The Ecological Society of America (ESA) held its annual meeting in Louisville, Kentucky, from August 11-16 this year. I depend on these annual events to catch up with the cutting edge of ecological research and thinking that is relevant to my ecological consulting, and with ecologist friends and colleagues from across the country. Field trips on the weekends before the meetings always give me a chance to visit local ecosystems and learn from local experts. In stories posted here on my Ecologia blog, starting in 2014, I have tried to capture some of the adventure of those ESA meetings and field trips (see links to those blog posts below under “For related stories see:”). This year’s meeting was as thought-provoking, stimulating, and excellent as ever. I joined an organized field trip to the Hoosier National Forest to learn about forest management firsthand on the Saturday before the meeting, and extended my stay to visit Mammoth Cave afterward.
I really wanted to visit Mammoth Cave because I knew that it was the core of a UNESCO biosphere reserve, one of the 27 biosphere reserves in the U.S. still participating in the UNESCO Man and the Biosphere Program’s international network. I spent four months last year at Cascade Head, on the Oregon Coast, another of those U.S. biosphere reserves, and have been in communication with the loose network of people involved in U.S. biosphere reserves. The word through the grapevine was that Mammoth Cave was one of the more organized and active of them, so I was curious to compare what I knew about the Cascade Head Biosphere Reserve with Mammoth Cave. I communicated with my contacts there – two hydrogeologists from Western Kentucky University in Bowling Green—and they enthusiastically facilitated my visit.
Biosphere reserves are supposed to be laboratories and models for understanding and improving the human-nature relationship. At Cascade Head, I concluded that biosphere reserves all around the world share some common principles. I came up with a framework of five stages, or components, that seemed to capture the history and current challenges of the Cascade Head Biosphere Reserve, each starting with the prefix “re”: resistance, research, restoration, reconciliation, and resilience. I wanted to compare Mammoth Cave with Cascade Head and test this framework in another setting. More on that later; but first…
Getting the lay of the land and understanding the ecology of Mammoth Cave, requires a plunge into deep time: the drifting of continents, the ecology of ancient seas, the effects of ice ages, the creation of karst landscapes and caves. Ready for the descent?
First thing to know is that 350 million years ago the area that is now Mammoth Cave in south-central Kentucky was the floor of a shallow, tropical sea about 10 degrees south of the equator, off the southwest coast of the giant supercontinent of Pangaea. Corals and meadows of crinoids – flower-like echinoderms called “sea lilies” – were common. Trilobites and brachiopods thrived. The skeletons of all these creatures, made of calcium carbonate, collected and compressed on the sea bottom for tens of millions of years, eventually becoming a layer of limestone more than 1,000 feet thick. Around 310 million years ago, a river flowing from the north deposited 50 to 100 feet of sand and silt, which eventually became a cap of shale and sandstone over the limestone.
Fast forward through a few hundred million years, filled with continental drift, uplift, and warping and cracking of rock layers. Six million years ago a ridge of sandstone and shale lay above the downcutting Green River in the area that is now Mammoth Cave National Park. To the south, where the limestone was not protected by a waterproof cap, precipitation drained into the ground, eating away limestone and flowing in underground channels, over eons creating an interconnected, underground river system flowing north toward the Green River. The Mammoth Cave system had begun to form. The most recent research, based on studies of cave sediments that can determine when they were last exposed to sunlight on the surface, places the age of the oldest cave passages at between four and six million years ago.
How can common rain eat away limestone? Earth’s atmosphere is made up mostly of nitrogen and oxygen, but contains a trace of carbon dioxide. Turns out that when rain falls through that skiff of CO-2 in the atmosphere, it absorbs some of it, and a weak acid, called carbonic acid, is formed. That normal process makes natural rainwater slightly acidic – something like 5.0 to 5.5 on the pH scale of acidity, so weak that our taste buds can’t taste its sourness. Nothing like vinegar, which is a hundred times more acidic. Nevertheless, its acidity causes rainwater seeping into cracks to dissolve the skeletons of ancient sea creatures compacted into limestone, eventually carving canyons underground.
The advances and retreats of continental ice sheets during the Ice Ages of the Pleistocene Epoch, which lasted from 2.6 million to 11,000 years ago, rerouted rivers across eastern North America, influencing the plumbing system of the Mammoth Cave area. During the pre-Illinoian glaciation from 1.8 to 1.2 million years ago, the ice sheet overran the ancient Teays River. Water then had to find a new route, and it captured the tiny old Ohio River, turning it into the major drainage system south of the ice, flowing westward. The last pulse of ice, the Wisconsin glaciation from 23,000 to 11,000 years ago, pushed an ice sheet to within 50 miles north of the Ohio River. As it retreated, the river cut down into its channel, and so did its tributaries, including the Green River. An online article on Mammoth Cave geology explains that, “As the Green River continued to cut deeper into its bed, the water table continued to drop. To keep up, new underground drains formed in the limestone bed, creating new channels beneath the original ones. The oldest cave passages are the closest to the surface and the youngest are the deepest underground. Now, at the present level of the Green River, cave passages continue to form in the deepest depths of Mammoth Cave.”
Young underground drainage channels were completely filled with water, and their eroded shapes are ovals, stretched horizontally. As passages with flowing water dried out, the rivers at the bottom of the passages cut slot canyons in the limestone, underground versions of those seen in western canyon country. Giant blocks of rock fell from the ceilings of some upper passages with rivers in their beds, creating tall rubble-filled canyons, and huge vaults with rounded ceilings developed at some junctions of these underground canyons. As water drained from the higher passages, it probed down vertical cracks and created underground waterfalls that eroded deep vertical shafts. In a few places, where the water-shedding sandstone capping the limestone let a trickle of carbonate-saturated water in, drips slowly deposited icicle-like stalactites from the ceiling, or cascades and curtains of flowstone.
Because of the cave system’s geological history, it is essentially a multi-layered, three-dimensional, underground canyon. One of the delights of Mammoth Cave, for me, was how it challenged my everyday, flat-surface brain to stretch to picture a 3-D underground world. A three-dimensional metal sculpture of the cave displayed in the Visitor Center helped – a little – to convey the topography of the amazing cave.
Because of its geological evolution, stacking layer upon layer of cave passageways on top of each other, Mammoth Cave is the most extensive cave system known on Earth. Over 400 miles of passageways have been explored and mapped, and geologists think there could be another 600 miles of undiscovered cave here.
I mentioned at the beginning of this story that my contacts at Mammoth Cave were hydrogeologists from Western Kentucky University. Now, after a brief tutorial about the links with water that are key to cave geology, it’s time to describe a magical peek into Great Onyx Cave with those speleoscientists. Dr. Chris Groves is director of the Crawford Hydrology Laboratory, associated with WKU, and Lee Anne Bledsoe is assistant director. They have been studying the connections between aboveground and underground water for decades, using a technique called “groundwater tracing.” Basically, they throw some high-tech dye into a stream, sinkhole, or sewer system on the surface, and then look for it with specialized collectors that they place in cave streams and rivers. They have traced water from the surface to expected and unexpected places in the cave plumbing system, from days to months later. The “watershed” of Mammoth Cave – the area from which surface water is gathered and flows through it – is about 95 square miles in area. It extends far outside the boundaries of the national park. Great Onyx Cave is the location of some of their water monitoring stations.
That was the reason we were gathered at the bottom of a steep gravel road, accessed through a locked gate, on a steamy Kentucky afternoon at the entrance to Great Onyx. We donned helmets and headlamps provided by our hosts, and one of the National Park Service staff accompanying our small group unlocked the heavy metal door at the cave entrance. A dozen steps down into the cool underground world we were greeted by a slim, orange, black-spotted salamander, about five inches long, the first of the unique cave species we would see. This was the cave salamander, whose scientific name, Eurycea lucifuga, indicates its preference for fleeing from light. It is sometimes found hundreds of meters into caves, far beyond the “twilight zone,” but has also been found around cave mouths and cave springs, bridging the belowground and aboveground worlds.
After descending past some dramatic flowstone formations, we entered a long, mostly level, tunnel-like passageway. This was one of those oval cave chambers that was once completely filled with water, which carved its characteristic shape. It was only a few feet taller than our heads – and sometimes less, requiring stooping and ducking – and now dry because the cave drainage had shifted to a lower level and the waterproof sandstone cap kept surface water out. It was precisely those dry conditions that created what was most surreal and magical about Great Onyx Cave. For long stretches, the low ceiling was a layer of gypsum. In some areas it was gray and smooth, but with starlike sparkles in our headlamps. In other places, gypsum had extruded from the surface, creating arcing curls of white crystals that formed improbable shapes, ranging from corkscrews to angels, flowers to fountains, and much in between. Here, in the midst of a real world that was more unimaginable than a dream, it was hard to focus on “hard core” science. For someone like me, torn between the perceptions of a scientist and a poet, Great Onyx was a surreal, star-spangled underground heaven.
We never reached underground water in Great Onyx Cave that first afternoon, but with a ranger-led tour the next day I descended to the lowest level of the current cave system, the River Styx – named after the river which, in Greek mythology, formed the border between Earth and the Underworld. We couldn’t reach the shore of the river because of damage to the trail from the last cycle of cave flooding, but we looked down on its eerie blue-green waters flowing at the bottom of a gray limestone canyon from a viewpoint above. From that point, the Styx flowed another half-mile underground before emerging in a spring along the Green River that was only a short walk from the national park visitor center. I walked there the following day, and tried to get my mind to grasp the underground-aboveground connection at the lower end of the cave system.
It is precisely the aboveground-underground connection, so tricky to imagine, that makes Mammoth Cave – an ancient cave system probably at least four million years old, and maybe up to six million – such an important place for understanding cave ecology and evolution. Sinkholes and entrances to the upper levels of the cave system are, and have been for millions of years, points of contact with surface species and ecosystems, especially because they carry in organic material and nutrients. And from the bottom, springs where cave rivers flow to the surface – like the River Styx Spring – are also points of ecological contact. Life is abundantly creative and entrepreneurial – an understated way to describe how it has poked into every crack and crevice of the planet, from thermophilic bacteria in Yellowstone geysers to the Ebola virus. So, it is no wonder that underground rivers and passageways in caves have their own ecosystems and unique species. But the caveat, of course, is that life is driven by energy, and the main source of energy on Earth is the Sun, and… well… caves don’t get a lot of sunshine. So, the source of energy to run cave ecosystems has to sneak in from aboveground photosynthesis, either from nutrients flowing in from sinkholes, or pushed into the springs at the outflows of cave rivers from below when the rivers into which they flow flood. Or, in another scenario, brought in by cave dwellers who forage out from cave entrances to tap energy from surface ecosystems, and bring it back into the dark.
With at least four million years to play around with these options, evolutionary entrepreneurship has come up with about 130 species that comprise the cave ecosystem at Mammoth Cave — one of the highest diversities of cave-adapted organisms in the world. One Mammoth Cave species even caught the attention of Charles Darwin as he was struggling to explain his theory of evolution by natural selection. The northern cave fish, Amblyopsis spelea, was the first species of blind cave fish to be described in the scientific literature, in 1842, by the American zoologist James Ellsworth DeKay. In On the Origin of Species, published in 1859, Darwin wrote that “It is difficult to imagine conditions of life more similar than deep limestone caverns under a nearly similar climate; so that, in accordance with the old view of the blind animals having been separately created for the American and European caverns, very close similarity in their organisation and affinities might have been expected. … On my view we must suppose that American animals, having in most cases ordinary powers of vision, slowly migrated by successive generations from the outer world into the deeper and deeper recesses of the Kentucky caves, as did European animals into the caves of Europe.”
Darwin interpreted the loss of vision in both American and European cave fish and other animals as a case of relaxed selection, in which complex structures like eyes that were maintained by natural selection were gradually lost as selective forces that favored them disappeared. A delightful, if arcane, scientific paper by Jonathan Armbruster and colleagues published in 2016 and titled “Morphological Evolution of the Cave-, Spring-, and Swampfishes of the Amblyopsidae,” unravels the evolutionary relationships in this North American family of fishes. The seven species in the family illustrate the evolutionary progression imagined by Darwin pretty well. Five of them “are obligate inhabitants of caves and have evolved a suite of troglomorphic characters, most notably non-functional, degenerate eyes and almost no pigmentation.” In other words, they are blind and white. (Don’t you just love that word “troglomorphic”?) Turns out that this family of fishes is the American claim to fame among cave faunas worldwide, in that “the Amblyopsidae is unusual in that nearly all species in the family are stygobiotic (obligate subterranean).” (Don’t you just love that word “stygobiotic”? I do – with its stem word a reference to the River Styx!) The story goes deeper and deeper.
One of the seven species in the family is the abandoned — or aboriginal — aboveground cousin: the swampfish (Chologaster cornuta), fully sighted and pigmented, found in the swamps of the Atlantic coastal plain. Another is the evolutionary link between aboveground and belowground, the spring cavefish (Forbesichthys agassizii): the spring cavefish is “a stygophile occupying karst regions of the Eastern Highlands where they generally occur in spring-fed streams and springs; they feed at night and retreat inside the springs or within dense vegetation during the day,” write Armbruster et al. (What a wonderful word, “stygophile” – not an obligate dweller on the River Styx, only a “Styx-lover.” Who said scientists weren’t poets?)
In Great Onyx Cave on that first afternoon, after being greeted at the entrance by the cave salamander, we also encountered two other stygophilous species. One was the cave cricket, Hadenoecus cumberlandicus – another great scientific name, the genus name meaning “living in Hades.” Who says scientists aren’t poets? Cave crickets, with their spindly long legs, are close relatives of the camel crickets that love the dark crawl space under my house, and are more closely related to grasshoppers than true crickets. Because they move in and out of caves, bringing nutrients created by photosynthetic plants in the aboveground ecosystem into the dark reaches of the underground world, they are what ecologists call a keystone species. With their energy imports, they nourish the stygian food web and stabilize the “arch” of the cave ecosystem, like the masonry keystone of an architectural arch does, metaphorically speaking.
We also encountered one of the cricket’s predators in Great Onyx that afternoon. After a 20-minute walk from the cave entrance, far into the dark, our headlamps started to reveal spiders hanging in webs in alcoves along the cave passage. These were the cave orb-weaver, Meta ovalis, a good-sized spider with a pale, bulbous abdomen and brown legs that reached out to almost an inch long. This species produces an egg case in which it protects its eggs – we saw two of those, suspended in web, which resembled white, fuzzy jelly beans. On the cave wall behind one, a flock of baby spiders crawled, apparently having hatched only recently. Not much is known about their ecology, but there have been a few studies, which show that the spiders eat cave crickets.
The scant scientific literature suggests that Meta ovalis is usually found in the entrances of caves and in the twilight zone, but occasionally in the deeper parts of caves. Rick Toomey, a National Park Service scientist at Mammoth Cave, told me that the spiders we encountered deep in Great Onyx were first spotted a few decades ago, and that at first, they wondered whether a spider population so deep in the cave would survive. Now, studies by Dr. Kathleen Lavoie and her colleagues – mostly cave-cricket ecologists – have seen that this population of Meta ovalis has become self-sustaining. Perhaps this is a new deep-cave “island” population, reproductively isolated from its more timid cousins, which in another million years will spawn a new, unique species – the Great Onyx Cave orb-weaver. Meta onyxi, anyone?
So far, this story has focused mostly on “research,” the second of the five “re”s that I proposed earlier were common to all biosphere reserves – and that I found at Cascade Head in Oregon, and was searching for at Mammoth Cave. What about my the other five things I hypothesized are common to biosphere reserves?
At Cascade Head, the first “re,” resistance,” referred to the history of actions that kept the natural ecosystems of the place from being trashed. Those included being incorporated into a national forest in 1908, designated as a U.S. Forest Service experimental forest in 1934, included in a larger Forest Service protected area in 1974, and named a UNESCO biosphere reserve in 1976.
What about Mammoth Cave? What is the history of its “resistance” and conservation? It’s a mixed review. Local people began touting the wonders of the cave soon after its discovery and people began visiting the cave around 1816. Entrepreneurs like Stephen Bishop, a slave who became a cave explorer and guide, began taking tourists into Mammoth Cave in 1838.
Ralph Waldo Emerson, prominent essayist and mentor to Henry David Thoreau, visited to Mammoth Cave in 1850, and it’s a remarkable story. While lecturing in Cincinnati, having heard about the cave, Emerson made special plans to go there. The expedition took about a week, as he described in detail in a letter to his wife written after the fact, on 16 June. He first traveled to Louisville, then set out from there on a riverboat on June 4 with a party of “seventeen gentlemen & ladies, including three Englishmen,” down the Ohio River to Evansville, then up the Green River and Barren River to Bowling Green, and then by coach to the Mammoth Cave Hotel, arriving at night. Early the next morning, Sunday, June 9, Emerson wrote, “our ladies appeared in short dresses and Turkish pantalette & turban indespensable to the adventure. We entered the grand old cavern at 7-1/2 o’clock, a chilly descent into the earth.” He described many of the cave passages and features that can still be visited on ranger-guided tours today. Emerson’s group made it to Echo River, and boated on it. It was a long day. “From the mouth of the cave to Serena’s Arbour, which was our farthest point, is nine miles, and we returned all the way on our own steps, an 18 miles’ walk performed in 14 hours. . . It was a long & trying tramp certainly for ladies to make,” he wrote.
Emerson and some others of his party returned the next day for a four-hour trip into another part of the cave, and he describes being particularly impressed with the “Star Chamber,” where the all the lamps except that of the guide were extinguished, and the the cave ceiling reflected the single light source, creating the illusion of a starry sky. Later Emerson wrote an essay, titled “Illusions,” about how he “lost the light of one day” in Mammoth Cave. “I saw high domes and bottomless pits; heard the voice of unseen waterfalls; paddled three quarters of a mile in the deep Echo River, whose waters are peopled with the blind fish,” he wrote. But the illusion of the stars shining underground was what seemed to strike him most, and what launched him into another of his wonderful, rambling, philosophical crawls through the labyrinth of his unique mind.
In a strange way – at least it seems strange to me now, from my modern vantage point – Mammoth Cave exemplifies a common feature of American conservation: tourism was often a driver for the protection of nature. Can we say that the fascination with stunning, unique, “sublime” scenery during the first three-quarters of the 19th century was really a key in the “resistance” to the destruction of wild and natural places? A bit reluctantly, I have to say yes: sublime scenery has protected important natural places, like Yellowstone, Yosemite, and Mammoth Cave.
But even before the first tourists started coming to Mammoth Cave, slaves were sent to mine the deposits of bat guano, accumulated over who knows how many millennia, for its high concentration of potassium nitrate, or “saltpeter” as it was called, needed to manufacture gunpowder. Mammoth Cave was probably one of the largest bat hibernacula in the world before the mining activity not far inside the historic entrance of the cave disrupted the bat colonies. Gunpowder made from Mammoth Cave bat shit killed British soldiers in the War of 1812 and probably Union soldiers in the Civil War. For the bats, the later protection of the cave that was motivated by tourism was too late. Gunpowder had already won out.
By 1924, a group of wealthy Kentuckians formed the Mammoth Cave National Park Association to advocate for the creation of a national park; by 1926 they had succeeding in getting the park officially authorized. At that time, approximately 45 percent of the surface land in what is now the national park was small farms and settlements, and although the farms were often poor and marginal, local residents were generally not happy with the creation of a national park on their land. Some farms were purchased with private donations, but others were acquired by the federal government through the messy process of eminent domain. Hundreds of farms were acquired and thousands of people displaced, and it took until 1941 for the federal government to acquire enough land to officially declare and establish the park.
OK, so that is a bit about the “resistance” aspect of Mammoth Cave. We’ve already discussed the scientific research dimension, both geological and ecological. What about the third “re,” restoration? At Mammoth Cave, restoration was an intersection of research and resistance. Hydrological research, like that still being done by my contacts at Western Kentucky University, showed that even though Mammoth Cave National Park was protecting the surface above most of the visited cave system, the “watershed” of the underground river system in the park reached far beyond the boundaries of the park. Agriculture and towns located on the “Sinkhole Plain” to the southeast were dumping fertilizers, pesticides, sewage, and other pollutants into the surface waters that quickly found their way through the porous karst into the underground watershed of Mammoth Cave. So, the question became how to restore the health and integrity of the connected aboveground-belowground watershed? Science could show the connections, but social and political action was needed to take the necessary actions to clean up and restore the aboveground watersheds that drained underground into the cave system.
Restoration? Part of it is straightforward: Don’t put your shit into surface waters! The Caveland Environmental Authority was created in 1998 as a step toward the needed restoration. It installed a regional sewer system with over 112 miles of main lines, and replaced four failing municipal sewer systems and hundreds of poor septic systems. Because of the Caveland Environmental Authority, “Water quality was greatly enhanced for the core protected area, Mammoth Cave National Park, and also for the buffer and transition zones of the Mammoth Cave Area Biosphere Reserve,” according to the Mammoth Cave Area Biosphere Reserve Periodic Review (2016).
But beside cleaning up the aboveground watershed so it doesn’t pollute the underground one, restoration presents other challenges. Mammoth Cave formerly supported one of the largest bat colonies in the world, until bat-guano mining at the historic natural entrance chased away most of the bats. It would be hard for any ecologist to think that it would be easy, or even possible, to restore the original ecological condition. Thirteen species of bats still utilize the many small entrances of the Mammoth Cave ecosystem, including Dixon Cave, just a short walk around the corner from the historic main entrance. But a bat disease called white-nose syndrome has been spreading through North American bat colonies, including Mammoth Cave, and is a significant threat. Visitors taking park service tours are now required to walk through foot baths to wash their shoes upon exit from cave tours, in an attempt to control the spread of white-nose syndrome.
Reconciliation. What about that? The surface ecology created by the karst geology led to the Mammoth Cave area being one of the largest “barrens” – more or less treeless prairies – east of the Mississippi River. In this karst-barrens area, farming wasn’t that good and economic opportunities weren’t that good, but tough Euro-American settlers and African-American slaves eked out their livings here. In the late 1960s, Kentucky established fifteen multi-county planning and development organizations called “area development districts” to encourage and support local economic development. The Barren River Area Development District (BRADD), is one of those, made up of ten counties centered around the Barren River, a tributary of the Green River. In 1990, BRADD teamed up with Mammoth Cave National Park to successfully propose to the UNESCO Man and the Biosphere Program that they should become a biosphere reserve, part of the international network. To me, this shows an attempt to reconcile the protection of a national and global treasure – Mammoth Cave – with its location in one of the poorest regions of Kentucky. “Biosphere reserves are about reconciling all people with the lands and waters,” Eleanor Haine-Bennett, director of the Canadian National Committee for the UNESCO Man and the Biosphere Program, told me in a phone conversation. At Mammoth Cave, the people and the “lands and waters” are not only on the surface, but also below ground. Deep ecology indeed!
And as a final note in this story, in the title of which I deliberately used the words “deep ecology” – trying to play on the edge of punning and ecological philosophy – we should talk about the final principle I claim is common to biosphere reserves: resilience. Cave critters have been adapting to the aboveground environment through the last dramatic cycles of climate change for millions of years, partly by going underground, going “deep,” where the surface changes of climate don’t reach quite so much. Let’s be honest: creatures who can move between aboveground and underground, hiding from all the shit going on on the surface, crossing the River Styx, spanning heaven and hell – they are likely to be the survivors, through nuclear wars and even until our Sun burns itself out. Resilient, as much as possible. Blind cave fish, cave crickets, cave spiders, and their few hundred cave-specialist relatives will certainly outlast humans. So we should humbly ask ourselves: What can we learn from them?
Go to Mammoth Cave. Go deep in. Strolling through its deep-time corridors, whose limestone walls are compressed corals and sea-lily meadows of the shores of the Pangaean Ocean, puts our tiny lives and lifespans in perspective. Feel yourself immersed in the rock layers hundreds of millions of years old, and be astonished to be walking and breathing in cave channels millions of years old and seeing creatures who fled the sunlight that long ago. Come back up with a new perspective on our strange species and the ephemeral world we take for granted.
For related stories see:
- Report from a Season at Cascade Head, February 2019
- Canoeing Louisiana’s Manchac Swamp with Ecological Aliens and the Voodoo Queen, September 2018
- Walking on the Trembling Prairie, August 2018
- Facing the Anthropocene in Florida: The Ecological Society of America’s 2016 Annual Meeting, August 2016
- An Interconnection of Ecologists: The Ecological Society of America’s 2012 Annual Meeting, August 2012
Sources and related links
- Mammoth Cave National Park
- Mammoth Cave Area Biosphere Reserve
- Mississippian (geologic subperiod)
- Mammoth Cave National Park Geology
- Crawford Hydrology Laboratory
- Evolutionary History of the Northern Cavefish, Amblyopsis spelaea. Niemiller, et al. 2012.
- Morphological Evolution of the Cave-, Spring-, and Swampfishes of the Amblyopsidae. Armbruster, et al., 2016.
- Ecology of North American Cave Crickets. Lavoie, et al. 2007.
- Ecology of the Cave Spider Meta ovalis. Rector, 2009.
- Ralph Waldo Emerson’s visit to Mammoth Cave in 1850 described in letter to his wife
- “Illusions,” essay by Ralph Waldo Emerson in The Atlantic, November 1857
- Mammoth Cave National Park history & creation of the park
- Caveland Environmental Authority