anchors free from the frame.

The attachment points are giving way.

[metal cracking]

- Imagine the dampers as joints

and the steel framework as bones.

The bones are starting to fracture,

which renders the joints powerless.

- Now, all 738 feet is truly vulnerable

for the first time.

- At this point, after all these years,

it wouldn't even take a massive quake to bring it down.

It's far from its former glory.

- [Narrator] The basin rumbles,

not with a powerful earthquake,

but a string of minor tremors.

[debris crumbling]

The ground rumbles,

and without those dampers running,

the Torre Mayor sways and sways,

and eventually topples over entirely.

[building crashing] [dramatic music]

- [Narrator] The tower finally succumbs.

Once hailed as one of the strongest structures ever built,

now its body lies, broken bones exposed,

to be reclaimed by the jungle now growing unchecked.

Almost no trace remains of the free port built

to protect humanity's treasures.

Deer, fox, and badgers roam

where masterpieces once lay in vaults.

But inside one of the innermost basement storage rooms,

there are some relics of mankind that remain untouched,

and in some ways, in better condition than ever.

Bottle upon bottle of the finest wine.

- The Geneva Free Port is said to hold around

three million bottles of high-quality collectible wine,

and some of it is still good, maybe even better than ever.

- Some wines are actually made to last.

Cabernet sauvignon and merlot, for example,

have high levels of tannins and high acidity,

which means they age really well, sometimes for centuries.

- And some wines have lasted even longer.

In 2019, in Spain,

a 2,000-year-old glass urn was found

in a Roman necropolis still containing liquid wine.

- [Narrator] Once, the most sought-after

vintages on the planet,

now these bottles wait for drinkers who will never come.

[tense music]

[birds chirping]

In Kentucky, centuries after people,

the most secure vault mankind ever built still stands.

But fuel from its long-dormant power plant

has been seeping through the surrounding ground

and spreading like a virus.

- You've got gas escaping out of the ground

and an environment

that's completely overgrown with vegetation.

So you only need one more ingredient

to create a truly epic wildfire, and that's one spark.

[thunder rumbling]

- Kentucky ranks in the top 10 states

for cloud-to-ground lightning strikes,

averaging half a million ground strikes per year.

That's over 1,400 strikes each day.

- [Narrator] A bolt of lightning arcs toward a tangle

of brush, and it bursts into flame.

What follows is utter destruction.

- The flames are spreading

and eventually collide with the leaking methane,

creating the mother of all Molotov cocktails.

[explosion blasts]

- [Narrator] An explosion rips through one side

of the abandoned fort,

and a fireball tears through the empty columns.

But will it be able to breach the vault?

[tense music]

- [Narrator] An explosion has rocked Fort Knox

and a fire has charred its walls,

but buried deep in the earth,

the vault built to store America's wealth

still guards its treasure.

- This scene would look like something

out of an Indiana Jones movie.

You would have vegetation concealing

the remnants of human activity.

And if you looked carefully,

you might see a pile of something shiny and gold.

- The thing about gold is that it doesn't rust or oxidize.

It essentially never degrades.

If it ever is exposed to sustained heat,

it simply melts and then re-solidifies.

So Fort Knox could look like it was flooded

by a river of gold.

It'd be like the lost city of El Dorado,

except there's no one left to go searching for it.

[tense music]

- [Narrator] Around the world,

the mightiest skyscrapers we left behind have crumbled.

Only the ghost of the Torre Mayor

haunts the flooded plain that was once Mexico City.

- Torre Mayor had redundancies built on top of redundancies,

but nothing is immune to time,

especially when nature never stops testing the limits.

- [Narrator] The Sydney Opera House is gone,

with no one to remember it.

- These buildings survive because we maintain them.

They're not built to last on their own.

So, without people, without that constant upkeep,

these amazing structures are eventually going to crumble

back into the earth.

- [Narrator] In New York, the Manhattan skyline

has been reduced to rubble.

But at the island's southern tip,

a reminder of humanity's engineering prowess

still stands:

the remaining floors of the World Trade Center.

- The bottom 20 stories are still there.

They're practically like a bunker,

half a million tons of high-grade concrete

that can withstand seven times the pressure

of regular concrete.

- That base isn't going anywhere.

It may not be supporting the mighty Freedom Tower

like it once was,

but what's left is still very much a monument.

It's a symbol of human ingenuity.

It is built to stand the test of time,

even in a world without us.

- For these structures, like the Freedom Tower,

the point was to be the best we've ever built.

And even when surrounded by ruin,

these towers are still hanging on.

- Imagine New York City,

now, mostly forest.

Just green everywhere.

But this iconic tower still looms over it.

If you're not getting "Lord of the Rings" vibes

from this, I don't know what you're thinking of.

- [Narrator] One by one, humanity's footprints

are being wiped from the earth.

In spite of all of our best engineering,

Mother Nature is patient and unyielding.

After people, even the fortresses

we built to last forever

eventually crumble into dust.

- We like to think that we build for eternity.

But nature doesn't care about concrete and steel.

Leave it long enough,

and even the strongest structures will fail.

[tense music]

- Forever's a human idea,

and we forget that Earth has its own clock.

And without us here to fight back,

even our greatest monuments become ruins.