GWEN IFILL: Finally: big questions about a big fossil find.
That's the subject of tonight's "NOVA." It focuses on an unexpected dig near the Snowmass Ski Resort in the Rocky Mountains of Colorado. In 2010, a bulldozer operator found huge bones that turned out to come from a watering hole dating to the ice age, a small lake used by ancient elephants such as mammoths and mastodons.
Here's an excerpt.
NARRATOR: The mastodon's watering hole would have been ringed by fur and spruce, much more lush than today's aspen, making Snowmass an elephant resort 100,000 years before it became a human one.
But was it a resort where bad things happened? Among the debris fields left by the landslides, those deposits of dirt, rock, and boulders, the scientists find teeth that indicate mastodons of every age lived here and died here.
KIRK JOHNSON, Vice President of Research and Collections/Chief Curator, Denver Museum of Nature and Science: There are a lot of animals that didn't make it through their whole lived span at Snowmass. Something caused them to die prematurely. It wasn't just a nice lake to go drink and occasionally drop dead and get buried. It was a lake where, if you were there at the wrong time, something might kill you.
DANIEL FISHER, University of Michigan: What we in fact have at Snowmass is something that looks more like a snapshot of a living population. It really looks like what you would get if you took a whole family unit and just flipped the switch on them all at the same time.
NARRATOR: What could flip the switch on a family of mastodons? The landslides look like they could have been caused by an earthquake, but how could an earthquake kill an entire population of mastodons?
Particularly deadly is an earthquake phenomenon called liquefaction. When earthquakes hit an area where the water table is high, it mixes solid ground with water and creates a kind of quicksand into which even entire buildings can sink.
Dan Fisher wonders if liquefaction could explain how all these mastodons died.
DANIEL FISHER: And the scenario that I began to entertain -- and all of this, you realize is just, hmm, what if -- was that animals could have been, say, down to drink for the day, to drink, to bathe, to play in the water of the lake, the ancient lake. Everything was fine.
They walked in, and the substrate was firm, as it always had been in their experience. But if an earthquake then struck and we began to get that trembling, and if then the sediment began to be to be -- to undergo this process of liquefaction, it would then change the pudding, essentially. And animals that had been standing comfortably on a substrate could begin to sink down into it, maybe only to their ankles, maybe only to their knees.
But when the shaking would stop, within a matter of seconds, that sediment would change back again to its firm state. And if they were in up to their knees, there'd be no way that they could work themselves out.
JUDY WOODRUFF: From tonight's "NOVA."
Hari Sreenivasan talked with one of the key researchers leading the project.
HARI SREENIVASAN: More now about these mammoth findings. It comes from Kirk Johnson, vice president and chief curator at the Denver Museum of Nature and Science, who's featured in the "NOVA" documentary.
Kirk, thanks for being with us.
First, how -- who stumbled upon this?
KIRK JOHNSON: You know, it was a bulldozer operator named Jesse Steele, who was pushing dirt on October 14 of 2010, and he ran his bulldozer right through a young female mammoth skeleton.
HARI SREENIVASAN: And he obviously didn't know it. So he does the first, best thing, which is, what, call the museum right away?
KIRK JOHNSON: Well, he didn't call the museum. He called the supervisor, who called the property owner, who called the museum.
And we ran out and looked at the site. And it was a beautiful little skeleton. We thought, well, that's great. And then, as we looked around the site more, we realized there was more than one skeleton. There were bones all over the place.
HARI SREENIVASAN: And now we know that this is one of the largest sites -- dig sites in American history. Put it in perspective for us. How many bones did you find in how much time?
KIRK JOHNSON: We dug for 69 total days. We had enough volunteers digging that we worked about 3,200 person days of effort. We moved about 8,000 tons of dirt by hand, and we found over 5,000 bones.
And I've got one right here with me. This is the tibia of a mastodon. It's beautifully preserved. And when we found these things, you could pull them right out of the ground and hold them up in the air. And it was just like a "Flintstones" moment. You could just think of yourself as being back in the ice ages. It was an incredible experience.
HARI SREENIVASAN: So, how is it possible that they're so well-preserved?
KIRK JOHNSON: The site is actually preserved at 9,000 feet. It's capped by a layer of clay.
And, clearly, this site was cold and poor in oxygen. So when we were breaking over the layers of peat that were covering the first mammoth, we were finding green leaves that we later found out were more than 50,000 years old. Once they were buried -- they were buried rapidly in green 50,000 years ago.
And, at that point in time, what happened was, they were preserved from being exposed to oxygen, and that kept them green. So, in fact, when we pulled the peat open and saw the leaves, then they oxidized immediately and turned black right in front of our eyes.
HARI SREENIVASAN: Wow.
So what's the reason that all of these different types of bones are clustered together in one area?
KIRK JOHNSON: That's sort of a $64,000 question at the site and that's what we addressed in the "NOVA" special. We talk about one hypothesis.
But it's very unusual to find that many bones in such great condition in such a close proximity to each other. And we've been looking at a number of different options. But one of the options we explored for the burial of the mastodon bones in particular is a series of earthquake-driven liquefaction or quicksand events, followed by landslides.
And the bones, as you saw when I held this thing up, are really in pristine condition. There's no marks on them at all. Yet, they were totally separated from each other. So it's a bit of a forensic mystery how so many animals died and their bones are preserved in such great condition.
And that's really part of the ongoing science of the site is, what did happen here? And we really opened up the questions in the "NOVA" special, but we have a team of 45 scientists from 19 institutions around the world that are right now working through the science of the site and understanding what happened at this lake near Snowmass Village.
HARI SREENIVASAN: So if these animals were stuck in quicksand let's say for days at a time before they died of starvation, why wouldn't any predators come in and take care of their bones?
KIRK JOHNSON: It's a really interesting question and a good question.
And one of the things we're going to do to test that over the next six months, we have this amazing array of bones. We have over 5,000 bones from the site. And what we're going to do now is look at every single bone and catalogue any potential predator marks or scavenger marks on these bones.
And it may turn out -- and we haven't done that yet, so we don't know what the result is, but, in general, we're not finding as many scavenger marks as we would expect for this scenario. So, like any good scientific project, this is hypothesis A. And we're going to test it by looking at the bones and see if we can find another explanation that maybe is a better explanation.
Remember, this site was discovered on Oct. 14, 2010. We dug through the middle of last summer. And we're just getting the bones out of their wrappings right now. So, the TV show happened in concert with the dig, and it's all happening really rapidly.
HARI SREENIVASAN: So, now, what's next? What's going to happen with all these bones? All these institutions around the world are studying it. Is there a report that you publish that is the definitive answer?
KIRK JOHNSON: We'll have bone scientists looking at the bones. We have amazing preservation of fossil plants, so we have palynologists and fossil seed specialists and fossil wood specialists looking at the wood.
And our goal is to put together the story of what happened at this site. We have -- the bottom of the lake deposits was deposited about 120,000 years ago, the top at about 40,000 years ago. So, we have about 80,000 years of history from the Colorado Rockies preserved in this site.
And not only do we find out about the animals and plants, but we actually have a pretty good read on what was happening with the climate in the Rockies at that time, as the glaciers came and went during the ice ages.
HARI SREENIVASAN: All right, Kirk Johnson from the Denver Museum of Nature and Science, thanks so much for your time.
KIRK JOHNSON: Hey, my pleasure. Thank you very much.
GWEN IFILL: "Ice Age Death Trap" airs tonight on "NOVA" on most PBS stations.
Learn more about the phenomenon called liquefaction on the NewsHour Science page.