"Mysterious Life of Caves"

PBS Airdate: October 1, 2002
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NARRATOR: Undaunted by deadly hydrogen sulfide fumes, two explorers venture into the dark recesses of a wild cave. Inside, toxic gases bubble up through the stream. Gas monitors warn the cavers of concentrations that could kill in minutes.

LOUISE HOSE (Geologist, Chapman University): We'll have to keep an eye on that.

NARRATOR: Yet, to their amazement, everywhere they look, life is flourishing, unfazed by the poison in the air or relentless darkness.

Then the explorers make a discovery even more bizarre. Dangling from the ceiling are soft, gooey strings of mucus that drip like a runny nose. To their disbelief, dripping from the strange slime is caustic acid strong enough to dissolve the cave walls right before their eyes. Suddenly the gas monitor beeps incessantly.

DIANA NORTHUP (Microbial Ecologist, University of New Mexico): And then we happened to look at it and realized it's carbon monoxide that's going off. And our respirators don't protect against carbon monoxide. I mean this cave is just full of good ways to die.

NARRATOR: The explorers get out, but they are stunned! How are the creatures here able to live at life's edge in acid, toxic gases and darkness? What mysterious process is eating away this cave? And is it unique? Scattered across the earth over 50,000 caves have been found. Most have been formed by water. Others have been carved by flowing glaciers or sculpted from volcanic lava. But the origin of some caves remains a mystery. Today, explorers are penetrating further than ever into the depths of this underground universe, transforming our understanding of how caves and the hidden life within them began.

The Mysterious Life of Caves up next on NOVA.

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NARRATOR: Just beneath the surface of the earth lies a vast unknown wilderness—a world of timeless darkness and eerie silence; of vaulting halls and maze-like passages; of mineral formations so exquisite they seem sculpted for some great cathedral, yet have been taking form by themselves for eons. They bear names both exotic and whimsical: fishtails, draperies, bacon, stalagmites and stalactites.

PENNY BOSTON (Geomicrobiologist, University of New Mexico): It's such a different environment, it's almost like going to another planet. In a sense, it is another planet. It's the part of our own planet that we have not explored. Just like the deep oceans are something that's only become accessible to us in the last century or less, now we're finding that kind of pioneering effort going on in caves.

NARRATOR: The quest to understand one of the world's most mysterious caves began in the Guadalupe Mountains of New Mexico. Two hundred fifty million years ago, this range was a massive barrier reef called El Capitan. It lay beneath an inland sea covering the American Southwest. As the sea evaporated, the reef hardened into a rock called limestone. Over time the earth shifted, lifting the rocks 8,000 feet above the sea floor.

Deep inside this ancient reef lies the world-famous Carlsbad Caverns. Its most striking feature is the Big Room, 750 feet below the surface. It's large enough to hold over six football fields, and has ceilings as high as a 30-story building.

Geologists believed that the Big Room and all limestone caves were formed by rain and underground water penetrating cracks in the rocks. As the water absorbed carbon dioxide from the soil, it formed a weak carbonic acid, similar to a carbonated drink. Over millions of years, this flowing water dissolved the rock away leaving networks of underground chambers. But this theory of cave formation would be challenged by geologists working in Carlsbad's Big Room—among them Dave Jagnow and Carol Hill.

CAROL HILL (Geologist, University of New Mexico): There were a number of mysteries in the cave that we couldn't figure out. One is why are these caves so big? And why was there no place that it looked like the water had entered the caves and had gone out of the caves? The Big room just ends absolutely abruptly.

NARRATOR: But what else besides flowing water could have removed such a massive quantity of rock? And there were other riddles. Why were some rocks so full of holes they looked like Swiss cheese? And other rocks, 20 feet high and covered with mud, turned out to be something else entirely.

DAVID JAGNOW (Consulting Geologist): The most unusual thing we noticed on the floor of the Big Room were these massive blocks—100-ton blocks the size of houses—of gypsum, the mineral gypsum. And this is a mineral that is formed by evaporation of seawater. And in most caves you wouldn't expect to find gypsum because it's so easily dissolved, and I would have expected it to be flushed out of the cave here long ago by the water dripping into this cave.

NARRATOR: It would take years to unravel the mystery of these gypsum blocks. In the meantime, gypsum would be found in other Guadalupe Caves, including one discovered only a few miles away from Carlsbad. Following the sound of the wind to the bottom of a pit in Lechuguilla Canyon, three cavers uncovered an extraordinary place.

Deep inside, their lights illuminated a bizarre underworld glistening with gypsum crystals. Passageways snaked through tunnels encrusted with the snow-white mineral. In contrast to Carlsbad, every inch of Lechuguilla was covered by the most spectacular gypsum formations ever discovered. The most dazzling crystals were found in the Chandelier Ballroom, where delicate branches, 20 feet long, hung from the ceiling. Because Lechuguilla had been sealed from the surface, its gypsum remained sparkling white.

DAVID JAGNOW: The gypsum is just beautifully preserved in Lechuguilla cave. But gypsum is a rare mineral to find in any cave, and yet these caves were full of these unique formations. And so that was another one of the clues that made us think these caves must have a very unique origin.

NARRATOR: Perhaps the gypsum had washed into the caves from the surrounding gypsum plains. Deposited by the ancient sea, these plains stretch for miles beside the Guadalupe Mountains. But when Carol analyzed Carlsbad's gypsum, she discovered it was chemically different from the plains. There was only one other way it could have been formed: when sulfuric acid reacts with limestone it dissolves, leaving gypsum behind.

DAVID JAGNOW: Sulfuric acid is much more powerful than carbonic acid. It can dissolve out about eight times the volume that a normal cave would be as a result of carbonic acid solution. So it's much more powerful, but you very rarely find it in nature.

NARRATOR: Where could acid as caustic as that in a car battery have come from? Carol and Dave began to search for a source. They knew that around the Guadalupe Mountains lay major deposits of oil and gas thousands of feet underground. They also discovered that an undesirable chemical tainted these deposits.

DAVID JAGNOW: These deposits are actually what we call "sour" because they contain hydrogen sulfide. And any time you drill a well around the Guadalupe Mountains, you smell this rotten egg smell. It's hydrogen sulfide gas.

CAROL HILL: And I knew that if hydrogen sulfide mixed with oxygen it would create sulfuric acid. So that had to be the source.

NARRATOR: Carol knew that something deep within the oil fields was producing the hydrogen sulfide gas, but what? A chemical analysis revealed it bore the telltale signs of life. Something living was eating the oil and giving off hydrogen sulfide.

But what kind of life could survive at depths of 15,000 feet, where temperatures soared to 165 degrees Fahrenheit? Clues were found at Yellowstone National Park when biologists discovered bacteria growing in a hot spring at 176 degrees Fahrenheit. That was close to the boiling point of water, which scientists always assumed would destroy a living cell. But these organisms had a chemical structure that protected them from the heat.

ANNA-LOUISE REYSENBACH (Microbiologist, Portland State University): It's pretty surprising to think that microbes can actually grow under these conditions at these high temperatures. And I don't dare touch this hot water because it's so hot. The temperature is fluctuating, but it's about 180 degrees Fahrenheit. That, in Celsius, is 83 degrees Celsius. That's really hot! And water boils in Yellowstone at 93 degrees Celsius, so what you're seeing here is gas, gas bubbling out from deep within the earth.

NARRATOR: Scientists would soon discover an even harsher environment where microbes could survive. The revelation came when a research submarine stumbled on volcanic cracks in the seabed, spewing out minerals and chemicals. Around these cracks, colonies of microbes clumped together in thick mats lived in total darkness. Instead of using energy from the sun, they fed on chemicals and seemed undaunted by temperatures that far exceeded those of Yellowstone.

DAVID WOLFE (Author, Tales from the Underground): We discovered at these deep-sea hydrothermal vents, organisms living at temperatures—thriving at temperatures—well above boiling—230, 240 degrees Fahrenheit. And the reason they can do this is because the pressures are so great at that depth that the water's not actually boiling. So water is still in a liquid state. So it really proved the point that indeed life could thrive under some rather extreme conditions.

NARRATOR: These extraordinary microbes were nicknamed "extremophiles," because they could tolerate extremes of pressure, heat and darkness. As long as there was water and a chemical source of energy, life could indeed survive deep beneath the surface of the earth. But could it set in motion the formation of a cave?

Back at Carlsbad, geologists proposed a simple but radical theory. Tiny microbes, feeding on oil far beneath the cave, produced hydrogen sulfide gas as a product of their metabolism. The gas seeped up into the Capitan Reef where it mixed with oxygen in the ground water and formed sulfuric acid. The acid ate away vast amounts of limestone, carving out the caves and leaving gypsum behind. It all made sense, but at first geologists were skeptical.

CAROL HILL: Everyone for 100 years had just assumed that it was only carbonic acid which carves out caves. Nobody had even thought of sulfuric acid being the agent by which this cave and the other caves in the Guadalupe Mountains could have formed. It was a completely new idea.

NARRATOR: And a startling concept. Biology, not just geology, was at work. Life itself was helping to shape the world beneath our feet.

Back at Lechuguilla, a team of microbiologists get ready to rappel into the cave to hunt for this hidden life. Only a small number of scientists dare to enter this underground wilderness. Among them is Diana Northup.

DIANA NORTHUP: For some people it's easy. For me it's an incredibly difficult cave to go into because I'm afraid of heights and Lechuguilla is pits. It's just one hole after another. But what draws me is the fact that there is incredible science to be done in this cave. It is full of scientific mysteries.

NARRATOR: At the bottom of the pit, Diana climbs through the hatch door that protects the cave. As the air pressure changes, winds can blow though the entrance tunnel at speeds up to 50 miles per hour. Once inside, she will be plunged into absolute darkness and totally dependent on battery-powered lights.

DIANA NORTHUP: When the door slams, all the wind shuts off, and so this wind tunnel that you've been in just comes down to a murmur. And then as you move away from it, the cave becomes completely silent. So you get no concept of what's around you, because you're just sort of sitting in there in a bubble of blackness.

NARRATOR: Next, microbiologist Penny Boston rappels into the pit.

PENNY BOSTON: I had done a lot of extreme environment work in surface environments. But I had never caved before—in a wild cave. And so Lechuguilla was my introduction to caving. And the first couple of times I went on trips there, I kept thinking, "All I have to do is live long enough to get out, and then I never have to come back." And that is what kept me going long enough to actually, sort of, endure it. And after those first couple of trips, when all I wanted to do was get out, the beauties of the cave and the sense of otherworldliness that I got really soaked into my soul.

NARRATION: Lechuguilla is the deepest known cave in the continental United States and the world's fourth longest. Over 100 miles have been mapped in this underground labyrinth. Names of places like Freak-out Traverse, Death Pit and Chasm Drop reflect the challenge of this underground terrain. One of the hardest hurdles is to descend Boulder Falls.

DIANA NORTHUP: Correct rigging says that you rig it so that you have to get onto the rope at the very edge of this pit. You have a safety to help you, but for me, who's afraid of heights, that's not a lot of comfort. And it's a 150-foot drop that is against the wall for a little way and then free. So as you come down this you are just hanging out in space.

NARRATOR: Boulder Falls is named after all the rocks that came loose the first time explorers ventured down. To reach the bottom, Penny and Diana lower themselves on a single rope tied to an anchor and control their speed with a hand brake. If they don't apply enough friction, they could spiral into an uncontrollable deadly descent.

At the bottom, the team begins the arduous climb down to the lower depths of the cave, where they will begin their hunt for hidden life. Although it's hard to imagine anything living in this barren, rocky terrain, all the necessary ingredients for microbial life are here. Lechuguilla is rich in chemicals they can eat like sulfur, manganese and iron, and, with 100 percent humidity, has plenty of water.

PENNY BOSTON: The past 30 years has seen a really major revolution in microbiology. Partly this is because we have now tools where we can study these little tiny guys that we couldn't study before. But also because we have begun to realize that wherever we seem to look, where there's energy sources for them to live on, we actually find life forms, and that maybe they are not really obvious and maybe you have to look really hard, but they exist.

NARRATOR: Penny and Diana wonder if odd formations hanging from the ceiling could be fossilized bacteria. They call them "u-loops" and take a sample to study back in the lab. Lechuguilla is considered a dead cave because it's no longer being etched out by sulfuric acid, but this doesn't mean that it's devoid of life. Although the microbes that set in motion the cave's formation are long gone, Penny and Diana believe that others have taken their place. They decide to investigate why the walls are decaying and covered by rust-colored patches.

PENNY BOSTON: The walls of the cave still seem to be enlarging. And what has happened, we believe, since the cave finished forming, is that there are other populations of microorganisms that can use other energy sources in the parent bedrock, and they might be actively participating in the breakdown of this material.

NARRATOR: To test this theory, Penny and Diana place chemicals on the walls that will react if they come in contact with living microbes. Where they get hits they collect samples. Back in the lab they hope to prove that bacteria are here, making a living in this extreme environment.

DIANA NORTHUP: When you go into a cave and shut that door or get beyond the entrance zone, there is no sunlight. So the normal system that we think about fueling life is not there. Now, in other caves, they have rivers that run through them that bring in carcasses or dead leaves or twigs and things like that that can fuel microbial processes. That doesn't occur in Lechuguilla. You have a system that has only isolated pools, not rivers or streams, and so it's a very low-nutrient environment, and that makes it more extreme.

NARRATOR: If these samples show that extremophiles are surviving here, far from the life-giving sun, they must be getting their energy by eating minerals in the rocks.

After eight exhausting hours, the team reaches Lake Labarge. They're a thousand feet below the entrance and over a mile in. Protected from surface contamination, the lake is so pristine that at certain angles its water seems almost invisible. Although the cavers can safely drink from the lake, all of their food must be carried in. Everything about the human presence poses a threat to the cave.

DIANA NORTHUP: We have a huge impact on caves by the very fact that we go in and bring ourselves. Now this may seem like a foreign concept, but as we walk through a cave we shed tens of thousands of skin fragments a minute. So you are a veritable little snowstorm and around you is this pile of debris. Your hair is falling out and littering the cave.

NARRATOR: But the greatest contamination comes from the necessity to camp underground for days.

DIANA NORTHUP: And that means you're eating and dropping food crumbs, which to you are just crumbs, but to a microorganism it's as through you dropped a supermarket on top of their head.

PENNY BOSTON: These organisms have been living in this really low-energy environment where food has really been hard to come by. They are adapted to that. When you overfeed them by giving them these sources of contamination you can really change the balance of who's there. Some organisms are actively poisoned, and so you are really mucking up the environment.

NARRATOR: Urine is left in designated places, but solid waste and trash have to be carried out. To conserve precious batteries for the long trek back, everyone turns in early. Since there's no difference between day and night, some find it hard to sleep—a reminder, perhaps, of how out of place humans are in this underground world.

PENNY BOSTON: The idea of an extreme environment has been very useful to us, because it has pushed us to look at environments where we wouldn't have imagined at one time that there would be living things. But it really is a human-centered idea. We think of anything that we wouldn't be comfy in as an extreme environment. But from the point of view of the organisms that live there this is home. So for them it's not extreme; it's only extreme for us. And so we have to expand our view of conditions within which we now understand that life can exist.

NARRATOR: But does life exist in Lechuguilla cave? Back in the lab, Penny and Diana examine their samples with an electron microscope.

DIANA NORTHUP: Scanning electron microscopy allows you to see incredibly tiny things. And we spend hours and hours lost among the sand grains, you know, as we just sort of cruise with the electron beam from particle to particle, hunting. And it's very hard sometimes, because there are minerals that look like microorganisms. So we use size and certain cellular structures or dividing cells or things to say, "These are actual cells."

NARRATOR: Magnified a hundred thousand times, the images reveal the extreme life that thrives in Lechuguilla's darkness. Here, tiny microbes appear as distinct circular shapes. Many appear firmly attached to the rocks.

DIANA NORTHUP: We found a really exciting thing when we looked at some samples and saw what looks like a string of pearls that are widely spaced. And these are shapes that have been seen in other cave samples, once we search the literature, and they're consistently found in all the corrosion residues.

NARRATOR: Penny and Diana hope to figure out exactly how the microbes are enlarging the cave walls. They find clues in a rock sample rich with the mineral manganese.

DIANA NORTHUP: What we're able to see is a very unique cell shape with an elongated tail that is associated with some bacteria that eat manganese. And manganese-eating bacteria are able to produce acid, which could be part of the corroding of this rock. Also, as they pull the manganese out of the rock wall to eat it, they're also leaving holes in the crystal lattice. So these two pieces together provide some of the evidence that we have that microbes are actually eating the rock walls.

NARRATOR: To be sure, they need to grow their samples in the lab, but less than one percent of all microbes can be cultured with standard techniques.

PENNY BOSTON: In many of the cases, we are looking at organisms that are so different from any living, known organisms, that we have to guess what these guys even eat. Is it a metal? How much do they want? If we add sugar to the medium, are we killing a whole bunch of them? Well, it's a very laborious and slow process to sort of work this out.

NARRATOR: For microbes that won't grow in culture, Diana extracts DNA to figure out who is there.

DIANA NORTHUP: Some of the organisms that I'm finding in Lechuguilla Cave, they are very, very dissimilar to any other known organism. We know them only because of their gene sequences, and they're incredibly diverse.

NARRATOR: But Lechuguilla's rock-eating bugs have one important trait in common.

PENNY BOSTON: They need very little, and yet they aren't inactive. Originally we thought that perhaps many of the organisms that we might find would be either dead or just shut down metabolically. But what we have found is that there are very large numbers of these guys actually active, metabolically active and reproductive. And so, to me, that was one of the most amazing things.

NARRATOR: If microbes are still eating the walls of a cave as old as Lechuguilla, what role might they be playing in a younger cave still forming?

News of just such a cave draws Diana Northup and geologist Louise Hose to the lush rainforests of southern Mexico. They've made the journey in the hopes of catching a cave in the act of being formed. As they approach, they discover that the stream that runs through the cavern is milky white, rich with sulfur and gypsum. Hydrogen sulfide gas bubbles up in the water, a sure sign that the cave poses a deadly, invisible risk to anyone who dares to enter.

LOUISE HOSE: The hydrogen sulfide combines with the moisture, the humidity, in the cave and forms sulfuric acid. And that sulfuric acid is in the air, and you breathe it into your lungs, and it will start to deteriorate the linings of the lungs. So we wear these gas masks to help filter out the hydrogen sulfide, and that's protecting my lungs from the sort of damage that can be done.

NARRATOR: Because of the toxic gases, few venture beyond the stone staircase that descends to the mouth of the cave. Diana and Louise carefully follow the stream that pierces the heart of Cueva de Villa Luz, which means lighted house in Spanish. It's named for the many skylights that connect the cave to the surface. But this influx of fresh air is no match for the deadly gases seeping into the cave.

As in Carlsbad, the hydrogen sulfide bubbling up in the stream is made by microbes deep underground. In addition to this gas, Louise and Diana's monitors detect other poisonous fumes.


LOUISE HOSE: Sometimes you have hydrogen fluoride.

DIANA NORTHUP: What do those do to you?

LOUISE HOSE: Kill you.

NARRATOR: As they venture further into the black recesses of the cave, their monitors beep incessantly, signaling a rise in gas levels. But despite the increasing darkness and toxic fumes, they find more life.

DIANA NORTHUP: Usually, when you walk into a cave, life is something that you have to look pretty hard to see. And so the thing that just strikes you, just slaps you in the face in Via Luz, is the fact that everywhere you look there's life: there's spiders, there's insects. And, as you go deeper into the cave and into the dark areas, there are rocks in the stream that are covered with this green coating. It's not being driven by sunlight—it has to be driven by other things. So you think about, "Wow, what's really driving this system?"

NARRATOR: Deep inside the cave they find clues. Diana is mesmerized by strange formations dangling from the ceiling. At first glance they look like stalactites, but unlike stalactites, which are hardened limestone, these are soft, gooey strings of mucus. Because they drip like a runny nose, they're nicknamed "snottites." Diana is struck by how similar they look to u-loop rock formations found in Lechuguilla.

DIANA NORTHUP: You can imagine that one snottite maybe went over and touched its neighbor and then became rock over time—it became calcified—and so possibly snottites existed all those millions of years ago when Lechuguilla was formed.

NARRATOR: But there is something else amazing about this slime.

LOUISE HOSE: The liquid that is dripping off the end of each of the snottites is sulfuric acid. If you get it on your hands—I've had it drop in my eyes—it burns tremendously. Clothing will tend to dissolve when we have it drip on our clothes. When it drops onto the rocks, it helps to form the cave by dissolving away the limestone.

NARRATOR: The scientists are shocked. Not only are microbes beneath the cave producing hydrogen sulfide, it forms sulfuric acid as it bubbles up in the stream—bacteria inside the cave are making acid. Diana examines a snottite.

DIANA NORTHUP: When you look at them with the microscope and look at them magnified many, many times, they're just enormous bacterial colonies. And when I looked at the DNA from one of these snottites, it's just this thriving culture of a bacteria that is known to eat sulfur and produce sulfuric acid.

NARRATOR: Wherever sulfuric acid eats away the limestone, pasty white gypsum is left behind. Finally, scientists can observe microbes in the act of creating a cave.

DIANA NORTHUP: When you look at this cave, the walls are literally melting. It's like a birthday cake, left out in the rain, that's sort of melted. And so it just cries out to you that this is an enlargement process, because the walls are literally being dissolved before your eyes.

NARRATOR: But is life as powerful a force as the flowing water shaping the cave? Although the scientists are not sure, one thing is clear: these microbes play a vital role.

LOUISE HOSE: And in these snottites, not only are there bacteria, but when we looked at them closely we found that there were mites and worms and other little tiny bugs and insects that were probably grazing off of the bacteria.

DIANA NORTHUP: And right beside this snottite, which is so like battery acid, there's spiders with egg cases. And there isn't just one spider, there's gobs of spiders. So the whole wall is alive, not just with microorganisms, but with some of these insects and arachnids, that you wonder, "How could they survive here? There's all this hydrogen sulfide. This ought to be affecting them, too."

NARRATOR: But this gas bubbling up from below not only drives the cave's formation, it helps fuel the food chain of this subterranean world. Cut off from the sunlight, bacteria feast on the hydrogen sulfide, and, in turn, become food for the insects and fish.

Suddenly the oxygen in the cave plummets to deadly levels.

LOUISE HOSE: There's a 10 percent drop in oxygen. You know we've seen it go from 20 and a half...

DIANA NORTHUP: Wow, 10 percent!

LOUISE HOSE: When you start to drop below 18 or 19, one worries. When it gets down to about nine and a half percent, it will kill you very rapidly, so we have to evacuate the area.

NARRATOR: The expedition ends abruptly, with many unanswered questions about the physical limits of life and its impact on caves.

The search for a cave that's easier to study leads a new group of scientists to northern Wyoming. Hidden on a rocky embankment is the entrance to Lower Kane Cave. From its narrow opening flows the familiar rotten smell of hydrogen sulfide fumes. Because the gas levels here are much lower than at Villa Luz, microbiologist Annette Summers-Engel and geologist Phil Bennett find Kane a safer place to work. Even sliding through its 30-inch crack is a relatively easy feat by caving standards.

Lower Kane has none of the exquisite mineral formations of Carlsbad or Lechuguilla, or the abundance of life of Villa Luz. Its gypsum-coated walls enclose three spring-fed pools linked by a stream that runs through the middle. The ceilings lack even the humblest stalactite. Yet this plain looking cave holds another treasure that has drawn these scientists underground for days on end.

Deep inside Kane's dark realm, the researchers have discovered a rare microbial world. Although the microbes themselves can't be seen, their colonies can. Just below the surface of Kane's springs, stretch tapestries woven by trillions of microbes banding together. This stringy matting, changing from subtle shades of blue-black, gold and brown, to garish Day-Glo orange, offers a glimpse of what early life might have looked like. Ancient rock records reveal that the earth was once covered with microbial mats like these. As life evolved, predators wiped them out. But, unlike Villa Luz, little else survives here so the microbes dominate. Since energy from the sun never reaches Kane, Phil Bennett tries to determine what chemicals in the water the microbes are feeding upon.

PHIL BENNETT (Geologist, University of Texas): Well, these bacteria don't actually use the sunlight for anything. What I mean by that is that they don't use carbon from trees or from things that were growing on the surface that came down in the cave and they're eating it. They're actually eating chemicals, chemicals that may not have ever needed sunlight to be produced. And so they're true dark life. They not only don't use light, but they never needed it at all.

NARRATOR: Just as at Villa Luz, hydrogen sulfide produced by microbes deep in the earth bubbles up though the spring and forms sulfuric acid. On the walls above, tiny droplets gleam in the light. They seem like ordinary condensation. But a needle-like probe reveals that the droplets are sulfuric acid. Although it's not as strong as the acid at Villa Luz, it's still dissolving away Kane's limestone and leaving gypsum behind. And like at Villa Luz, colonies of bacteria are making this acid. But how much did they shape the cave?

PHIL BENNETT: The bacteria may be doing the things that would appear to have made the cave, but also the cave may have been formed without bacteria and they are living here. And that's the specific question that we're trying to answer: did the bacteria do all this or did this all happen and they just moved in because it was a great spot to live?

NARRATOR: The scientists devise a test of elegant simplicity. Chips of pure calcite, the mineral in limestone, are placed in plastic tubes and submerged in the springs beside the microbial mats. In some, both acidic water and microbes are allowed to flow freely over the limestone chip. In others, water is allowed in, but a fine filter keeps out the microbes.

After submerging the chips in the spring for several months, the team studies them beneath a microscope. The difference is striking. The chip exposed both to the microbes and acidic water is far more eroded. Colonies of microbes have left scars across the surface. But the chip only exposed to acidic water hardly seems changed.

PHIL BENNETT: So if we had an absolutely sterile cave, would we see hydrogen sulfide converting to sulfuric acid? And the answer is, "Yes, but not very fast." When we have bacteria living off that hydrogen sulfide, producing sulfuric acid, it is much faster, we think, and more efficient, and so the cave formation process is very rapid.

PENNY BOSTON: We now know that microbes are not just along for the ride when it comes to caves and the subsurface in general—that they're active agents of geological change over time. This is really a new perspective, that the organisms and the geology cannot be separated over time. And to understand caves, we have to understand the life that they contain. And if we had a planet with no life on it, the caves that would form would be very different from the ones that we have on this planet. And that magic ingredient that we have here, that has to be figured out, is life.

NARRATOR: To understand the full impact of life on Kane Cave, Annette and her colleagues have spent years analyzing its microbial mats. To their surprise, instead of simple colonies, they contain communities as diverse as a city block.

ANNETTE SUMMERS ENGEL (Geomicrobiologist, University of Texas): Traditionally it's been thought that many of these mats are composed of one type of organism. But we're finding in these mats, after studying them for several years, that they're composed of a wide range of microorganisms using iron, using sulfur, using carbon in different ways. And we never really thought that that would be possible. And each one of the mats has a different assemblage of microorganisms doing specific things for the cave.

NARRATOR: There are microbes eating hydrogen sulfide, making sulfuric acid and dissolving the limestone. Below them are bugs that live off the waste of the rock-eaters, making methane gas or even possibly new minerals. But the greatest surprise is the discovery of a primitive extremophile closely related to those found thousands of miles away at undersea volcanic vents. There are hints that these organisms descend directly from the earliest forms of life that emerged on Earth some 3.5 billion years ago.

DAVID WOLFE: There is a growing consensus that life probably began in the subsurface environments where organisms would have been safeguarded from the rather horrendous conditions going on at the surface at the time, with meteor bombardments, volcanic eruptions, high ultraviolet radiation loads. So surface life really, quite possibly evolved from subsurface life and not the other way around.

NARRATOR: If extremophiles represent ancient life, they also give us a glimpse of how this life survived here on earth and perhaps elsewhere.

PHIL BENNETT: Life can live off simple chemical compounds. And what's important about that in early life and perhaps in other environments or other planets, that's all there is. So when we look at the bacteria in Kane and how they're living off of simple elements, this may be how bacteria are living on Europa, for example, or on Mars.

NARRATOR: Both astronomers and geologists believe that other planets and moons in our solar system have subsurface conditions similar to Earth's. If extremophiles can survive in harsh environments here, why not beneath the surface of some distant planetary body where liquid water exists?

PENNY BOSTON: We have found organisms in more places than we could have ever dreamed. And so, by looking at the extremes of what life can do on our planet, we may be looking at a condition that's representative of the average condition on another planet. And it helps us to be able to stretch our imaginations and figure out just how to go about looking for life elsewhere in the universe.

NARRATOR: It's hard to imagine a more alien, exotic world than the underground realm of caves. While their sculpted beauty is breathtaking, the invisible dark life they contain is even more extraordinary. These microbes, with their unique strategies for survival, raise the possibility that there is far more life in the universe than we ever imagined. And on earth, much of it remains undiscovered beneath our feet.

DIANA NORTHUP: We can't see microbes, and so therefore, we tend to dismiss them. But they are not only active, they're powerful. They carve out caves. They're why we're not up to our necks in dead trees and plants. They produce even the very air that we breathe. Without them we'd be nothing. They actually really are the engineers of our planet. They are worthy of our respect.

Lechuguilla cave is open only to scientists, but on NOVA's Website, you can explore this pristine subterranean world yourself, at or America Online, Keyword PBS.

To order this show or any other NOVA program, for $19.95 plus shipping and handling, call WGBH Boston Video at 1-800-255-9424.

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Mysterious Life of Caves

Written, Produced & Directed by
Sarah Holt

Edited by
Sarah Holt

Narrated by
Edd Gasper

Associate Producers
Jennifer Callahan
Jennifer Lorenz

Sid Perou
Stephen McCarthy
Tom Zannes
Slawomir Grunberg

Michael Bacon
Adrian Burch
David Whittaker

Sound Recordists
Michael Becker
William Doll

Dan Nutu

Assistant Editor
Dan Van Roekel

Online Editor
Ed Ham

Mark Kueper

Audio Mix
Richard Bock

Sound Editor
Rob Todd

Archival Material
Aerial Extreme
Channel 4 - London
Discovery Communications
Finley-Holiday Film Corp.
Getty Images/The Image Bank
Al Giddings Images, Inc.
Lava Video Productions
Ka'Io Productions
Karst Productions
Kurtis Productions, Ltd.
Mark Jenkins
Sekani Moving Ideas
Speleo Projects
Woods Hole Oceanographic Institution

Special Thanks
Rick Bridges
John Brooks
The Carlsbad Caverns Guadalupe Mountains Association
Cornell University
Larry Fish
National Marine Sanctuaries
National Park Service
The New Mexico Museum of Natural History and Science, Albuquerque
Megan Porter
Libby Stern
United Airlines
Urs Widmer

NOVA Series Graphics
National Ministry of Design

NOVA Theme
Mason Daring
Martin Brody
Michael Whalen

Post Production Online Editor
Spencer Gentry

Closed Captioning
The Caption Center

Production Secretary
Queene Coyne

Jonathan Renes
Diane Buxton

Senior Researcher
Ethan Herberman

Production Coordinator
Linda Callahan

Unit Managers
Holly Archibald
Denise Drago

Nancy Marshall

Legal Counsel
Susan Rosen Shishko

Post Production Assistant
Patrick Carey

Associate Producer, Post Production
Nathan Gunner

Post Production Supervisor
Regina O'Toole

Post Production Editor
Rebecca Nieto

Coordinating Producer
Laurie Cahalane

Supervising Producer
Lisa D'Angelo

Senior Science Editor
Evan Hadingham

Senior Series Producer
Melanie Wallace

Managing Director
Alan Ritsko

Executive Producer
Paula S. Apsell

A NOVA Production for WGBH/Boston

© 2002 WGBH Educational Foundation

All rights reserved


Jewel of the Underground

Jewel of the

A self-guided tour of spectacular Lechuguilla Cave.

Journey into Lechuguilla

Journey into

Journalist Michael Ray Taylor on caving 1,200 feet down in Lechuguilla.

The Lives of Extremophiles

The Lives of

Microbiologist Diana Northup on bacteria that live where nothing else can.

How Caves Form

How Caves Form
Watch as rainwater, waves, lava, and bacteria create four different types of caves.


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© | Created September 2006