- Hey, smart people, Joe here.

I'm in the Scablands of Eastern Washington right now and, well, that's a waterfall behind me.

It's a pretty nice waterfall, but as waterfalls go, it's actually pretty average.

It's height, the volume of water that falls over its face, they don't rank anywhere near the top 10 as waterfalls go, but this waterfall does hold one important record among all waterfalls.

It was created in what is perhaps the largest flood to ever happen on planet Earth, at least that we know of.

Whatever you're imagining as this flood, you need to think bigger, much, much bigger, because the flood that created this and the entire landscape around us is bigger than anything that could happen on the planet today, by a long shot.

(playful music) Across what is today a dry and aired landscape, there are clues of an epic flood hidden in rocky scars and strange land forms, if you know how to read them.

The pieces of this mystery are hard to make sense of on their own, but together they tell a story that's, well, almost impossible to believe.

In fact, it took decades for scientists to finally accept that these cataclysmic events really did happen.

And in the process, this story completely changed science, forcing geologists to totally rethink their ideas about the powerful forces and events that have shaped the earth throughout deep time.

This discovery also set the stage for other discoveries about other violent events that have shaped not only our planet, but others.

But we're getting ahead of ourselves.

Our story begins hundreds of miles to the east in a beautiful river valley where today we find the town of Missoula, Montana.

- [Kallie] Here we are.

Here it is.

- Okay, what's it?

- This is the Glacial Lake Missoula High Water Mark.

So this marks the highest level that Glacial Lake Missoula got in Missoula Valley, so this would've been the shore of a giant glacial-fed Lake.

- You guys know Kallie from PBS Eons, right?

She hiked us up here to see this, which is why I'm so not sweaty at all.

- Yeah, you gotta hike to see these high water marks.

- If Kallie and I had stood in this spot 15,000 years ago, we'd have found ourselves in the shore of an immense lake.

At the end of the last Ice Age, the valley below us set beneath 300 meters of peaceful blue water.

- As far as you can see would've been basically underwater, except for where we're standing and the tops of some of these taller mountains, all of it would've been underwater.

- These would've just been little pointy islands.

- Just little islands, yeah.

- It was called Lake Missoula, a fresh water inland sea, more than 600 meters deep at its deepest point, holding more water than Lake Ontario and Lake Erie combined, but why was there a lake here?

About the past million years, massive ice sheets advanced and retreated across the Northern Hemisphere during natural cycles of climate change.

During the last Ice Age, an ice sheets stretched unbroken from Alaska to today's US-Canada border.

And one little finger of that ice sheet sat here at the end of this river valley, and that massive mountain of ice would've been right here.

You need to picture something more like the wall from Game of Thrones than like a big iceberg.

We're talking 2000 feet tall of ice, 30 miles wide at its widest, and the south end of it ran up against these mountains right here, blocking the river and forming Glacial Lake Missoula.

It was big, real big.

Just like a stopper filling a bathtub, river water, rain, and melted snow backed up behind this ice dam for centuries, forming a lake covering nearly 8,000 square kilometers, but there's just one problem.

Ice floats in water.

First, a few leaks began to form in the ice dam, and with the weight of thousands of cubic kilometers of water pushing against it, one day... - Just boom!

It catastrophically failed.

The explosion, the sound of it probably would've carried throughout the entire Northwest.

It drained probably in a couple of days.

- It's just impossible to imagine like a 2,000 foot wall of ice, like Game of Thrones style, just like- - Like failing.

- Just it cracks - Just gone.

Yeah, yeah, it would've been pretty spectacular, to say the least.

- A glacial lake twice the size of Rhode Island emptied in just a couple days, and the torrent of water escaping the broken ice dam was equivalent to more than 10 times the flow from all the rivers on the planet.

That is insane.

This kind of sudden glacial outburst is called a joökulhlaup.

Icelandic words are awesome and kind of scary.

It's almost impossible to put into scale the unimaginable power of these floodwaters, but right behind me, we can see proof of them.

If you've ever seen those little ripples that form in the sand at the bottom of a little stream, well, there's ripples at the bottom of this valley caused by the flood waters.

It's exactly the same phenomenon as in that little tiny stream, enlarged to the size of an entire prairie.

It's mind boggling.

As thousands of cubic kilometers of water emptied out of this giant bathtub, flowing more than 120 kilometers per hour, it's set off the worst and most destructive flood that we know of in Earth's history.

We think of erosion as something happening slowly over eons, but these Ice Age floods cataclysmically reshape the landscape in just a matter of days.

Water swept the land clean of top soil, every bit of gravel, every bit of sand, gone.

It even ripped away bedrock in some places.

From above, we can see the scars left by this violent flood, branching channels and coulees carved deep into the earth.

They give this region its nickname, the Channeled Scablands, and there's nowhere that demonstrates the unimaginable scale and power of these flood waters better than here, Dry Falls.

About 50 kilometers south of the Grand Coulee Dam, we find what looks today like a deep, broad canyon, slowly carved into the earth over eons, perhaps, but that's not what happened here.

Everything you see was carved almost instantly by the violent flood waters from Lake Missoula.

During the floods, this was the largest waterfall in the world, five times the width of Niagara Falls.

The water was more than 30 meters deep as it rushed over the edge, ripping away rock from its 120 meter face and carrying it downstream.

Elsewhere, powerful flood surges cut deep vertical canyons and cataract cliffs in just days.

The raging waters were powerful enough to redirect entire rivers, like at Palouse Falls.

The ancient path of this river flowed west through the Washtucna Coulee, but during the floods, the river overflowed its banks, raged south and cut this new waterfall.

All across the Scablands, raging whirlpools of flood water carved potholes and craters into the bedrock.

The floods were so powerful, they carried giant granite boulders embedded in icebergs, dropping them hundreds of kilometers from the mountains where they originated.

This here puppy is a found piece of Canadian granite.

Got no place here, though.

These so-called erratics are scattered throughout Eastern Washington today, some even settling as far as Western Oregon, but there's another mystery to solve.

When we think about eroding rocks, that seems like something that should take thousands, millions of years, even.

How did a flood that only lasted a couple of days do so much damage to this landscape?

That's 'cause it's made of a very special kind of rock that came from another kind of flood millions of years before the Ice Age floods.

It was a flood of lava.

All across this area, massive eruptions happened that laid down lava 2,000 feet thick in some places and that cooled into this, basalt.

These are giant basalt columns behind me.

As that lava cooled over decades, even centuries, it contracted, and as it contracted, it broke.

It fractured along these amazing geometric faces.

This one's super cool.

You can see it's like a perfect hexagon.

They're almost like beehives for giant lava-eating rock bees or something.

So when that flood came through here, these columns were already pre-broken, ready to be swept away in 80 mile an hour tidal wave tsunamis of flood water.

It's also a place that rattlesnakes like to hang out, so I think I've been here long enough.

So thousands of years ago, a wall of ice was violently destroyed, emptying 20 million cubic kilometers of water per hour into Eastern Washington, scouring and reshaping the surface of the earth, causing eons of erosion in mere days, and eventually emptying out into the Pacific Ocean where huge amounts of sediment and rock from Montana, Idaho, and Washington can still be found today.

And what's crazy is this didn't just happen once.

It's now thought that this ice dam repeatedly melted and refroze as many as 25 times over several centuries, each time devastating the land with epic floods.

We can see evidence that Lake Missoula repeatedly filled and emptied in the so-called strandlines marking the hills around the town, each an ancient shoreline of this Ice Age lake.

And elsewhere, striped deposits called rhythmites show layers of sand and sediment laid down in flood after flood.

These violent events all happened surprisingly recently in human history.

Archeological evidence tells us that humans had already crossed into North America by the time of the floods.

The creation stories told by the Nez Perce and Palouse Indian tribes even contain similar flood elements.

- We don't have a lot of human artifacts or fossils associated with the lake emptying, but we know we had animals and humans in the area during this time.

- And then you didn't.

(both laughing) - Well, I'm just saying that human eyes probably saw the lake and experienced the effects of the flood and the catastrophic failure of it.

And it took a lot of work, some really smart people to go against the grain and be like, "No, no, no, this is a catastrophic flood."

And they'd be like, "Nah, no, no, no, no.

That's not how it happened."

- When geologists, J. Harlen Bretz first proposed the idea of the Missoula floods in the 1920s, people thought he was nuts.

At the time, geologists just didn't think that large catastrophic events like this really happened.

Instead, everyone figured that Earth's geologic formations were carved only by the same slow, gradual processes still happening today, like rivers, glaciers, and weather.

The old slow and steady idea called uniformitarianism now stood alongside a new idea called catastrophism, where rapid and violent events could sometimes shape Earth's geology in ways that slower processes couldn't explain.

And suddenly geologists began to see catastrophism as an answer to other unanswered questions, like the formation of our moon by a planetary collision, and the asteroid impact that caused the dinosaur extinction, and more.

The Ice Age floods of Lake Missoula changed the face of our planet in violent and dramatic ways, but they also change science.

Our planet's shapes and scars tell stories written in time and stone, if you know how to read them.

So listen to the earth, keep your eyes open, and stay curious.

Thank you, Kallie, for hiking us up here.

And if you want to be taken on more great prehistoric adventures where you don't have to sweat as much as I did.

- Yeah.

- Go check out PBS Eons.

- For sure.

- That massive mountain of ice would've been, Sports Center update.

(Joe laughing) There's a puppy, okay.

Allow me to model for scale.

- [Kallie] Yes, always need a scale bar.