ARTICLE Sinuous draperies adorn the walls, appearing to glow warmly from First discovered in the mid-1950s, Musk Ox Cave on BLM lands in New Mexico was still being surveyed and mapped several decades later. Cave explorers have found fascinating formations as well as numerous fossil vertebrates dating from the Late Pleistocene, some 1.5 million years ago. JIM GOODBAR, BLM within as your light strikes them. A spiky, chandelier-like object reaches down from the ceiling, its irregular pendants shining wetly in your lantern's beam. Stately columns line your path, and crystalline gems sparkle from murky alcoves. A limpid, blue pool ripples gently as ghostly, translucent creatures dart for cover. Compact, furry forms hug high, gloomy corners, their black eyes peering from beneath folded, leathery wings. Is this some unearthly, long-abandoned palace hall, its windowless expanses now inhabited only by odd denizens of the dark" Surprisingly, you're actually in a place that's extremely down to earth-quite literally, you've just stepped inside the Earth. Welcome to the underground. This article will explore many of the fascinating aspects of the most common types of caves, from their development and geology, to the animals that call them home, to the ancient humans who left their marks in beautiful wall art long ago. Since caves are such multifaceted, complex environments, speleologists-scientists who specialize in the study of caves and their resources - must be well-versed in a variety of scientific disciplines, from geology to biology, from paleontology to archaeology. For teachers, caves can offer the perfect focal point for interdisciplinary education in subjects as diverse as biology, archaeology, language arts, and, of course, earth sciences. The "otherworldly" world of caves is one of Earth's last true frontiers, and the only one that is entirely underground. According to the Federal Cave Resources Protection Act of 1988, a cave-or cavern-is "any naturally occurring void, cavity, recess, or system of interconnected passages which occurs beneath the surface of the Earth . . . and which is large enough to permit an individual to enter, whether or not the entrance is naturally formed or manmade." While underground streams do not always flow as torrentially as in this Wyoming cave, caves are nonetheless a major vehicle for groundwater replenishment in many regions. © SID KEAR The deepest and longest cave systems have been explored only by a hardy few; most noncommercial caves remain largely unseen and unknown by the majority of humans. And those are only the caves that scientists know about. The possibilities for new and significant cave discoveries are both real and very intriguing--particularly on some remote public lands managed by the Bureau of Land Management (BLM), an agency of the U.S. Department of the Interior. New cave discoveries are being made each year. Remote and seemingly disconnected from the sunlit world, caves are, in fact, intricate and delicate ecosystems that shelter unique organisms. Though they may appear to be glistening but barren worlds of stone and water, they not only bristle with living organisms but also house evidence of past life forms, both human and nonhuman. Caves are also critical components of the hydrologic cycle in the areas that host them. In fact, cave areas provide 25 percent or more of our nation's water supply and are particularly important in the arid climates of the American West. Cave-related aquifers are highly vulnerable to pollution, however. The geologic nature of these areas -fractured, soluble bedrock overlain by thin soils-allows for rapid vertical and horizontal movement of water, including any pollutants it may carry in from the surface. Contaminants introduced into this system are not well filtered by the thin soils, and once in the water table, are moved along with great rapidity. First discovered in the mid-1950s, Musk Ox Cave on BLM lands in New Mexico was still being surveyed and mapped several decades later. Cave explorers have found fascinating formations as well as numerous fossil vertebrates dating from the Late Pleistocene, some 1.5 million years ago. JIM GOODBAR, BLM Although small-scale in terms of passage length, Wyoming's Natural Trap Cave is of great interest to scientists. The roof of its one large room collapsed thousands of years ago, opening the cave to the surface and allowing animals and plant materials to fall in. The cave is now filled to a depth of more than 5 m with these remains, which EarthWatch volunteers excavated and studied for several years. BLM WYOMING Limestone to Lava: Types of Caves Solution caves, by far the most common, extensive, and largest type of U.S. caves, are formed by the chemical action of weakly acidified water upon soluble rocks. These include limestone (whose principal constituent mineral is calcite, CaCO 3 ), dolostone (principal constituent: dolomite, CaMg(CO 3 ) 2 ), and the mineral gypsum (CaSO 4 ·2H 2 O). Limestone and dolostone caves are clustered along the Appalachian Mountains, and in several midwestern states and south-central Texas. They are also sprinkled throughout states west of the Dakotas. The much rarer caves carved in the evaporate mineral gypsum are located mainly in northern Texas and New Mexico. Other evaporate minerals, such as rock salt (NaCl or KCl), may also host caves. Other, less prevalent types of caves include erosion caves, which are hollowed out by lake and seashore waves and river meanders, and volcanic caves in lava tubes. Volcanic caves, formed when a lava flow's surface cools and hardens while molten lava still flows beneath, are most numerous in the northwestern United States and in Hawaii. All in all, there are more than 40,000 known caves of all types in the United States. About 130 U.S. caves have been developed commercially for public visitation, and at least 81 national parks and monuments contain noteworthy caves. Kentucky's Mammoth Cave National Park, for example, features the world's longest cave, with more than 567 km of mapped passages. Carlsbad Caverns National Park's "Big Room" is the world's largest known subterranean chamber: 400 m long, 200 m wide, and 90 m high. Also within Carlsbad Caverns is Lechuguilla Cave, which plunges to more than 485 m below the surface, making it the deepest solution cave in the United States. The BLM manages hundreds of caves on western public lands. Because most BLM caves are located in remote areas-often relatively removed from human interference- they sometimes present unique opportunities to view cave ecosystems in pristine condition. Untouched caves offer superb prospects for speleological research and protection of cave-related resources, including regional water supplies. BLM public lands also hold great promise for the discovery of new caves, particularly in their farthest reaches and within national monuments and other protected areas. WHERE ARE AMERICA'S CAVES? U.S. GEOLOGICAL SURVEY Karst and Caves: How They Form Many solution caves are part of a complex, irregular topography known as karst, named for the Kras region of the Republic of Slovenia; an astounding 44 percent of that nation is comprised of karst lands. Karst is a unique landform that hosts characteristic features. These include caves, sinkholes (funnel-shaped, closed depressions due to subsidence of surface soils into voids below), lost rivers and sinking streams (surface waters that disappear underground through holes or cave entrances), and springs, known as resurgences. Karst topography hosts few, if any, surface streams, and those that exist are very short. Sometimes, if a sinkhole drain becomes plugged, a pond as large in area as several hectares may appear; if the drain unplugs, a pond of that size may disappear overnight. In tropical areas, a spectacular variety of karst landscape, known as tower or pinnacle karst, features high, steep-sided, flat-topped limestone "towers" separated by areas of alluvium. The formation of karst lands is dependent upon underground solution of rock and the diversion of surface waters to sub-surface systems of interconnected drainage conduits. During the Paleozoic era 300 million years ago, many areas of the United States were covered by warm, shallow seas. The skeletons of dead marine animals and the remains of plants built up as sediments and reefs, the layers of which compacted into thick, hard calcium carbonate rock as the reefs grew larger. When the seas retreated and the land was uplifted by movements in the Earth's crust, the reefs and sediments-hundreds of meters thick-were ex-posed at the surface. In such areas, as weathering produces soil, trees and other plants begin to grow. Rainwater and snowmelt are naturally slightly acidic (pH 5.6) because atmospheric CO 2 combines with precipitation as it falls, forming weak carbonic acid (H 2 CO 3 ), the same acid as in carbonated soft drinks. Once on the ground, the moisture trickles down along plant roots and percolates through the soil, absorbing additional CO 2 from decaying plant remains. The carbonic acid and various other organic acids begin to dissolve underground conduits within the limestone bedrock. During the initial phases of cavern development, the majority of solutional activity occurs at the top of the water table, where the weakly acidic water is concentrated. As the area is further uplifted, or as the water table drops, solution conduits fill with air. At the same time, water continues to enter the cave system through joints and other fractures created during uplift and widened by the flow of acidic surface water. In some cases, these weak points eventually become cave system entrances. Cave passages develop further as the acidic water flows through the conduits, enlarging and deepening them by both chemical action (corrosion) and mechanical erosion (abrasion) by sediments transported by the water. Concurrent weathering by both corrosion and abrasion is called corrasion. Groundwater exiting caves through resurgences often has a higher concentration of calcium and bicarbonate ions than does water entering those caves, indicating that solution is actively occurring today. In karst areas, rivers often disappear underground, where they continue to erode and shape the landscape. JIM GOODBAR Appearing to defy gravity, a twiglike heligmite formation snakes upward amidst a forest of small columns overhung by dripping soda straws. (Cottonwood Cave, New Mexico) JIM GOODBAR Fine selenite-crystalline gypsum-needles form when a sulfate solution percolates up through cave floor silt. (Fitton Cave, Arkansas) JIM GOODBAR Centuries of calcite deposition may cause stalactites and stalagmites to grow together, forming huge columns. (Ogle Cave, New Mexico JIM GOODBAR "Popcorn " and "Pearls " Moving water from percolation and floods can transport materials into and through caves both physically and chemically. The physical process known as breakdown causes rocks to fall from cave ceilings. Sand, silt, and clay may be physically deposited, and secondary deposition of minerals may also occur. Caves offer an ideal environment for chemical deposition of minerals from carbonate- laden water entering the caves. As weak carbonic acid seeps through cracks and fractures in the bedrock, the acid dissolves the surrounding calcium carbonate rock. The rock fractures are generally very tight, keeping the carbon dioxide in solution, much like the cap on a soda bottle keeps its contents bubbly. As the calcite-rich water enters air-filled cave passages, it loses excess carbon dioxide to the atmosphere (much like what happens when the soda bottle cap is removed), or the water itself may evaporate. This causes dripping water to precipitate minerals from solution, creating "cave decorations" known as speleothems-from the Greek for "cave deposit." Speleothems formed by dripping water are collectively termed dripstone. Common dripstone speleothems include stalactites and stalagmites, icicle-shaped masses of precipitated calcite that, respectively, hang from cave ceilings and protrude upward from cave floors. A stalactite and a stalagmite that have grown into each other form a column. When dripwater flows down cave walls and over the floor, it creates sheet-like flowstone or rimstone dam deposits. If it seeps from a rock joint and then trickles down a cave wall, complex, sinuous deposits known as draperies are formed. Speleothems form at varying rates based on a number of factors. The temperature outside the cave has a major effect, as it regulates the rate of decay of surface plants and animals (higher temperature, higher decay rate), and hence, the amount of carbon dioxide in the surface soil. Higher outside temperatures also increase evaporation rates, which affect the amount of water percolating through fractures and cracks into the cave. Another important factor is the amount of rainfall, which governs water flow into the cave (more water, faster speleothem growth). The warm, red-to-orange-to-brown colors of speleothems are the result of plant acids, iron oxide compounds, or bacteria brought in from the surface. (By contrast, pure calcite is white or clear.) Other oxides of iron and manganese may impart a deep brown or black color. Speleothem shapes are determined by the acidity of the water that enters the cave by dripping, seeping, or splashing, and whether the water stands, flows, or drips after entering the cave. Speleothems often appear in bizarre forms. For instance, "cave popcorn" is clusters of calcite balls that can form along the walls, floor, or ceiling of flooded cave passages. "Cave pearls" are loose calcite balls that, like actual pearls, grow around a tiny grain of sand or other impurity, sometimes to the size of ping-pong balls. Cave pearls are prevented from cementing themselves to the floor by the agitation of dripping water. Helictites and heligmites are thin, twisting speleothems that may grow out from walls, up from the floor, or down from the ceiling. Cave formations have so captured the human imagination that in many U.S. commercial caves, uniquely shaped speleothems are often likened to animals or humans, with elaborate stories woven around them. Colored lighting is sometimes used to enhance the fantasies. The water in cave pools is often supersaturated with calcium carbonate. Under these conditions, calcite crystals may initially crystallize around bits of debris in the water and eventually build impressive features such as these "Christmas trees." The process is similar to that by which sugar crystals form "rock candy" along a string submerged in sugar water. (Madonna Cave, New Mexico). JIM GOODBAR, BLM "Soda straws" grow as calcium carbonate-rich water flows through their hollow cores. Calcite deposits may build up on the ends of soda straws, forming "lemon drops" such as these. (Wind Cave, New Mexico) JIM GOODBAR, BLM Stalactites take on a reddish-brown color from plant acids, iron oxide compounds, or bacteria brought in from the surface. These stalactites are surrounded by white gypsum. SID KEAR Deep Breathing Caves provide a relatively constant climate, with stable temperatures and high humidity. The temperature deep inside a particular cave is equal to the average annual temperature of the surrounding surface environment. Therefore, a cave in the Montana mountains might have a constant temperature of 3ºC, while a cave located in the low desert of New Mexico might have a constant temperature of 26ºC. Cave temperatures may vary by one or two degrees from summer to winter, with temperatures near cave entrances varying much more because of the effects of outside weather. The humidity within caves is generally 95-98 percent. Caves breathe-there is a constant exchange of air between the cave and the surface. This can occur in two ways. One is as a result of barometric pressure changes. If the barometric pressure outside a cave is higher than that inside, air moves into the cave. When a weather front moves over the area and the outside barometric pressure falls, the cave "breathes out." Even slight air pressure changes due to diurnal air temperature fluctuations are reflected in a cave's airflow. The second mechanism of air circulation in caves is called the "chimney effect." If a cave has more than one entrance, the lower entrance will pull in cold air during the winter. As the air enters the cave, it warms and rises until it exits from the upper entrance. This phenomenon reverses in the summer, when warm air enters the upper entrance, cools, sinks, and finally exits via the lower entrance. Cave circulation provides a natural form of air conditioning that blows warm air in the winter and cool air in the summer. The Twilight Zone and Beyond The cave environment varies as one moves farther away from the surface. This variation is used to classify parts of the cave into ecological zones. The "entrance zone" is similar to the aboveground environment, with attendant variations in temperature and humidity. This zone receives plentiful sunlight and green plants are present. The entrance zone is moderated somewhat by warm, moist air blowing out of the cave in winter, and cool, moist air blowing out in summer. The little brown bat (Myotis lucifugus) is one of the more common bats in the United States. It is typically found in forested areas and often roosts in caves. An individual can consume up to 1,200 insects in an hour of feeding. JOHN AND KAREN HOLLINGSWORTH, U.S. FISH & WILDLIFE SERVICE The average length of this cave salamander (Eurycea lucifuga) is around 15 cm. The young of the species is often yellow, whereas adults are generally bright orange with black spots covering most of the body. They are generally found around limestone outcroppings, feeding on a diet of small arthropods and insects. BLM An inhabitant of isolated caves in Texas, the Tooth Cave spider (Neoleptoneta myopica) is a typical troglobite. It is on the Federal Endangered Species List. WYMAN MEINZER, U.S. FISH & WILDLIFE SERVICE Therefore, this zone often has a unique microclimate that supports a greater diversity and density of plant and animal species than does the surrounding surface area. In the "twilight zone," a little farther into the cave, sunlight is greatly reduced and plants do not grow. Temperatures still vary quite a bit but are more stable than at the entrance. This dimly lit zone is home to a mix of surface-adapted species and cave-adapted species. Still farther into the cave is the "middle zone," where one first encounters total darkness. This zone still experiences some temperature and humidity fluctuations in response to surface weather changes. In deeper, more extensive caves, the final area is the "dark zone." This area, with almost constant temperature and humidity, is home to an array of unusual, uniquely adapted organisms. Life in the Dark The diversity of animal species living in caves is very small compared to animal life on the surface, and invertebrate diversity is much greater than vertebrate diversity. For example, the huge Mammoth Cave system houses a vertebrate biodiversity of only 43 mammal species, 15 reptiles, 19 amphibians, and 3 fish. The number of cave animal species that are endangered is therefore relatively high. The Nature Conservancy estimates that 95 percent of the animal species found exclusively in caves are vulnerable or imperiled. Even in the darkest areas of a cave, all life ultimately depends on sunlight. Since green plants cannot live deep in caves, living plants are not at the bottom of the cave food chain. Cave animals must rely on occasional floods to wash leaves, twigs, and other plant debris from the surface into the cave. Another food source is droppings from animals, such as bats and crickets, that venture outside the cave to feed and then return to sleep or raise their young. In some caves, these droppings may provide the only food source. However, few animals can feed directly on the droppings; instead, bacteria and fungi decompose these materials into simple nutrients. Tiny cave-dwelling insects, such as springtails, feed on the plant debris, fungi, and bacteria and their products, and then become a food supply for caves' larger predators, such as mites, pseudoscorpions, and crayfish. The larger animals' droppings then replenish the food supply for fungi and bacteria. Cave animals are divided into four categories, based on the amount of time they actually spend in caves. "Accidental species" are those that wander, fall, or are washed into caves, providing an important source of nutrients if they die there. Trogloxenes (Greek for "cave guests") are temporary cave residents that move freely into and out of caves. They seek out the habitat at times-say, for hibernation or rearing young-but do not complete their entire life cycle in caves; in fact, many show no special adaptations to the cave environment. Bats, bears, skunks, raccoons, pack rats, moths, and humans are all trogloxenes. Mexican free-tailed bats (Tadarida brasiliensis mexicana), for example, roost and reproduce in Carlsbad Caverns during a seven-month period each year, and then migrate south in early autumn, returning in the spring. (For more about bats and their critical ecosystem functions, see the poster back. Trogloxenes other than humans play an important role in providing food for those cave animals that never visit the surface. Troglophiles (Greek for "cave lovers") can live in the dark zones of a cave but can also venture outside in search of food. This group includes some kinds of earthworms, beetles, frogs, salamanders, and crustaceans. The cave cricket (of the order Orthoptera)- actually, a kind of grasshopper- is one of the most commonly found organisms in this group. Troglobites (Greek for "cave life") are the true cave dwellers. They spend their entire lives in caves, living permanently in the dark zone. They cannot survive outside caves and have developed special adaptations to help them live in this stark environment. Since cave food sources are meager, the sense organs and physical resources of troglobites are primarily devoted to survival and finding food. In a lightless environment, unnecessary senses, such as sight, have degenerated or been eliminated. Most troglobites are transparent or white to pinkish in color, because they have no need for protection from sunlight or for visual camouflage from predators. Many have no eyes or poorly developed eyes because these organs are unnecessary in the dark, are prone to injury, and take precious energy to maintain. An eyeless cave fish such as the northern cave fish (Amblyopsis spelaea), for instance, can survive longer in a cave pool or stream with less food than can a fish that has eyes, an example of regressive evolution. What many troglobites have lost physiologically, they have made up for with the development of such adaptations as longer legs or antennae, finely tuned olfactory senses, extreme sensitivity to vibrations, or metabolisms that enable them to go for long periods of time with little food. Animals that have completely adapted to cave life include cave fish, cave crayfish, cave shrimp, isopods, amphipods, millipedes, some cave salamanders, and insects such as roaches. In tropical caves, troglobitic roaches such as the giant cave roach (Blaberus giganteus) may blanket the cave floor in huge masses beneath bat nurseries, feeding on guano and quickly devouring any bat pups unlucky enough to slip to the ground. Cave microbes are currently being intensely researched in caves. In Lechuguilla Cave alone, more than 1,000 strains of microbes have been isolated from pools, soils, speleothems, corrosion residues, and sulfur deposits. Microbes from Lechuguilla Cave, Mammoth Caves, and other U.S. caves are being studied for possible use in fighting certain types of cancer. The precarious lifestyles of most cave animals highlight both the fragility of the cave environment itself and its inter-connectedness with the surface. Ultimately, the energy and resources that feed cave animals and cave environments- including water supplies-come from the Earth's surface. Surface land use practices therefore directly impact water quality and life forms within caves. The systems are so delicate, in fact, that even recreational cave visitors can impact the underground world. These are some of the factors that make cave management a daunting task. Exploring New Depths Understanding of cave and karst systems can be enhanced by articles such as this and by ongoing efforts on the part of the public to "dig deeper." Caves are clearly more than just awesome repositories of underground formations and curious creatures. They are part of a complex geological and biological system that has extended from the surface of the Earth to the underground and back for millennia, evolving continuously. Caves represent one of the last frontiers of the American landscape to be explored. The fact that an underground wonder as extensive as Carlsbad Caverns was discovered little more than a century ago should serve as inspiration to those who might be tempted to think there's nothing left to discover. And the realization that it was a 16-year-old who made the discovery should serve as a particular incentive to young people to keep exploring the world around them.

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