NOVA scienceNOW: September 1, 2009

PBS Airdate: September 1, 2009
Go to the companion Web site

NEIL DeGRASSE TYSON (Astrophysicist/American Museum of Natural History): On this episode of NOVA scienceNOW: Everybody knows earthquakes happen in places like California, but there may be a looming threat where you least expect it.

Memphis, Tennessee? That's right; land of Elvis, barbecue, and Beale Street.

These folks don't look too worried, but maybe they should be. Some of the most powerful earthquakes in history happened right under their feet.

Now, geologists are deep underground, uncovering a frightening pattern.

BRUCE FOULKE (University of Illinois at Urbana-Champaign): And that layered stalagmite is what lets us recreate the timing of earthquakes.

NEIL DeGRASSE TYSON: Could the next big one strike America's heartland?

Also, why are these fruit flies being rolled and bumped on a machine called the deprivator? It's all in the name of sleep research.

MATTHEW WILSON (Massachusetts Institute of Technology): Sleep is an enigma. What is its purpose? That's something that we do not understand.

NEIL DeGRASSE TYSON: But new studies indicate that one purpose of sleep may be to help us learn, that while we snooze, our brains replay memories, maybe even editing and enhancing them.

ROBERT STICKGOLD (Beth Israel Deaconess Medical Center): When we wake up in the morning, in some way, we have a different brain.

NEIL DeGRASSE TYSON: And in our profile, you'll meet "the Stephen Hawking of South Korea."

SANG-MOOK LEE (Seoul National University): He looks into the universe; I look into the bottom of the sea.

NEIL DeGRASSE TYSON: A geophysicist and professor, struck down in his prime.

SANG-MOOK LEE: My van flipped, I was crushed.

NEIL DeGRASSE TYSON: He struggled for acceptance in a society that shuns the disabled.

SANG-MOOK LEE: People with disabilities tend to hide in a shadow.

NEIL DeGRASSE TYSON: Fueled by his passion for science, he fought his way back, inspiring others to overcome their disabilities.

SANG-MOOK LEE: I ask myself, "What can I do, actively, to give back?"

NEIL DeGRASSE TYSON: All that and more on this episode of NOVA scienceNOW.

Funding for NOVA scienceNOW is provided by the National Science Foundation, where discoveries begin. And...

Discover new knowledge; biomedical research and science education; Howard Hughes Medical Institute: HHMI.

And the Alfred P. Sloan Foundation, to enhance public understanding of science and technology and to portray the lives of men and women engaged in scientific and technological pursuit.

And the George D. Smith Fund.

And by PBS viewers like you. Thank you.

SLEEP

NEIL DeGRASSE TYSON: Hello. I'm Neil deGrasse Tyson, your host for NOVA scienceNOW.

We all know that dreams can be, dreams can be a little weird, sometimes filled with bizarre events that would never happen in real life. Nobody really knows why we dream. In fact, nobody really knows why we sleep.

Here are some folks who are trying to figure it out.

Amita Sehgal likes her flies, fruit flies, to be precise.

AMITA SEHGAL (University of Pennsylvania and Howard Hughes Medical Institute): I do have a genuine affection for them.

NEIL DeGRASSE TYSON: But sometimes, she has a strange way of showing that affection—especially, when she puts them into this thing.

AMITA SEHGAL: We use this piece of equipment we call "The Deprivator."

NEIL DeGRASSE TYSON: The Deprivator? It's like riding a roller coaster during an earthquake.

What's interesting to Sehgal is what the flies do after spending a whole night in here. The flies on the left were undisturbed last night, and they look fine. But the flies on the right, they were jostled all night long in the Deprivator. Now, some of them look dead, but they're not. According to Sehgal, they're catching up on lost sleep.

AMITA SEHGAL: If we keep flies awake at night, they need to make up for the sleep they have lost, and so will sleep in the morning, at a time when they're normally active.

NEIL DeGRASSE TYSON: But why would flies need to sleep? Could it be for the same reason we need to sleep? Maybe. But if you ask an expert what exactly that reason is...

MATTHEW P. WALKER (Harvard Medical School): We actually know very little about what sleep is doing for the brain.

AMITA SEHGAL: We spend a third of our lives sleeping. If you don't sleep, you die.

MATT WILSON: Sleep is an enigma. What is its purpose? That's something that we do not understand.

NEIL DeGRASSE TYSON: Looks like a waste of time. But then why would so many creatures do it?

MATT WILSON: Sleep is something that, the more we look at it, the more we see that it is fundamental. It's fundamental to essentially all organisms.

NEIL DeGRASSE TYSON: Including, it seems, organisms like fruit flies. When they're not being knocked around all night, Amita Sehgal's flies follow a pretty familiar schedule.

AMITA SEHGAL: They're active during the day and they sleep at night, for the most part, although there is an afternoon siesta as well, especially in males.

NEIL DeGRASSE TYSON: Trying to pinpoint the reason for a fly to snooze up to 12 hours a night, Sehgal's lab studies the fruit fly's brain.

AMITA SEHGAL: What we were doing was trying to figure out which part of the fly brain was important for sleep.

NEIL DeGRASSE TYSON: Sehgal's experiments pointed to the mushroom body, a part of the brain found in creatures like insects and spiders, but not in humans. Biologists have known about the mushroom body for years, but they associated it, not with sleep, but with something else entirely, an insect's memory.

AMITA SEHGAL: There is, then, this structure in the fly brain, which we already knew was required for memory, and we now find that it controls sleep.

NEIL DeGRASSE TYSON: The finding's intriguing because, for a long time now, sleep researchers have been debating a possible connection between sleep and memory.

Bob Stickgold has been looking into this possibility, sometimes in unconventional ways. For him, video games are research tools that can help reveal how our brains learn.

ROBERT STICKGOLD: Do you remember, when you first started playing Tetris^®,...

NEIL DeGRASSE TYSON: Oh, yeah.

ROBERT STICKGOLD: ...that you went to bed at night, and you lay in bed, and you closed your eyes, and you saw little Tetris pieces floating around in front of your eyes?

NEIL DeGRASSE TYSON: How did you know that? How did you know that...

ROBERT STICKGOLD: Because...

NEIL DeGRASSE TYSON: ...I dreamed Tetris shapes?

ROBERT STICKGOLD: ...because everybody does.

NEIL DeGRASSE TYSON: After taking a few rides on a ski machine, Stickgold's research subjects fall asleep, and then he promptly wakes them up.

ROBERT STICKGOLD: If we wake you up just two or three minutes after you fall asleep and ask you, "Neil, what's going through your mind?" You'll say, "Seeing those suckers somersaulting down when I crash."

NEIL DeGRASSE TYSON: And why would I dream of this embarrassing moment?

Stickgold is convinced that while you sleep, your brain is reviewing what you've learned and strengthening your memories.

ROBERT STICKGOLD: The brain is being modified while we sleep, so that when we wake up in the morning, in some way, we have a different brain. And it's a brain that functions better.

NEIL DeGRASSE TYSON: At least it seems to function better on some kinds of memory tasks. Recent studies show that after a single night's sleep, sometimes even after a nap, we can do a better job recognizing visual patterns and even solving some math puzzles.

MATTHEW WALKER: What we're going to have you do is try and type out a short, five-digit sequence.

NEIL DeGRASSE TYSON: I saw it first-hand when I took a simple typing test, typing a string of five numbers over and over again, as fast as I could.

After a night's sleep, I could suddenly type the numbers faster and more accurately. And research backs this up. Most people improved their typing by about 20 percent after sleep.

MATTHEW WALKER: Practice doesn't make perfect. It seems to be practice with a night of sleep that makes perfect. Sleep is enhancing that memory so that when you come back the next day you're even better than where you were the day before.

NEIL DeGRASSE TYSON: But exactly how could sleep enhance your memory? We don't know. But possible clues have been showing up, not just in the brains of flies, but in the dreams of rats.

M.I.T. researcher Matt Wilson says he can read rats' minds, including their dreams, with tiny electric probes.

MATT WILSON: What it means is that we're able to, at any time, plug in our electronics, and...

NEIL DeGRASSE TYSON: Figure out what they're thinking.

MATT WILSON: ...read their, read their mind.

NEIL DeGRASSE TYSON: Wilson's mind-readers are actually thin wires, about a tenth the width of a human hair, that pick up the electrical signals among dozens of brain cells.

The wires—painlessly implanted in the rat's brain and held there by a kind of hat—carry the signals right into Wilson's computers.

That information comes up back through these connectors into your computer, and you're sitting there watching a map of the thoughts of this rat?

MATT WILSON: Exactly. That's exactly right.

NEIL DeGRASSE TYSON: It's remarkable.

Wilson is most interested in mapping the rat's thoughts in a part of the brain called the hippocampus. Like the fruit fly's mushroom body, the hippocampus of a rat or a human plays an important role in memory, including our sense of space and location.

Wilson uses a specially designed rat maze. If the rat follows the right route, he's rewarded with some chocolate syrup. And as he moves through each different spot in the maze in search of his goal, a unique pattern of cells fires in his brain.

MATT WILSON: So we can tell where the animal is, simply based upon which cells in the hippocampus are active. That pattern will be unique for a given location in a given environment.

NEIL DeGRASSE TYSON: What's amazing is that the same patterns turn up again, even after the rat drifts off to sleep.

That's right, Wilson eavesdrops on the rats' dreams. And they aren't about cheese, they're about running the maze.

MATT WILSON: So when the animals would go to sleep, we would see these patterns of brain activity that were expressed while the animals were running on the maze, being replayed, in the same sequence, the same order in which they had been experienced.

NEIL DeGRASSE TYSON: But the replay wasn't exactly the same as when the rat ran the maze. Sometimes it was like an extreme fast-forward; quick flashes of the experience.

MATT WILSON: Now, at the time, you never know what is going to be important and what is not important. So you may re-evaluate or edit those memories to identify the things that were important.

NEIL DeGRASSE TYSON: And this fragmented replay wasn't just happening in the hippocampus. Wilson also detected it in the visual cortex, meaning the rats were likely seeing the maze in their sleep.

What's more, the visual cortex is part of the larger neocortex, which, in humans, is responsible for, among other things, long-term memory.

MATTHEW WALKER: The hippocampus is replaying the events of the day. The hippocampus is almost, sort of, reactivating the memories at night and playing them out to the neocortex. It's almost as though the hippocampus is having a therapy session with the, with the neocortex. And it's almost saying, "Okay, here's what we learned during the day."

MATT WILSON: What are rats and what are people doing during sleep? They are processing memory. They are replaying memory. Now, we could ask, "Is this about learning?" And I believe that's exactly what it is about, that animals are, and humans, are trying to learn from past experience.

NEIL DeGRASSE TYSON: So, the idea here is that the sleeping brain might be reviewing and strengthening new memories it wants to hold onto for the long-term. And it might identify certain goals we want to work towards.

So could it be that "sleeping on it" isn't just an old saying, but a biological process that consolidates and organizes important information?

MATT WILSON: These are pretty big concepts. And they certainly are controversial. The function of sleep, as it relates to learning and memory, that's something that, at this point, remains speculation. We're making a leap.

NEIL DeGRASSE TYSON: Not everybody is leaping into bed with this idea. And researchers have a long way to go before they know what sleep is really doing for our brains. But if the speculation turns out to be true, then you'd have to wonder, "What is our 24/7 culture doing to our ability to think straight?"

MATTHEW WALKER: Sleep is not just something that we can choose to sort of dabble in every now and again. It's not a luxury; it's a biological necessity.

MATT WILSON: My sense is that disruption of sleep is much deeper than simply, you know, robbing us of rest. My guess would be that we lose the opportunity to gain understanding, a deep understanding of our past experience, that what we sacrifice, in a sense, is wisdom.

On Screen Text: Amita Sehgal discovered something about jet lag by studying fruit flies. A mutation in a single gene caused symptoms resembling jet lag. When Sehgal restored the gene the flies returned to a normal sleep cycle.

FIRST PRIMATES

NEIL DeGRASSE TYSON: As inhabitants of Earth, we humans are relative newbies. In fact, our branch of the evolutionary tree may have split with these apes only about 6,000,000 years ago.

But what if we look further back in our primate family tree? There must have been some great and wise ancestor who founded this wonderful line of creatures, right? Well, as correspondent Peter Standring reports, the latest research is revealing that our origins may have been quite a bit humbler than we thought.

PETER STANDRING (Correspondent): The Badlands of Wyoming: some of the largest dinosaur bones, ever, were found right here. But University of Florida paleontologist Jonathan Bloch is hunting for a set of bones that are nothing like the giant bones of T-Rex.

JONATHAN BLOCH: Here's a little piece of bone here. Here's a little piece of bone. I think that's a little vertebra.

PETER STANDRING: Tiny mouse-sized bones, buried in limestone, that just might be the fossil remains of our earliest primate ancestors.

An age-old mystery surrounds the origin of primates. No one knows exactly where we come from or how we got our evolutionary start.

Here's what we do know: giant dinosaurs once ruled this basin, where they dined freely in a lush forest. But then, around 65 million years ago, the dinosaurs die off when a massive comet slams into the planet. Ten million years later, something extraordinary happens. The fossil record suddenly shows a new kind of mammal, with unique characteristics: the primate, our ancient ancestors.

So what is a primate? What is it that separates us from the rest of the evolutionary pack? Well, maybe it's our good looks or our superior intelligence.

The truth is brain size does come into play. We primates, even Noah here, have larger brains than our mammal relatives. It's a feature that evolved to help us learn complex social behavior and how to do things like make tools or even outwit our prey.

We also developed forward-facing eyes with stereo vision. It's a feature that allows us to judge the world around us in 3D. Over time, we also developed the ability to leap, basically to jump from branch to branch, where grasping hands, or in Noah's case, grasping feet, equipped with nails instead of claws, enable us to reach that tasty piece of fruit.

Our earliest ancestors developed these unique characteristics, some time after the extinction of dinosaurs. The question is, "When and why?"

So let me get it straight. If the dinosaurs became extinct 65 million years ago, and then primates suddenly appeared around 56 million years ago, what happened in between? I mean that's almost 10 million years that's unaccounted for.

JONATHAN BLOCH: Right. That's the $6,000,000 question. And I don't think they just appeared on the face of the planet, they evolved.

PETER STANDRING: But from what? I mean, something the size of a mouse?

JONATHAN BLOCH: Exactly.

PETER STANDRING: Jonathan believes the evidence to support his theory and help solve this ancient primate mystery can be found here, hidden inside the limestone of the Bighorn Basin.

JONATHAN BLOCH: A tiny little piece of broken bone can connect up with an entire skeleton of a mammal.

This looks like a pretty good limestone. It should be...should be full of fossils, but we really won't know until we get it back to the lab.

You see a tiny little piece of bone, and you hope that there's more inside, you have no guarantees, so it's a little bit of a gamble.

PETER STANDRING: But a gamble worth taking, because these stones might hold ancient clues.

Back in his lab, Jonathan, along with graduate student Doug Boyer, gets to work. Their goal? To free the delicate bones from the rock-hard stone. They begin by placing the limestone under a microscope.

JONATHAN BLOCH: That immediately starts to open up the world of the block. We identify all of the bone that's outcropping on the surface.

PETER STANDRING: Doug carefully coats the tiny bones with plastic to protect them from the powerful acid bath they're about to take.

DOUG BOYER (Stony Brook University): We leave the block in acid for, at the most, two to two and a half hours, and that'll remove about a millimeter-thick rind of limestone.

JONATHAN BLOCH: We repeat the process, again and again and again and again, until all of the bone is exposed.

PETER STANDRING: Much to their surprise they find hundreds of tiny bones.

Doug devises a method to meticulously document the relationship between each and every bone. The process will take months, but when complete, it will reveal far more than they ever anticipated: dozens of tiny mammals never before seen, including these three extraordinary skeletons.

And what are these?

JONATHAN BLOCH: These are plesiadapiforms.

PETER STANDRING: Plesiadapiforms are tiny mouse-like creatures that lived during the mysterious 10-million-year period between the extinction of dinosaurs and the appearance of primates. It's a very diverse group, with more than 120 species, including these three.

JONATHAN BLOCH: They represent the most complete skeletons of plesiadapiforms known in the world.

PETER STANDRING: An extraordinary find, for sure, but will they help Jonathan solve this primate mystery? Are plesiadapiforms our earliest ancestors?

JONATHAN BLOCH: If we look here, this nail-like structure makes you think, because the presence of a nail is a hallmark characteristic of living primates.

PETER STANDRING: This is an enlarged image of the extraordinary nail Jonathan found. Next to it, the claw he expected—a startling difference.

JONATHAN BLOCH: This nail might actually be the first nail in the history of primate evolution.

PETER STANDRING: Concrete evidence to support his theory of primate evolution. Could there be more hidden within these tiny bones?

To find out, Jonathan enlists the help of Mary Silcox, evolutionary anthropologist at the University of Winnipeg. She's been busy zapping primitive skulls with an industrial-strength CAT scanner, large enough to fill an entire room. Mary takes the skull of one of the limestone skeletons and prepares it for scanning.

MARY T. SILCOX (University of Winnipeg): The x-ray goes through the specimen, and we collect 2,400 separate views, which produce a cross-sectional image.

A structure that had been identified as just a little piece of bone in the middle ear actually had the form of a tube. And the reason that was exciting was because there's a structure running through the ear of particularly primitive primates—things like lemurs—which is a tube for a large vessel that goes to the brain.

PETER STANDRING: A tiny tube, a tiny nail, the evidence is mounting. But to prove his theory of primate evolution, Jonathan still needs more. He adds another member to the team. Eric Sargis, professor of anthropology at Yale University, and the world's leading expert on tree shrews. Why a tree shrew expert? Scientists believe that tree shrews—a primitive species of tiny tree-living mammals—are actually related to early primates.

ERIC SARGIS: Tree shrews are not primates, but they're close relatives. They share a number of characteristics that separates them from other groups of mammals.

PETER STANDRING: Would plesiadapiforms pass the ultimate primate test? Are they the first step on the primate family tree or just another relative on the tree shrew family tree?

The team goes to work bringing together all the information they had collected independently into a single comprehensive study.

Detail by detail, feature by feature they comb through all the data using a numerical system to compare and contrast.

JONATHAN BLOCH: After we studied the different characteristics of these animals, and reduced them down to numbers—you know, absence of a nail is a 0, presence of a nail is a 1—we then ran this through a computer algorithm.

PETER STANDRING: Using this information, the computer was programmed to create family trees illustrating the potential relationships each mammal has to the next. The team expected the computer to come up with several possible scenarios in the form of several possible family trees. Instead, the program came up with only one.

JONATHAN BLOCH: I was a little surprised to see it so unambiguous.

PETER STANDRING: This single family tree could lead to only one conclusion.

JONATHAN BLOCH: I think the evidence, as it stands today, is pretty compelling that yes, in fact, these are primates.

MARY SILCOX: Every new piece of data that we had coming out of our study of this material seemed to be consistent with that idea.

PETER STANDRING: Not only that. One of the plesiadapiform skeletons Jonathan and Doug painstakingly etched out of limestone, a species by the name of Dryomomys, turns out to be far more primitive than the other two, possessing only one primate characteristic, the shape of its teeth.

ERIC SARGIS: It's sort of a transitional specimen between more primitive things, like tree shrews, and later primates.

PETER STANDRING: One part primate, other parts not.

ERIC SARGIS: I mean, it really starts to tell us something about the base of the primate tree, what the earliest primates look like. So, if we're one leaf on the branch, so are chimpanzees, gorillas, orangutans, among apes; all the different monkeys in the old world and the new world; lemurs from Madagascar; lorises and galagoes; all those animals are living today, but you can trace it all back to a single common ancestor. And as you get closer and closer to that common ancestor, dryomomys is one of the animals that's closest to the base there. It's the most primitive primate skeleton ever found, to date.

PETER STANDRING: Jonathan had evidence to support his theory. Primates didn't just appear on the planet, they evolved over a 10-million-year period. And just as he thought, the earliest primates were the size of a mouse. Still one question remains. What sparked this amazing transformation? The team believes our ancient ancestors evolve on the heels of a mass extinction. Without the mighty T-Rex around, the tiniest of mammals are free to forage and explore, and they discover a world filled with flowering plants and succulent fruit.

With tempting fruit growing at the end of tiny branches, our ancestors have plenty of motivation to change. So they begin to evolve, developing long fingers for climbing trees, specialized teeth, hands and feet, uniquely designed for grasping and eating the tiniest, tasty berry. Over 10 million years, they slowly develop unique characteristics that we recognize in our primate relatives and ourselves.

ERIC SARGIS: So that if plesiadapiforms don't evolve, we're probably not standing here talking about this right now.

On Screen Text: Our 6 favorite distinguishing characteristics of primates: opposable thumbs; reduced litter size (usually just one baby); color vision; expanded cerebral cortex; greater facial mobility and vocal repertoire; one pair of mammae in the chest.

EARTHQUAKES IN THE MIDWEST

NEIL DeGRASSE TYSON: We've got a pretty good idea of what causes earthquakes in places like California. It's all about movement along the big cracks in Earth's crust. North America is part of a giant rigid plate of crust moving this way, and the Pacific Ocean sits on another plate moving this way. Most of the time, the edges are just kind of stuck together, but every now and then, the tension gets too high...boom! The edges slip; you get an earthquake.

So you might think, "Hey, if I just stay away from here, I can avoid the big shake up."

But not so fast. As correspondent Peter Strandring reports, they're now realizing that there's another kind of quake. It's big, it's dangerous, and it could be headed straight for the middle of America.

PETER STANDRING: Sam Panno is on the hunt. He's looking for evidence, hidden deep inside a cave in Illinois. There's over 14 miles of wet, slippery, bat-filled tunnels down here.

After hours of hiking, he spots what he's come for.

SAM PANNO (Geochemist, Illinois State Geological Survey): Looks good.

PETER STANDRING: These lumps of rock might not look like much, but for Sam, they could help answer a frightening question: "Is the American heartland about to be hit with a cataclysmic earthquake?"

When I think earthquake, I think California, the San Andreas Fault and fears of "the big one." And that makes sense, because most big earthquakes happen in places like California, where big plates of the earth's crust collide with each other. But if you ask the experts, they'll say one of the biggest looming earthquake threats in the US is far from California. In fact, it's here, in Memphis, Tennessee.

It's Saturday night, and the crowds on Beale Street seem pretty, well, carefree. But about 200 years ago, the entire region was rocked by enormous earthquakes, and experts like Gary Patterson say Memphis has something to worry about.

So here we are on Beale Street on a Saturday night, and, obviously, the place is packed. From what I understand, luckily, Memphis didn't exist when the big quakes were happening in 1811 and 1812.

GARY PATTERSON (University of Memphis): That's right. We didn't have instruments in the ground, but we know they were really big. They really happened and they could really happen again.

What happened was that the midwest was hit by not one, but three epic earthquakes, with epicenters somewhere near the town of New Madrid, Missouri.

According to eyewitnesses, parts of the Mississippi River changed direction.

TISH TUTTLE (U.S. Geological Survey): Geysers of water and sand went shooting into the air. Fissures were forming in the ground. Some of them were so large that they were afraid they'd be swallowed up by them.

PETER STANDRING: In fact, one report claimed eight Indians were killed by the opening and shutting of the earth.

There weren't any big cities in the region, and no one knows how many people died, but the tremors were so strong, they were felt from Mexico to Canada, over a thousand miles away.

How is it possible that some of the biggest quakes in American history would strike in the middle of the continent, where there are no visible faults, and away from the plate boundaries?

GARY PATTERSON: It's an enigma. It's, it's, it's a, it's a puzzle.

PETER STANDRING: To help solve the mystery, geologists are trying to uncover the truth of what happened here in the past. And that's why I joined geologist Sam Panno and his team on their little cave adventure, to track down rocks that bear the imprint of ancient earthquakes.

And I expect we're going to get muddy?

SAM PANNO: A little bit.

PETER STANDRING: If we want to find the right rocks, it turns out we need to go underground, and we need to get a little wet.

Hey, Sam, I guess that trail of popcorn that I dropped behind us isn't going to do us any good at this stage.

SAM PANNO: I guess not.

PETER STANDRING: Lucky for me, Sam knows his way around these tunnels, all 14 miles worth. He's been trekking through midwestern caves for years, on the hunt for stalagmites, those spikes of rock that grow up from the cave floors.

Since they build up gradually, from minerals dripping from above, each stalagmite carries a unique geological record, sometimes stretching back tens of thousands of years.

If earthquakes happened here, the stalagmites will prove it.

SAM PANNO: Stalagmites grow layer upon layer.

PETER STANDRING: Break a stalagmite in half, and you can see rings of growth, kind of like tree rings.

SAM PANNO: You have the youngest rings on the outside, the oldest ring on the inside.

PETER STANDRING: So a stalagmite like this one, for example, may have taken tens of thousands of years to grow to that height?

SAM PANNO: That's about right.

PETER STANDRING: For years, Sam has been taking samples of these rocks to hunt for signs of earthquakes hidden in their crystal rings.

Back in the comfort of a warm, dry lab, the collection starts to tell a story.

BRUCE FOULKE: That one layer would have been a split.

PETER STANDRING: Bruce Foulke, at the University of Illinois, studies the rocks Sam gathers. Even the rough shape can reveal past earthquakes.

BRUCE FOULKE: You can see that the outermost layer here is shifted. Instead of growing here, it's moved down and shifted off to here. The reason that's happened is the stalagmite is growing from a point source of water that's dripping, dripping, dripping. Then we had earthquake activity, and the position of the drip moved.

And now the new drip started accumulating and growing a new stalagmite in this position, which is down and off the shoulder of the old stalagmite.

PETER STANDRING: Whenever they spot a shift like this, the team cuts a paper thin slice of stalagmite. With a powerful microscope, Bruce can isolate individual crystal layers...

BRUCE FOULKE: Wow.

PETER STANDRING: ...and see exactly where the earthquake happened.

BRUCE FOULKE: That specific line is represented here on the microscope screen by this dark area of crystal growth.

PETER STANDRING: To date the earthquake, they don't count the rings like you do with tree rings. Instead, high precision chemical analysis of the crystals reveals how long ago they formed.

When the results for this stalagmite came back, the team discovered that this major shakeup dates to about 200 years ago, a remnant of the great quakes of 1811 and 1812.

And with other samples they've gathered, the team has uncovered more.

In fact, the rocks reveal a pattern of violent midwestern earthquakes stretching back at least 15,000 years. The 1811, 1812 quakes weren't a freak one-time event. And if they happened many times before, then they will probably happen again.

The big question is when.

Today, the whole region, called the New Madrid Seismic Zone, is carefully monitored by seismic sensors. It turns out the ground here is shaking almost every day.

GARY PATTERSON: We have about 200 small earthquakes per year, 90 percent of those too small to be felt by humans. It's the most active seismic zone east of the Rockies.

PETER STANDRING: So why is the midwest made of such shaky ground when there are no visible faults on the surface?

According to one theory, it's because the region lies over an ancient wound, deep down in the earth's crust.

GARY PATTERSON: Half a billion years ago, the North American continent was being pulled apart by plate tectonic forces. And it created a rift.

PETER STANDRING: But at some point, the continent stopped breaking up.

GARY PATTERSON: For some reason, that rift failed.

PETER STANDRING: The failed rift created a weak spot in the crust, which might make the region more vulnerable to quakes, but no one really understands how.

To try to get a better picture of what might be going on, Beatrice Magnani is probing the earth's crust beneath the Mississippi River.

Chugging along in a tugboat, her team sends powerful sound waves down through the water, and a mile deep into the earth's crust. As the sound waves bounce back, they're picked up by dozens of small microphones trailed by 80 yards of cable in the river.

The echoes reveal distinct layers of sediment.

BEATRICE MAGNANI (University of Memphis): Reading the sediments is like reading a book. The sediments tell us the story of what happened with time.

PETER STANDRING: What happened was these layers became deformed and folded by major earthquakes.

But as Magnani discovered, these folds were outside the currently active New Madrid Seismic Zone.

BEATRICE MAGNANI: Nobody knew about these faults. It's actually a discovery that...it's even more puzzling than not finding anything.

PETER STANDRING: For Magnani, it means that over the eons, the danger zone in the midwest may jump from one area to another, which makes it even more difficult to predict future big earthquakes in an area that is underprepared.

This city wasn't even founded until seven years after the great earthquakes of 1811 and 1812, so it's hard to imagine what would happen if something of the same magnitude occurred in this region today.

If the 1811 and 1812 quakes are repeated, they will affect much more than Memphis, but also the entire region stretching to the north, including St. Louis.

GARY PATTERSON: A huge area would be affected, possibly an area containing 12 million people. There're a lot of old buildings.

PETER STANDRING: Whenever it happens, the next big midwestern earthquake is guaranteed to be catastrophic, destroying billions of dollars in property, and affecting millions of lives.

Yet, unlike Californians, many people here remain unaware that their cities and towns stand on potentially very shaky ground.

On Screen Text: A few theories throughout history explaining earthquakes: Aristotle thought violent winds trapped underground. Two millennia later, the Enlightenment prompted new theories: lightning, really strong lightning or maybe subterranean chemical reactions. And our current theory? Giant chunks of Earth rubbing against each other; we'll see how long this one lasts.

PROFILE: SANG-MOOK LEE

NEIL DeGRASSE TYSON: Scientific research is filled with challenges like will your experiment work or won't it? People can invest years of their lives, not knowing if they'll get the payoff of a big discovery.

In this episode's profile, we meet a man who met all the ordinary challenges to become a successful researcher, and then one day, quite suddenly, was handed a whole new set of overwhelming obstacles that his passion for science empowered him to overcome.

Meet Sang-Mook Lee. Where he's from, he's known as the "Stephen Hawking of South Korea."

SANG-MOOK LEE: "Sang-Mook Lee, professor of Seoul National University, also known as Stephen Hawking of Korea." That's like a fixed quote. Every time I'm cited, they cite like that.

He looks into the universe; I look into the bottom of the sea.

NEIL DeGRASSE TYSON: Sang-Mook is an esteemed professor at South Korea's top university, and one of the foremost geophysicists in his country. He studies underwater volcanoes and tectonic plates deep beneath the ocean.

Yet, just three years ago, he was in a near-fatal car accident that left him paralyzed from the neck down. Many thought he would never work again.

MIKE GURNIS (California Institute of Technology): He was heavily sedated; he couldn't move; the machines were keeping his lungs going, but you could just see a light in his eye and that there was a future out there.

NEIL DeGRASSE TYSON: He made an impossible comeback.

He has since emerged as a national hero.

KUNWOO LEE (Seoul National University): By showing his attitude, will become a hope for every handicapped people.

NEIL DeGRASSE TYSON: He credits his return to his passion for science.

SANG-MOOK LEE: I just love science. I knew I had to get back because it was something I really wanted to do.

NEIL DeGRASSE TYSON: Sang-Mook's love for science began as a child, when he dreamt of exploring the world.

SANG-MOOK LEE: That's when I dreamt about being a globetrotter and going around the world at very exotic places.

NEIL DeGRASSE TYSON: He went after this dream. Getting a Ph.D. in geophysics from M.I.T., he began a life on the sea.

SANG-MOOK LEE: When I returned back to Korea, I became the marine geophysicist in Korea, and we had the ship. That was when my science got really, really exciting, because I was the only one in charge of this big ship. You're being like pirate.

NEIL DeGRASSE TYSON: His research involves studying the submarine activity of tectonic plates, those massive slabs of rock that make up Earth's crust.

SANG-MOOK LEE: We use indirect ways to image what is below the seafloor, because all the records are preserved in the ocean.

NEIL DeGRASSE TYSON: Shooting seismic waves through the ocean floor creates a picture of what lies inside the earth. These pictures allow him to monitor underwater earthquakes and volcanoes, and learn more about how Earth's crust was formed.

SANG-MOOK LEE: What I do is I study the past 200-million years of Earth's history, to understand how Earth has behaved in the past four and a half billion years.

NEIL DeGRASSE TYSON: Sang-Mook hopes that by studying the tectonic activity in uncharted regions, we might someday be able to forecast geological disasters, like tsunamis and earthquakes.

SANG-MOOK LEE: I looked at the map of the western Pacific, and I tried to choose targets where nobody has gone before. I think that is the real essence of science, discovery, like, climbing up the peak of a mountain that nobody has gone before.

NEIL DeGRASSE TYSON: But everything changed on July 2, 2006, on a field trip to California, with a group of students.

SANG-MOOK LEE: It was toward the end of our trip, and we were working toward Death Valley.

I don't remember anything, that day, nothing at all. People will tell me, "This is what happened. This is what happened."

"Oh, is that so?" I don't recall anything. My van flipped, and I was crushed in the seat.

NEIL DeGRASSE TYSON: Sang-Mook's fourth cervical vertebrae had been fractured, and he was in a coma.

SANG-MOOK LEE: When I was still in coma, I knew that I was injured. And I said, "Oh, Sang-Mook, you're in trouble." And then, I said, "But Sang-Mook, you have been in difficult situations before, so you can come out of this trap. Figure out what to do."

NEIL DeGRASSE TYSON: He awoke from his coma after three days, paralyzed from the neck down.

SANG-MOOK LEE: Immediately after the injury, I worried that I wouldn't be able to work.

NEIL DeGRASSE TYSON: And he worried about the challenges that South Koreans with disabilities commonly face, challenges like economic hardship and family problems. Married with three children, Sang-Mook Lee and his family would ultimately separate, and although financially able to provide for his own care, he knew he would have to contend with the stigma that so often accompanies disability, especially in South Korea.

DAN SHIM (Massachusetts Institute of Technology): People with disabilities are not well accepted in this society. And they tend to hide in a shadow and, you know, they don't want to expose themselves. Even taxi drivers will refuse to, to get you on, if you have disability.

SANG-MOOK LEE: In Korea, people with my level of injury would stay in the hospital, or would shun from the society, but I was not embarrassed to come back to my work and to try to do what I used to do.

NEIL DeGRASSE TYSON: Using all the technologies available, he was out of bed within a month of his accident.

SANG-MOOK LEE: Science played a very, very important part for me to come back after the injury. Because of my interest in science, I just thought, "I must go back to work."

MIKE GURNIS: Suddenly—and it was, it seemed like it was suddenly—now he was in motion again. He had this chair, and he had all of the gizmos. And he could literally move the wheelchair around, turn it, and then also use computer devices, almost immediately, by breathing into a straw. He also could move his head back and press a button by moving his head. I mean it was extraordinary to see this happen so quickly.

NEIL DeGRASSE TYSON: Just six months later, Sang-Mook returned to teaching.

SANG-MOOK LEE: One day, a reporter noticed me. This reporter was from one of the major newspapers in Korea. He heard about me, but when he saw me, it shocked him,

and he knew from reporter's instinct that this could be a big story. And the next day, on March 5th, this very big newspaper in Korea ran my story on the front page.

NEIL DeGRASSE TYSON: The attention made him immediately recognizable throughout his country.

MIKE GURNIS: As time went on, he then assumed this greater and greater role to become a spokesperson for disabled people within Korea.

NEIL DeGRASSE TYSON: He now hosts his own radio show, writes a column on disability in South Korea's biggest newspaper, and he's written a popular book about his work in the sciences.

WOMAN: I was incredibly moved by your book.

SANG-MOOK LEE: I feel very fortunate that I had an opportunity to reevaluate myself at that moment in my life. I asked myself, "What can I do to give back? What can I do actively to give back?"

NEIL DeGRASSE TYSON: Sang-Mook's biggest project is a multimillion dollar undertaking, funded by the South Korean government, and it's finding ways to help other handicapped South Koreans get back in the workplace.

SANG-MOOK LEE: Why do so many disabled students only take the civil service exam? We need to lead more of these students to go into computers and science.

NEIL DeGRASSE TYSON: As part of this program, he's working to discover and develop technologies for the handicapped, including Korean voice recognition software and robotic limbs.

SANG-MOOK LEE: You and I may run a shop.

NEIL DeGRASSE TYSON: In a city where Sang-Mook often has to bring his own ramp just to get around, there's still lots of work to do.

SANG-MOOK LEE: It is very difficult, you know? Basically, we have to remodel Seoul.

NEIL DeGRASSE TYSON: But this hasn't stopped him. His love for science has him globetrotting once again. He recently made his first overseas trip since his accident, to an international science conference in San Francisco.

MIKE GURNIS: That he can come to an international meeting, fly across the Pacific, interact with his colleagues, present his scientific work, it does symbolize he is now part of the community of scientists, once again.

NEIL DeGRASSE TYSON: In fact, Sang-Mook has major research trips planned to explore underwater volcanoes in Tonga and mid-ocean ridges in the Pacific Antarctic. And although he no longer sails the seas himself, Sang-Mook can still be the captain of his crew.

SANG-MOOK LEE: Now that I'm confined to wheelchair, I cannot go directly to those places. But nowadays with the development of Internet, satellite communication, I'm able to participate in many of the cruises, without going directly to sea. I can enjoy science without getting seasick.

Many people heard of my story, that I was severely injured, and they doubted that I would ever come back. But with the new programs that are opening up, they will see a lot more of me.

NEIL DeGRASSE TYSON: Before we leave, we'd like to hear your perspective on this episode of NOVA scienceNOW. Log on to our Web site and tell us what you think. You can watch any of these stories again, download additional audio and video, explore interactives and hear from experts. If you want to get the scoop on upcoming broadcasts and find out what goes on behind the scenes, you can sign up for the weekly e-newsletter at PBS.org.

That's our show. We'll see you next time.

Stay tuned for more about NOVA scienceNOW, but first...

Funding for NOVA scienceNOW is provided by the National Science Foundation, where discoveries begin. And...

Discover new knowledge; biomedical research and science education; Howard Hughes Medical Institute: HHMI.

And the Alfred P. Sloan Foundation, to enhance public understanding of science and technology and to portray the lives of men and women engaged in scientific and technological pursuit.

And the George D. Smith Fund.

And by PBS viewers like you. Thank you.

This NOVA program is available on DVD. To order, visit shoppbs.org, or call us at 8-800-PLAYPBS.

NOVA is a production of WGBH Boston.