NARRATOR: 75,000 years ago on the Indonesian island
of Sumatra, the supervolcano Toba erupts,
sending the entire planet into climatic chaos.
It unleashes a volcanic winter that may have pushed the planet
into a thousand-year Ice Age and brought human beings
to the brink of extinction.
But could this type of mega disaster happen today?
Is there another sleeping giant out there waiting
to erupt and wreak havoc?
And could our modern world and race survive?
STANLEY AMBROSE: You can't stop volcanoes.
Disasters of that scale are beyond human control.
What could we do to prevent another volcanic winter?
Well, nothing.
I mean, you really can't.
Nature has a way of doing what it wants.
And really, humans just have to adapt
to what nature throws at them.
NARRATOR: Today, the site where Toba erupted is showing
signs of increased volatility.
Workers are keeping a watchful eye on the seismic activity
in the area.
In the last million years, Toba has erupted four times.
Could it erupt again?
We believe there is magma still beneath Lake Toba.
There are small volcanoes inside the Toba Caldera.
And there are hot springs in there.
NARRATOR: For the time being, Toba does not
seem to be an imminent threat.
But elsewhere around the globe, supervolcanoes
are growing, from Italy to Japan to New Zealand.
They erupted in prehistoric times,
and they will erupt again.
When that happens, they will trigger disasters that
could cripple modern society.
ALAN ROBOCK: If an eruption the size of Toba
erupted today or tomorrow anywhere in the world,
there's nothing you can do.
Most people would die.
It's hard to imagine people that would survive.
If you had a fallout shelter with 10 years' worth of food
and enough weapons to keep all the other hungry people out,
you could probably live, if you call that living.
NARRATOR: What most people don't realize
is that some of the largest supervolcanoes in the world
are actually forming in our very own backyard.
GREG ZIELINSKI: There are regions in the Western US
that have the potential for mega eruptions.
NARRATOR: In California, the Long Valley
Caldera sits just next to Mammoth
Mountain, a popular ski resort.
When Long Valley erupted 750,000 years ago,
its ash covered everything as far south as Texas
and as far east as Nebraska.
In Northern New Mexico, the Valles Caldera
erupted more than one million years ago
and devastated the land as far east as Iowa.
But the most notable and active supervolcano
is actually one of America's favorite family destinations,
Yellowstone National Park in Wyoming.
The entire park is a gigantic caldera,
with boiling magma pulsing just beneath visitors' feet.
The magma that forms the supervolcano in Yellowstone
is actually what heats the geysers
and the mud pools and the other natural formations you see.
NARRATOR: Scientists believe that an eruption
at Yellowstone and its aftermath would be comparable to Toba.
GREG ZIELINSKI: Well, the interesting aspect
about Yellowstone is that when it erupts,
it looks like it erupts big, real big.
CRAIG CHESNER: Yellowstone has had two supervolcano eruptions
in the past few million years.
NARRATOR: In fact, it seems that Yellowstone
is on a 600,000-year cycle.
The last time Yellowstone erupted?
650,000 years ago.
What would happen to modern society
if one of these supervolcanoes in the Western United States
erupted?
If the same amount of ash from the Toba eruption
were to, say, erupt from Yellowstone,
that would cover the Continental United States
in about four inches of ash.
Imagine the effects that that would have.
It would just be horrible.
NARRATOR: Just as with Toba, the area
closest to the super eruption would face
the most immediate devastation.
Although Wyoming is a relatively rural state,
there are urban centers like Las Vegas within striking distance.
CHRISTINE GIBSON: If a supervolcano
were to erupt in the Western United States,
the odds of surviving in Las Vegas
would really be against you.
NARRATOR: Massive amounts of ash would be only
the first obstacle to overcome.
CHRISTINE GIBSON: Las Vegas would
be under inches of this really, really thick ash,
and it's heavy.
So it can collapse roofs.
It can bury animals.
It can bury people.
It can bury cars.
NARRATOR: As the ash spreads across the country,
the air would be too toxic to breathe.
People would be forced to wear protective masks
over their eyes and mouths.
Crops would fail, and water sources would become polluted.
Car engines would clog.
And any plane flying would come hurtling to the ground
as the engine stopped working.
Jets can't handle that volcanic ash.
It causes the engines to flame out,
especially the bigger jets, the 747s.
NARRATOR: But the ash fall would be just the beginning.
As we saw with Toba, while the ash settles,
the destructive sulfur gases would
be making their way into the stratosphere
and circling the rest of the world.
It wouldn't just be a case of, oh, poor old America suffers.
The world would suffer.
ALAN ROBOCK: The devastation of a natural disaster
like a hurricane or a tsunami or an earthquake
can be dealt with because even though the people there
are shocked, we can bring in food
from other places in the world.
We can bring in medical supplies.
If a supervolcano erupted today, there
wouldn't be anybody from anyplace
else that would come help us, like in these smaller
scale disasters.
It would affect everybody in the world.
NARRATOR: Scientists agree it's not
a question of if another supervolcano will erupt,
it's a question of when.
There's really no way to prepare for the eruption
of a supervolcano.
They're just so massive that this
is not a power that we can contain
with any known technology.
SUSAN JOY HASSOL: The Earth will survive.
I'm not sure that humanity would bounce back
from such a tragic kind of event,
such a dramatic and catastrophic kind of event.
So Earth, it turns out, is fairly resilient.
It's the conditions for human life that may be more at risk.
NARRATOR: Supervolcanoes do not just erupt.
They are catalysts for global climate change and devastation.
Unlike other natural disasters that are often localized
and even avoidable, a natural disaster
like the one unleashed by the Toba eruption is inescapable.
These volcanic winters and the consequences
of such apocalyptic change may only happen
once every 100,000 years.
But when they occur, the planet and its inhabitants
are never the same.