One day, about 12 million years ago in the heart of North America, herds of rhinos were grazing on wide-open grasslands, while turtles swam in a nearby watering hole.

Early dogs hunted in the underbrush, and long-legged cranes searched the water for food.

I like to think that it started out as a nice day.

But it definitely ended pretty terribly.

The air quickly filled with volcanic ash, dimming the sky and falling to the ground in clumps, forming drifts like snowbanks.

The ash coated everything, including the plants the animals ate, causing their teeth to become scratched and worn as if they'd been chewing mouthfuls of sand.

And they unavoidably inhaled it - the sharp, glassy edges slicing at their lungs.

After days or weeks of suffering, the rhinos and other animals eventually succumbed, and were slowly buried in ash.

The watering hole where that took place is now known as the Ashfall Fossil Beds in... Nebraska.

Yes, Nebraska -- where there are no volcanoes within a thousand kilometers.

So what caused this really bad day in the Miocene?

Where did the ash come from?

The answer rests inside one of the most dynamic, transformative, and potentially dangerous features in North America, a feature that's also responsible for some of the continent's most amazing fossil deposits: A volcano we now call Yellowstone.

The Yellowstone volcano is actually a supervolcano.

That means it's more explosive than your run of the mill volcano, and the effects of its eruptions are much greater.

One of its eruptions, 2.1 million years ago, was about 6,000 times as big as the eruption of Mt St Helens in 1980.

Today, Yellowstone slumbers beneath the ground in Wyoming.

But 12 million years ago, it was in Idaho.

And 16 to 14 million years ago, it was on the border of Oregon, Idaho and Nevada, making a brief stop in its long, slow migration across the continent.

The volcano itself isn't actually moving.

Instead, it's fed by a mantle plume - a massive body of magma that originates deep inside the earth before upwelling toward the crust.

That plume holds relatively still, while the continent above moves over it.

Geologists refer to the volcanoes and craters made by these-plumes as hotspots.

As North America is slowly pushed across the Yellowstone plume by plate tectonics, it creates a string of volcanic scars above where the plume once was.

And no matter where it's been, every few hundred-thousand years, the supervolcano erupts, churning out remarkable amounts of lava and ash.

But, as big and impressive and even destructive as those eruptions have been, none of them have actually caused any major extinction events.

In fact, for paleontologists, what makes the Yellowstone plume so special is not just the magnitude of its eruptions - it's the fact that those eruptions have turned out to be really, really good at preserving fossils.

So how does a volcano preserve fossils?

After all, lava plus animals equals, well... fire and burning.

Unless you're Chris Pratt, in which case you're somehow just fine.

So how can something like that be responsible for saving fossils rather than destroying them?

Well, let's talk about those Nebraskan animals first, because not only are they amazing, it's also where I did my undergraduate summer internship back in 2006.

Now 12 million years ago, the Yellowstone mantle plume was under Idaho, and the volcano there erupted, sending ash flying 1600 kilometers away, probably reaching as far as the Atlantic Ocean.

The ash killed and buried the animals, preserving them at Ashfall Fossil Beds in faraway Nebraska, creating the only fossil formation made entirely of falling ash.

And although the event sounds sudden, the process at Ashfall was painfully slow for those involved.

When the ash started to fall, smaller animals, with their smaller lungs, were quickly overwhelmed and died.

This is why the bottom of the Ashfall fossil beds contains a layer of small animals, like turtles and birds.

Their remains were then trampled on and scavenged, as other larger, hardier animals continued to visit the watering hole.

And next came a layer of medium-sized animals, including horses and small camels.

They would've survived longer than the turtles and birds - for weeks, rather than for hours or days.

As a result, the skeletons of these animals show signs of bony growths, which are caused by the long-term inhalation of ash.

An affliction known today as "Marie's Disease".

Then, on top of all this, is a layer of more than 100 individuals of the rhino species called Teleoceras.

Being bigger than all the other animals, the rhinos were able to live longer.

Their remains, too, show evidence of Marie's Disease, and they may have lasted weeks or even months, only to finally succumb to fever, pain, and irreversible lung damage.

Now, as painful as they were, the deaths of these animals were still scientifically invaluable.

Ashfall preserved so many articulated rhino skeletons that scientists are now able to tell males from females - males were larger had bigger lower tusks And the way the rhinos were preserved even taught us a lot about their behavior.

Because their skeletons were found together and mostly uncrushed, it seems that the rhinos died at about the same time, as a herd.

That herd was made mostly of juveniles and adult femalestheir mothers, some of whom were pregnant with their fetuses preserved.

There were only a few males in the formation, which suggests that there probably weren't many males in the herd.

Now, some modern rhinos form herds, but their herds are typically about a tenth the size of the one discovered at Ashfall.

And that sort of knowledge about herd structure is almost impossible to get in the fossil record, because it requires that a lot of animals die together, and very quickly.

Otherwise, it's hard to say if animals died in a group, or if lots of solitary animals died in the same area over time.

But it's not only volcanic ash that can preserve amazing fossils.

14 million years ago, lava from the Yellowstone Plume coursed through what's now Washington state, filling rivers and lake systems.

And while lava normally burns soft tissue - as anyone who's played "the floor is lava" will know - this lava was cooled enough by the water that it left a particularly brilliant fossil.

Lava spilled down into a lake and covered a dead, bloated rhino called Diceratherium, which was either on the shore of the lake or was already floating in the water.

And remarkably, the lava cooled so quickly that the rhino became entombed in rock - still floating upside down.

Eventually the tissue inside the rock rotted away.

And what's left today is an imprint of a rhino's body, upside-down, with fossil bone fragments found inside.

It's truly a one-of-a-kind specimen.

But though these sites are amazing, the Yellowstone plume's greatest claim to fossilization fame is a bit more... indirect.

In addition to preserving individual fossils, it's also responsible for preserving the formations that contain fossils.

For example, the John Day Formation in Oregon is famous for its fossil deposits, containing animals like 30 million year old dogs and sabertoothed cat-like animals called nimravids.

These rich fossil formations are made of ancient dirt, or paleosols.

And the paleosols of the John Day formation are pretty soft and prone to erosion.

When they're exposed to water, they -- as well as the fossils they contain -- are typically weathered away.

But!

15 million years ago, those gently eroding paleosols were coated in a thick layer of lava from the Yellowstone Plume, which at the time was sitting under where Oregon meets Idaho and Nevada.

And lava is much harder than the paleosols, so this new layer protected the fossil-bearing rocks below -- at least, until a river slowly wore down the lava and exposed the fossils.

But it's not just one formation that's been protected by the Yellowstone plume in this way.

The volcano's many eruptions have sent lava flowing all over the Pacific Northwest, and hot clouds of ash formed hard, glassy layers that spread even further south, into Nevada and California.

These eruptions preserved many important and impressive fossil formations, like the Ellensburg Formation in Washington and the Virgin Valley formation in Nevada.

And there are still plenty of parts of the continent that are covered in thick layers of Yellowstone's lava or ash.

So there are probably a lot of other fossil deposits that we don't even know are there!

Needless to say, the Yellowstone supervolcano isn't done yet.

As you might imagine, because it's so big and potentially life-threatening, Yellowstone is monitored verrrry closely by geologists.

I mean, ash clouds blocking out the sun and lava flows covering several states might be great for fossils, but it's not so great for ... anything that's currently alive.

Now, of course, volcanic eruptions aren't exactly regular.

But geologists have found that Yellowstone's most major eruptions tend to occur about every 600,000 to 800,000 years.

And its last major eruption was 640,000 years ago, while its last eruption of any size was a smaller outpouring of lava only 70,000 years ago.

So the Yellowstone supervolcano's next eruption might happen kind of soon -- but only in geologic terms!

And in any case, don't worry about what might happen the next time that thing blows.

Instead, think of all of the exquisite Holocene fossil deposits that will be preserved for future paleontologists, with the rich variety of North American life frozen in time, just like those rhinos were that fateful day 12 million years ago.

Hope that makes you feel better.