NOVA

66 million years ago,

planet Earth was very different from today.

Some of our ancestors at the time

might have looked like this furry creature.

The rulers of the land were giant reptiles.

Dinosaurs.

That's one of the most infamous, a carnivorous T. Rex.

And just behind are the bison of their time,

a common plant eater, Edmontosaurus.

But what happened to them all?

66 million years ago, an asteroid hit the Earth,

and scientists think that it was this collision

that wiped out the dinosaurs.

But no one has ever found the fossil of a dinosaur

that they know for certain died as a result of the impact.

However, a truly extraordinary dig site might change that.

Hell Creek Formation, North Dakota.

These sedimentary rocks are rich in dinosaur remains.

From triceratops...

to T. Rex.

Now, in a patch of land no bigger than two football fields,

a long-buried secret is coming to light.

Because this place might hold evidence of one of the most

dramatic events in all the four- and-a-half-billion-year history

of our planet.

Everything was fine

on Tuesday in the Cretaceous, and the next second,

the world just wasn't the same.

Any time that an asteroid

the size of Mount Everest smashes into the Earth,

that's not going to be a good day.

It's actually pretty remarkable

that anything survived.

Let me get down here between you.

For almost ten years, a team of scientists

has been trying to find out exactly what happened here.

You're at the edge

of your seat every moment

trying to dig this stuff up.

They call the site Tanis,

after an ancient Egyptian city,

and believe it could be a mass graveyard of creatures

which were killed

in the asteroid strike 66 million years ago.

This site might reveal the remarkable story

not just of how the dinosaurs lived,

but how they died.

The impact really was a worst-case scenario.

It's almost beyond

what we can imagine.

If the dig team is right,

Tanis could be a place where the remains

of a long-lost world are frozen in time.

A place that gives us, for the first time,

an unprecedented window...

into the lives of the very last dinosaurs.

And a minute-by-minute picture of what happened

on the day the asteroid hit.

"Dinosaur Apocalypse: The New Evidence"...

Right now on "NOVA."

This landscape is full of fossils

dating from the Late Cretaceous,

the period which began around 100 million years ago

and ended 66 million years ago, when the dinosaurs vanished.

Paleontologist Robert DePalma wants to find out more.

I think anybody who has ever liked dinosaurs in the past,

or still does, has thought at one point or another,

well, what happened to them?

Why are they not here anymore?

At the end of the Late Cretaceous,

fossil evidence tells us Hell Creek

might have looked like this.

There were low-lying marshy flood plains,

intercut by river channels,

and covered with horsetails, ferns, and trees.

Back then it was warm and wet here all year round.

If we go back to about 66 million years ago,

the Earth in some ways was very similar to today,

and in other ways it was an alien world.

The climate was very different, the temperature was different.

There were no ice caps at the poles.

Hell Creek is one of the most famous

and well-studied areas for digging up dinosaurs.

Hell Creek

is really the only place in the world,

at least right now, where we have a really good record

of the last surviving dinosaurs.

Hell Creek records the very, very last

days of the dinosaurs, and it's, it's the best information

that we have in the world about that extinction event.

This dig site lies in the northeastern corner

of the Hell Creek Formation.

66 million years ago,

instead of today's dusty prairies,

there were sandy, silty riverbanks.

Instead of rocky cliffs, there were forests.

And instead of the wildlife we know today...

Well, scientists are trying to find out what that was like.

One of the great things about paleontology

is also one of its most frustrating elements,

and that is that you can never be sure.

Until somebody builds a time machine

and we can go back in time and actually observe dinosaurs

in their natural environment,

we will never know for sure

whether our inferences are correct or not.

So it'll always be open to a bit of interpretation

and uncertainty because fundamentally

trying to pinpoint something that happened

on a given day 66 million years ago

is really, really tough.

And so a lot of paleontology

is putting forth theories

and then other paleontologists coming forward and saying,

"No, that doesn't make sense because here's another theory."

Every paleontologist can only hope

their site might uncover something new to debate.

A sand bank lying between a river and a forest

would one day become what Robert now calls Tanis.

The site had been explored by others in the past.

But it wasn't until after Robert and his team

started digging here in 2012...

So somewhere from between

there and down here is where that came from,

it's coming from up above. Hey, look at this.

What?

That anyone would know how important this site could be...

Here we've got this

freshwater environment of the Hell Creek formation, and this

shocking red, green colors coming from the shells

of ammonites, a marine organism,

kind of like a coiled snail in appearance.

So we've got this marine organism that's been thrown up

into this freshwater environment

and they do not belong here.

How they got there is a mystery,

but even more intriguing.

I'm just gonna go ahead and plane down some of this rock.

Sitting above the ammonite shells is something that

holds a crucial clue about the age of these rocks.

So this orange layer right here

is composed

100% of impact-related debris.

It is enriched in iridium.

Iridium is an element that's rare

in the Earth's crust,

but it's common in asteroids.

The layer it's in marks the K-Pg boundary.

The boundary is made up of dust and debris

from a huge asteroid impact.

It's been dated to 66 million years ago,

the time when dinosaurs disappeared.

Look at that. That's what you need?

Yeah, yeah, yeah, yeah, yeah, that's what we want.

Okay, so it's coming from this area here.

So somewhere within that region is where

these pieces are coming from.

And it has been found all over the world.

In this layer, the concentration of iridium

is a hundred times higher than the baseline

for the rest of the Earth's crust.

So, perhaps the simplest answer to that is that

it came from outer space.

And so we have this

wonderful marker

that is the iridium layer,

that coincides

with the extinction event.

So this is one of those few cases where you can

really tie what is often a fuzzy thing and kind of

bring it into focus because you have

this moment in time represented by that layer.

Having the K-Pg boundary here at Tanis

dates the site to around the time dinosaurs went extinct.

No rattlesnakes.

Once you see that layer,

once you identify it,

it really does stand out because it is a thin layer

of rock that caps one world,

the world of dinosaurs.

And it ushers in another world, a world where you never find

a single dinosaur bone or tooth or footprint again.

What makes this site even more exciting is

the rock layer right beneath the boundary,

in which Robert and his team found the ammonites.

The rock here is really not quite rocky,

and it just falls apart in your hands.

This crumbly rock isn't unique,

especially in Hell Creek.

But it's rarely found in layers like this one.

Over four feet thick, this layer contains

several geological features which, to an expert,

signify that it was deposited very rapidly.

As in a storm or a flood,

burying anything within it in an instant.

Which could mean that

anything in this layer would have been quickly entombed,

like the bodies in the volcanic ash of Pompeii.

Generally speaking,

the faster you get buried after you die,