NARRATOR: A young world.

The atmosphere toxic.

The land barren and forbidding.

Until the arrival of a life form that changed everything.

Plants.

DORI CONTRERAS: Everything about the landscape was influenced by plants moving onto the land.

NARRATOR: Evolving in the oceans, they eventually partner with fungi to turn an alien world... One of the most bizarre prehistoric landscapes of all.

NARRATOR: ...into one we recognize today.

(eruption roars) Along the way, they're subjected to cataclysm after cataclysm... LYNN SOREGHAN: Ice could have covered the entire planet.

NARRATOR: ...and alter the very Earth itself.

CHRISTOPHER JACKSON: This interplay between biology and geology has utterly transformed our land.

NARRATOR: How did an inhospitable rocky planet... ...become an oasis for life?

"Ancient Earth: Life Rising."

Right now, on "NOVA."

♪ ♪ ♪ ♪ NARRATOR: Viewed from above, planet Earth is a riot of colors.

But there is one color that is special.

One that reminds us our home is a living, breathing planet, unique in our solar system.

♪ ♪ Green.

(wind howling) This is the story of how Earth transformed... ♪ ♪ ...from a hostile and inhospitable world... ...into a haven of opportunity for life.

Our green planet wasn't preordained.

It's the result of an immense struggle over millions of years.

NARRATOR: How plant life rose from the oceans to dominate a rugged and desolate landscape.

ALY BAUMGARTNER: Everything that you know about the planet, everything that you think of when you look outside, is because of plants.

NARRATOR: And how plants reshaped Earth itself.

CONTRERAS: Without plants, Earth would be nothing like it is today.

JACKSON: Plants are literal terra-formers.

They transformed bare rock into life-giving soil.

♪ ♪ KIRK JOHNSON: Plants are elaborate, beautiful little machines.

These are the most amazing things in the world.

("Never Close Enough" by SIPHO.

playing) ♪ Oh, we won't ever hear the silence ♪ ♪ Or ever see the colors ♪ (exploding) ♪ That never lived in our minds ♪ ♪ ♪ ♪ Just a moment ♪ ♪ Never too far out ♪ ♪ Never close enough ♪ ♪ ♪ NARRATOR: Half a billion years after Earth was formed... ...the planet is covered by an endless ocean... ...broken up only by a few remote volcanic islands, with no trace of life on the surface.

But this is a living world.

To find ancestors of modern life, many scientists believe you would have to leave the surface behind... ♪ ♪ ...and travel to the depths of the oceans.

It's thought early life thrived in superheated geothermal vents.

♪ ♪ These single-celled organisms would've been uniquely adapted to life in this harsh environment, and would ultimately give rise to plants and everything that will ever live on Earth.

♪ ♪ But for now, they're stuck in the ocean, because large landmasses... (eruption roaring, water hissing) ...do not yet exist.

So there's a theory that four billion years ago, the only land on Earth were just a few black volcanic islands scattered in the ocean.

♪ ♪ This early land was made of basalt, which is cooled lava.

And it contains magnesium, calcium-- things that life requires.

AISHA MORRIS: The problem was that these small early volcanic islands were short-lived, because they were smashed to pieces by the tides created by the much closer moon at that time.

NARRATOR: For dry land to form and persist, one of the world's most powerful geologic forces had to begin: plate tectonics.

(eruption roars) MORRIS: Plate tectonics really tries to explain the land forms that we see on Earth: things like volcanoes, events such as earthquakes.

And it's really based on the idea that plates on the surface of the Earth move around in relation to one another, and sometimes go beneath each other, sometimes crash into each other, and sometimes slide past each other.

NARRATOR: This geologic process would prove to be the key that would change the surface forever.

♪ ♪ But its beginnings are shrouded in mystery.

HAZEN: The beginnings of plate tectonics is so controversial.

Some people think it was density differences and crust being pulled down by gravity.

NARRATOR: But some incredible new evidence has been discovered that suggests an extra-terrestrial origin.

Sometimes geology gives us great clues about how early processes occurred.

NARRATOR: Scientists have found microscopic structures called spherules, thought to be formed by asteroid impacts 3.2 billion years ago, hidden in ancient rocks.

These are located alongside rocks that show indications of plate tectonics.

So it's really hard to know for sure, because this happened so long ago, but one of the things that may have happened is that these impacts may have actually started the magma moving.

These asteroid impacts would've ruptured the crust.

They may have played some role in starting the plates moving and allowing plates to sink.

This may have started plate tectonics.

♪ ♪ NARRATOR: Data suggest that one of these giant asteroids is as much as 30 miles across.

♪ ♪ More than three times the size of the one believed to have wiped out the dinosaurs.

♪ ♪ (explosion roars) Early life forms in its path would be lost to oblivion.

But somehow, in its deep ocean hideaway, some early life clings on, and its odds for making it onto land have just gotten better.

Because with the advent of plate tectonics comes an entirely new kind of rock... ...that could allow life to get a foothold.

This rock forms during a process called subduction.

Subduction is when one slab of rock, a tectonic plate, is forced down beneath another tectonic plate.

EDMONDS: The subducting plate brings water down into the interior of the Earth, and that water is a bit like adding salt to an icy road.

So adding water to hot rock makes it melt, but this melt is really special.

It's really rich in silica and oxygen, and when it rises to the surface, it forms granite.

NARRATOR: A rock that shapes the surface of Earth as we know it today.

Now, you may think of granite as just a speckled gray rock, but it actually has a really special property that make it a wonderful land-building material.

NARRATOR: Granite has a low density.

HAZEN: So we all understand density because we put ice cubes in our drink, and when we do, about ten percent of that ice sticks above the water, 'cause ice is ten percent less dense than water.

And in the exact same way, granite is about ten percent less dense than basalt, so ten percent of the granite sticks up above basalt.

♪ ♪ This means that after granite's formed, this hard, heavy rock actually floats on top of the partially molten basalt beneath it.

(wind howling) ♪ ♪ NARRATOR: As plate tectonics continue, over billions of years, vast swathes of rock gradually emerge across the planet.

Earth's first landmasses are born.

A couple of billion years after the inception of plate tectonics... ...a multitude of continental-scale landmasses are strewn across the surface of the planet.

A vast frontier.

A rocky world with plenty of minerals containing nutrients like potassium and phosphorus, key elements for life.

♪ ♪ For any organism that can adapt to this new territory, incredible opportunities await.

♪ ♪ Dwelling in the shallows, within touching distance of the land... ...are recognizable, plant-like life.

Marine algae.

Descendants of the early life that originated in geothermal vents.

They bask in the light-filled shallows, having adopted an absolutely incredible way to harvest energy directly from the sun: photosynthesis.

Photosynthesis is a chemical reaction by which plants take light from the sun and combine it with water and carbon dioxide to make oxygen and glucose, the sugar they use for energy.

NARRATOR: Marine algae have evolved to use a crucial cellular innovation called chloroplasts, specialized structures filled with chlorophyll, a compound that helps harness the sun's energy and gives them their distinctive green hue.

But they are still confined to the water.

An environment which they are perfectly adapted to.

A watery environment is lovely if you're a plant.

It's like being surrounded by a bath of your food.

You've got nutrients available to you, there's no danger of drying out, and you don't have to worry about gravity or soil.

NARRATOR: Migrating onto land at this point would require a huge evolutionary leap.

For plants to live outside of water, it's a bit like me trying to live on Mars without a space suit.

NARRATOR: But while marine algae can't quite make it from the ocean onto dry land, they can adapt to another, more accessible environment: fresh water.

Eventually, the algae take up residence in the lakes, streams, and rivers.

From there, around half a billion years ago... ...the algae begin their slow journey onto the land.

We may never know the combination of factors that spurred green algae on land.

NARRATOR: Maybe it was the changing of Earth's landscapes to more clement conditions, or the time needed for profound genetic change.

But whatever the reason, green algae begins to come ashore.

The journey is likely to have started at the pebbly shorelines of freshwater lakes and rivers.

Some eventually evolve a thick waxy coating that stops them from drying out in their harsh new environment.

But this effective adaptation is a double-edged sword... ...making it more difficult to absorb nutrients from their surroundings, like they did in the water, and making success on land nearly impossible.

So, how did plants come to thrive on land?

♪ ♪ Though this may look like an ordinary field, it contains one of the world's most important geological sites, and the secret to what helped plants survive on land.

This is an incredibly exciting place for me to be, because more than a hundred years ago, this site where we are today was excavated to reveal the most incredible set of 400- million-year-old plant fossils.

What's special about the fossils from this particular site was, the degree of preservation of the plant material meant that we got some amazing clues as to how they actually made landfall in the first place.

♪ ♪ NARRATOR: Around 400 million years ago, this area looked completely different from how it looks today.

It resembled Yellowstone National Park, where boiling mineralized waters cascade down rocky terraces.

FIELD: So the ancient hot springs that were here at Rhynie turned any plant material that they touched into rock, and that got buried over millions of years.

The resulting bed of fossils is what we call the Rhynie Chert.

♪ ♪ This fossil is really amazing.

If you look closely, you can see these circles in the surface of the fossil.

And those circles are actually cross sections through the stems of ancient plants that grew all around here.

NARRATOR: And you can see something unusual when you look at them under a microscope.

What we see is, there's this light banding around the top, and that's plant tissue, but if you look even more closely, you see these dark brown thread-like structures weaving between the cells and actually invading some of the plant cells.

And those thread-like structures are actually fungi.

NARRATOR: This incredible evidence shows that plants actually teamed up with another kind of life form to help get onto the land-- fungi.

Rhynie Chert fossils are some of the only fossils on Earth where you can see this ancient link between plants and fungi.

♪ ♪ NARRATOR: Fungi, however, were not newcomers to the land surfaces.

Fungi made its way onto terrestrial land probably hundreds of millions of years before plants.

NARRATOR: They were able to live on the land consuming bacteria and drawing minerals directly from the rocks below.

FIELD: So fungi are able to get hold of these nutrients by exuding organic acids out of their fungal hyphae, which are thread-like filaments that form the majority of the fungus.

NARRATOR: Even though the Rhynie Chert fossils are around 400 million years old, this union of plants and fungi must have occurred millions of years earlier.

CONTRERAS: Plants benefited from the partnership because they got the nutrients from the rock's surface that were broken down by the fungi.

And then the fungi benefited because they got to use the sugars that were supplied from the plants doing photosynthesis.

NARRATOR: Bolstered by this new relationship with fungi, plants evolved the ability to exchange sugars for vital nutrients like nitrogen and phosphorus.

Even with their thick, water-retentive coating, they can still get all the nutrients they need.

The emergence of this newly evolved mutual relationship, or symbiosis, is a key turning point for plants' journey.

Now plants can finally survive out of the water on the land.

The symbiosis between land plants and fungi is super-important.

It's essential for how ecosystems evolved and how they work today.

BAUMGARTNER: After millions of years and lots of evolutionary tries, plants were finally onto the land.

♪ ♪ NARRATOR: Plants and fungi created one of Earth's first complex terrestrial ecosystems.

♪ ♪ And their partnership continues to this day.

♪ ♪ It's often easy to spot mushrooms, which are the reproductive part of some types of fungi.

But most fungi live underground, where we can't see them.

So this plant has grown with a fungal partner, and you can see that, with the plant roots being intermingled with fungal filaments, and these wrap themselves around the plant roots and form these intimate associations.

NARRATOR: These associations are so vital that nearly 90% of plants living today are dependent on them.

FIELD: It's really easy to overlook fungi because, for the most part, they live underground, whilst plants grow much taller and are more obvious.

NARRATOR: But some fossil evidence from around 420 million years ago suggests that this balance once looked quite different.

♪ ♪ Something utterly astonishing has happened to some fungi.

They have become giants.

♪ ♪ Colossal fungal spikes tower over the landscape.

They're called Prototaxites.

Standing over 20 feet high, they reproduce by releasing spores that are carried by the wind.

CONTRERAS: The Prototaxites landscape would've been an alien world.

NARRATOR: So alien that when the fossils were first discovered back in 1843, scientists were not even sure what they were.

It was a very strange and odd thing when people found it.

It was shaped like a chunk of wood.

NARRATOR: But when they took a much closer look, they discovered something incredible.

So what this is is a very thin slice of Prototaxites.

♪ ♪ And we find that, unlike a log, which would be full of woody cells, instead we find a mass of these fungal filaments, reminiscent of fungi today.

JOHNSON: It creates, in my mind, one of the most bizarre prehistoric landscapes of all, 'cause there's nothing like it today.

NARRATOR: The towering Prototaxites dominate the landscape.

Plants, by contrast, are still tiny, measuring just a few inches or less.

Stuck by the water's edge, near to where they first made landfall.

LYDON: So, even though plants have made it out of the water, they're still confined to the edges of lakes and rivers.

A lot of the land surface on Earth at this time was also quite dry, and maybe rocky or very sandy.

NARRATOR: With no way of holding on to that water inland, Earth could've remained a fungal paradise.

For plants to establish themselves away from the water, they were gonna have to change, to evolve new traits that allowed them to live in the new environment.

NARRATOR: But there is a vital ingredient missing.

♪ ♪ Something that seems like it has always been part of the fabric of Earth.

♪ ♪ Soil.

Just one teaspoon of this crumbly brown stuff includes more living organisms than there are people on the planet, and that is absolutely unbelievable.

Despite the fact that soils only make up a small portion of the Earth's surface, there are thousands of varieties, including more than 20,000 in the U.S. alone.

♪ ♪ NARRATOR: Soil is formed by the combined action of organisms like bacteria and fungi working on biological matter like fallen leaves.

Combined with particles like sand or clay, it is the perfect material to hold all the ingredients plants need to thrive.

Soil traps and holds on to water and nutrients so that plants can use them all year round.

If we look in the fossil record, we see something absolutely remarkable.

About 470 million years ago, soils as we know them today did not exist.

Instead, we had the earliest version of soils, what we call proto-soils.

They were only maybe a millimeter thick.

They were formed from very thin layers of bacterial or fungal mats; they just didn't have the same kind of building blocks that soils today have.

NARRATOR: Early fungi do not need soil to thrive.

But plants need something to support and nourish them if they are to survive farther away from the water.

Soon, they start making soil.

♪ ♪ They begin slowly, over millions of years, breaking down the bare rock with tiny root-like structures called rhizoids, mixing in nutrients from their fungal partners.

♪ ♪ But the true transformative ingredient... ...is the plants themselves.

Generation after generation breaks down after death, building up a bed of soil.

Turning once inert rock into a living layer.

♪ ♪ Fast-forward 20 million years.

And with the growing soil layers, plants are transforming.

♪ ♪ They're much bigger.

And their ability to move water through their tissues by a kind of plumbing has allowed them to live farther away from the water.

They're now more varied, with deeper roots to support their taller stems.

Prototaxites, the giant fungi, still tower over them.

But soil makes it possible for plants to thrive much farther inland.

Their wind-borne spores reach drier upland areas.

♪ ♪ Without the barrier that kept terrestrial plant life near the water's edge, plants scale new heights, and diversify as they occupy new environments.

♪ ♪ For the first time, significant parts of Earth's land are turning green.

FIELD: Plants changed our landscape by terra-forming bare rock into living matter.

Life and Earth, they're not separate entities-- they're totally interconnected.

So, these changes that plants caused to the Earth's surface had huge consequences.

Pretty much everything about the landscape was influenced by plants moving onto the land.

NARRATOR: But plants don't just alter Earth's surface.

♪ ♪ CARVALHO: As plants are creating this beautiful green planet, carbon dioxide levels in the atmosphere actually start going down.

Part of the reason has to do with plants.

As plants are growing, they're accumulating carbon dioxide as they're transforming it into sugars, and making up their plant bodies.

As plants had exploded across the landscape, CO2 in the atmosphere decreased by upwards of 25%.

NARRATOR: Ironically, this poses a problem to plants.

Because carbon dioxide is plant food.

They absorb the CO2 for photosynthesis through structures called stomata.

CONTRERAS: Stomata are these cellular complexes that have a pore in the middle, and these are the gas exchange pores for plants, so it's the way they move carbon dioxide in and out of the plant body.

If you look at stomata under the microscope, it almost looks like they're breathing as they open and close.

JOHNSON: In early land plants, between 420 and 390 million years ago, most land plants had their stomata on their stems.

The falling carbon dioxide levels had a huge impact on plants.

In order to get as much carbon dioxide as they were before, they would need more stomata.

But where to put them?

CONTRERAS: The answer was as elegant as it was revolutionary.

♪ ♪ Leaves.

♪ ♪ NARRATOR: Leaves came about as an adaptation to cope with the changing atmosphere.

Leaves make plants more efficient by providing greater surface area for photosynthesis and for taking up CO2 for gas exchange.

♪ ♪ NARRATOR: In a relatively short time, most plants had evolved leaves, but this seemingly small innovation would trigger an entirely new dynamic between plants.

When we have plants growing close to each other, these big surfaces that capture sunlight are creating competition because of the shading of each other.

NARRATOR: This started a race for light that will ultimately transform the landscape once again.

♪ ♪ With leaves supporting this increased photosynthesis, some plants begin building their bodies out of tougher biological material, wood, which leads to the rise of a whole new type of organism-- biological machines unlike anything Earth has seen before.

Trees are photosynthesizing powerhouses.

With strong wooden trunks, they can grow much taller, effectively pushing past the competition towards the sunlight they need.

Trees were a huge step forward for terrestrial plant life.

Trees were remarkable at exploiting resources.

They had enormous, deep rooting systems, allowing them to access water and nutrients, they had these giant stems for stability, and on top of that, a canopy of leaves for photosynthesis.

Archaeopteris, like this one here, was one of the earliest trees, and they formed many of the Earth's first forests.

Here's one branch of Archaeopteris.

It has big woody stems, but the leaves kind of look like ferns, but kind of not.

BAUMGARTNER: The ability of Archaeopteris to consume vast amounts of carbon dioxide allowed them to grow to be almost 100 feet tall.

♪ ♪ JOHNSON: I often like to say that trees are made out of gas.

They absorb carbon dioxide from the atmosphere to make the carbohydrate in their plant bodies.

♪ ♪ NARRATOR: Earth is on its way to becoming a forest world.

♪ ♪ The Prototaxites are gone, never to return.

♪ ♪ And the majority of the fungi retreats from the surface, thriving in giant networks below the ground, where they have remained ever since, a vital partner to the plant life above them.

The spread of plants from the water's edge across the planet's surface is dramatic.

♪ ♪ Plants transform the face of the planet.

But in doing so, they also set the stage for global upheaval.

♪ ♪ This beautiful coastal redwood forest is part of a dynamic ecosystem, with trees playing an essential role, even after they can no longer stand tall.

A fallen coastal redwood locals call the Phoenix Tree is a rare instance of being able to see the interconnectedness of the forests above the ground.

So this tree fell over about 86 years ago, and the special thing about it is that that wasn't a death sentence.

Instead, beneath each separate branch are roots that are forming, and these branches are eventually going to become their own independent trees.

♪ ♪ These spread-out, shallow roots allow them to basically hold hands with each other for stability, and they can even share resources like water and nutrients.

NARRATOR: But this tree is just one small piece of a vast and interconnected system.

It's a living bridge that connects the atmosphere with the soil and the life within it.

When trees are alive, they are homes, they are food, and they provide moisture to the environment around them.

But they actually provide the same things after they've decomposed.

NARRATOR: Dead trees are broken down by organisms that return the carbon that was locked in them back into the atmosphere through respiration.

And this equilibrium helps keep the atmospheric CO2 in balance.

But around 350 million years ago, that balance was interrupted.

Leading to a crisis that would threaten the survival of all life on Earth.

♪ ♪ The rapid spread of terrestrial plant life has left Earth a changed planet.

In the Southern Hemisphere, ice sheets form.

But in the equatorial regions, the climate is still very hot and wet.

♪ ♪ As the ice sheets expand and contract, over millions of years, sea levels fluctuate, creating huge deltas where vast swamp forests spring up... ...covering millions of square miles... ...and giving rise to a whole host of new plant species that thrive in these wet conditions.

The largest among them, Lepidodendrons, grow over 160 feet tall.

These are part of a group called lycopods, or scale trees.

It looks like it's not a plant.

People often find these things, say, "These thing's got scales.

"It looks like a garfish, or a dinosaur, or some kind of snake, a reptile."

And in fact, these are called scale trees.

NARRATOR: The swamp forests these trees grew in would've looked similar to today's, except those trees would've grown incredibly quickly, creating a huge amount of plant mass in a very short time.

HETHERINGTON: Lepidendron trees could be enormous, and one hypothesis is that they reach this amazing height in as little as 15 years.

So this rapid plant growth would've removed enormous amounts of carbon dioxide from the atmosphere.

NARRATOR: But this was not the only impact these trees had on the atmosphere.

CARVALHO: So when these massive trees fell into waterlogged conditions, which typically are very low in oxygen, they become really, really hard to break down.

Through time, all this organic matter accumulates.

NARRATOR: This huge amount of plant mass was not broken down and returned to the atmosphere, as typically happens today.

Instead, it was locked away in the sludge of the swamp forests.

This would have immense consequences for the Earth's climate.

♪ ♪ NARRATOR: The deep basins of the tropical swamp forests were log-jammed with fallen trees and partially decayed plant matter that formed peat.

This carbon-rich mixture was then buried and compressed under millions of tons of marine sediment, deposited by fluctuating sea levels.

All the ingredients were in place, slowly transforming... (birds squawking) ...into an incredible type of rock.

We can see that substance in the black seam behind me, within the cliff face, within the rock records.

And that substance is actually coal.

And this coal forms when that plant material is buried within the Earth and subjected to the Earth's intense heat and the intense pressure.

And it forms this midnight black, energy-rich material.

So coal is very, very rich in carbon, and it's this period of geological time which is associated with the deposition of these thick coal deposits, so it's no surprise that this period got its name, the Carboniferous, which means coal-bearing or coal-producing.

♪ ♪ NARRATOR: During the 60 million years of the Carboniferous, plant life locked away billions of tons of carbon in the form of coal.

JACKSON: We know that present day, the burning of coal is having a dramatic impact on the Earth's climate, but actually, the formation of coal back in the Carboniferous had an equally as profound effect.

NARRATOR: With so much plant matter locked away underground, atmospheric carbon levels dropped.

And this is why massive sustained coal production kicked the whole system out of balance.

NARRATOR: And the impact was dramatic.

♪ ♪ Over millions of years, CO2 levels plummet, and global temperatures nosedive, in a reverse of the greenhouse effect... (wind howling) ...causing vast icy expanses.

The consequences of carbon hoarding by the equatorial swamp forests leave plants teetering on the edge.

In the frozen south, the forests die off en masse.

SOREGHAN: As carbon dioxide continued to drop, it would've become very, very cold, and up to a quarter of the world's landmasses would've been covered in ice.

NARRATOR: If this process had continued, it would've had dire consequences.

It could've potentially caused a Snowball Earth, or what we call a runaway glaciation, where ice could have covered the entire planet.

NARRATOR: Which would've set plants back hundreds of millions of years.

But that isn't what happens.

Beneath the frozen surface, the giant tectonic plates are continuing their perpetual movement.

SOREGHAN: Over the 60 million years or so of the Carboniferous, plate tectonics had been creating a new supercontinent.

They were zippering together, essentially along the Equator, and moving northward.

Mountains were being formed, new weather patterns were being altered and adjusted as a result of this change in the landmass on Earth.

NARRATOR: And with this change... ♪ ♪ ...about 300 million years ago, most of the swamps dry up and cease their production of coal.

SOREGHAN: When coal formation essentially ended, it meant that we weren't locking away as much carbon dioxide, and yet, volcanoes were still belching out CO2, but we weren't drawing it down as much.

And therefore carbon dioxide began rising in the atmosphere, the Earth began warming... ...and the glaciers started melting.

♪ ♪ NARRATOR: The ice sheets recede and eventually disappear.

This set the stage for a plant renaissance.

Plant life exploded in diversity, leading eventually to the green world we see today.

NARRATOR: Over the following few hundred million years, plants would overcome one challenge after another, developing flowers and fruit, co-evolving with insects, and eventually forming grasses and grains.

Plants transform the land from green into every vivid color of the spectrum.

♪ ♪ NARRATOR: Earth's transformation from a barren, rocky world to the living planet we see today is spectacular.

Rocks breathed life into the land, but then life altered the rocks, and changed them, and diversified and enriched our planet in so many ways.

If plants hadn't conquered the land, we definitely would not be here today.

BAUMGARTNER: The chair that I am sitting in is from plants.

The food that you eat, the clothes that you wear, everything you know is because of plants.

♪ ♪ So, no matter how we try, we can't escape the fact that this is a plant planet.

♪ ♪ NARRATOR: On "NOVA: Ancient Earth"... PAUL WIGNALL: It's the mother of mass extinctions.

Something utterly catastrophic.

SURESH SINGH: Much greater than the one that ended the age of the dinosaurs.

Now we finally know the culprit.

(explosion pounds) JEFFREY BENCA: We are finally able to piece together clues from this ancient crime scene.

SINGH: We're talking wildfires, acid rain, deadly U.V.

radiation.

This was hell on Earth.

NARRATOR: "Inferno" on "NOVA."

Next time.

SINGER: ♪ Wish I could go back in time ♪ ♪ ♪ ♪ ♪ ♪ ♪ ♪ ♪ ♪ ♪