One of the most intriguing interpretive tours I’ve joined across the national park system was the “wild cave tour” offered at Mammoth Cave National Park.

For six or more hours in sections of the cave off-limits to the more traditional tours we scooted through tight places on our bellies when not able to get by on hands and knees. Headlamps lit our way, hardhats protected our skulls from low-hanging rock walls, and coveralls kept us relatively clean as we shimmied through cracks and crevices. The ranger led us through chambers, to waterfalls, and up and down the subterranean landscape. It was a tour I’d quickly repeat.

Then, too, I’ve gone on the Crystal Cave Tour at Sequoia National Park. Much, much shorter than the Mammoth Cave wild cave tour, this experience was enjoyable, but it really did pale in comparison to Mammoth Cave’s offering. But that’s not the point. Rather, it’s that there are some pretty interesting caves scattered across the national park system that are worth exploring.

Before we get to that, though, let Dr. Robert Janiskee, a distinguished professor emeritus in the Geography Department at the University of South Carolina’s Columbia campus, provide some geologic tidbits on these systems.

“A cave can be loosely defined as any cavity in the ground that is big enough to have at least a part of it unreachable by direct sunlight,” he explains in his course on national parks. “The National Park Service’s Cave and Karst Program reports that there are more than 3,600 caves in our national parks. By 2007, significant caves and related solution features had been identified in at least 77 parks, or about one out of five units in the system. “

How did these places form? Often, simply by dissolution.

“Caves can be produced by various other processes, but most are created by the dissolving of calcareous rocks – principally limestone, marble, or dolomite,” says Dr. Janiskee. “When rain water that has been slightly acidified by carbon dioxide in the air and soil percolates downward through cracks in limestone, the mild carbonic acid slowly dissolves the rock. Called carbonation, this limestone dissolution process eventually creates voids and passageways big enough to be called caves. Dissolution can also occur in 'bottom up' fashion (as at Carlsbad Caverns) when sulfur-containing brine and gas from deep below the surface move upward, encounter groundwater, and produce dilute sulfuric acid that dissolves carbonate rocks. Caverns, sinkholes, and related dissolution features are especially common in karst regions.

“Many areas underlain by limestone have caves and other distinctive dissolution features such as sinkholes, deep gullies, springs, disappearing streams, and underground rivers,” he adds. “A landscape with numerous features formed by the dissolution of limestone, marble, or other calcareous rocks is called a karst landscape or karst region. In pseudokarst regions, water erosion of gypsum formations can create gypsum caves.”

Now, what about the formations you find inside caves, the stalagmites and stalactites, the travertine and cave bacon? Properly termed speleothems, these features are created by the deposition of minerals dissolved in water that drips, seeps, flows, and forms pools within the caves.

“Most varieties of speleothems can be classified as either dripstone or flowstone. Dripstone is the general term for features formed by water dripping from the ceiling of a cave,” explains Dr. Janiskee. “Flowstone is the term applied to the various depositional features formed by water flowing or dripping on cave walls and floors. The stereotypical image of speleothems emphasizes three dripstone features -- stalactites, stalagmites, and columns. A stalactite looks like a stone icicle dripping down from the ceiling (note that stalactite has a “c”, as in ceiling). A stalagmite is a conical or carrot-shaped deposit protruding upward from the cave floor (note that stalagmite has a “g”, as in ground). When a stalactite merges with the stalagmite below it, a column is formed.”

Of course, not all caves are the result of dissolution. According to the professor, passageways called mud caves are found in the eroded shale formations of some badlands areas while massive shifts along fault lines can create tectonic caves. In mountainous areas, he adds, boulders may slump or cascade and create passageways called talus caves. Wind erosion can form eolian caves, wave action can form shoreline sea caves, and running water in glaciers and high-elevation snowfields can create glacier caves. Bedrock caves that contain ice year round are called ice caves.

Anyone who lives in a landscape crafted by volcanism knows about lava tubes, the underground passages created by streams of lava that, over a matter of hours or days, develop a hardened crust encircling the molten flow.

“Tube-fed lava can flow great distances because the walls and roofs of the tubes are good thermal insulators and the lava flowing through the tubes can remain fluid much longer than surface flows. When the lava flow ebbs and eventually ceases, the tube drains and leaves an empty passageway,” says Dr. Janiskee. “If the roof doesn’t collapse, the result is an elongated lava tube cave. Lava tube caves exist in Hawaii Volcanoes National Park, Idaho’s Craters of the Moon National Monument & Preserve, and several other park units in Oregon and California.

“Perhaps the best known of all the lava tube caves is the heavily-visited Thurston Lava Tube in Hawaii Volcanoes National Park. It is not the longest lava tube cave in the U.S., since that distinction belongs to Ape Cave, a remarkable 12,810-foot long lava tube cave in Mount St. Helens National Volcanic Monument (operated by the U.S. Forest Service).”

Most caves across the park system are managed as undeveloped wild caves because that’s the only feasible option, according to the professor. Some examples can be found in Sequoia.

“Heavy, careless, or inappropriate use can easily damage physical and cultural resources in caves. Indeed, some caves contain speleothems, microorganisms, rare wildlife habitat, human artifacts, and other resources that are extremely fragile and can be irreparably damaged by the slightest touch or even by the body heat, carbon dioxide, skin flakes, and food particles that visitors leave behind,” says Dr. Janiskee. “The Park Service’s mission to preserve and protect park resources, coupled with safety concerns and various federal mandates (especially the 1988 Federal Cave Resources Protection Act), compel park officials to severely restrict public access to caves and cave segments that cannot tolerate the stresses of public visitation. Visitors are rarely permitted to enter park caves unescorted, but there are lots of managerial arrangements for limited public access."

So, with that geologic foundation in hand, let’s take a brief look at the six national parks that we can think of as “cave parks.” These are the park units with “cave” or “cavern” as part of their name, and in which the primary resources under management are caves and cave-related features.

Carlsbad Caverns National Park in southeastern New Mexico contains more than 100 of the caves that were created in the 250 million-year old Capitol Reef formation. Among them are America’s deepest and arguably most beautiful cave, Lechuguilla Cave (closed to the public) and Carlsbad Cavern, a very popular tourist cave with one of the largest underground chambers in the world (the Big Room). Carlsbad’s public cave facilities, which are renowned for their scale and sophistication, include such remarkable features as elevators in a 700-foot deep shaft, an underground lunchroom, and an amphitheater for bat flight-watching. The Big Room is even partially wheelchair-accessible. The park, which now attracts about half a million visitors a year, was established as a monument in 1923 and upgraded to National Park status in 1930.

Jewel Cave National Monument in the Black Hills of South Dakota is one of the lesser-publicized parks, but it is a giant in the world of caves. In fact, Jewel Cave is the second longest cave in the world. By 2007 the cave’s tally of known passageways had reached nearly 141 miles and was expected to go even higher. The “jewel” in the cave’s name alludes to the sparkling calcite crystals that impressed the cave’s discoverers in 1900. The national monument was established in 1908 and turned over to the Park Service for management in 1933. Park management began offering guided tours in 1939. Today most visitors take the Scenic Tour that visits a formerly remote area of the cave that has been made accessible by a 300-foot elevator and has a half-mile loop trail with concrete walks and metal stairs and platforms.

Mammoth Cave National Park in Kentucky is one of America’s oldest and busiest tourist attractions. Although the national park wasn’t established until 1941, formal cave tours have been offered at the site since 1816. About two million people visit the park each year now, and 500,000 of them take a cave tour. Mammoth Cave is by far the world’s longest cave system (more than 365 miles explored so far) and has many scenic delights and interesting cultural-historical artifacts. A variety of general and special interest tours is available. Reservations are strongly recommended, especially from April through October. Sadly, the park is plagued with a variety problems originating outside its borders. Park air is routinely fouled by city smog and coal-burning power plants, and the park’s subterranean waters have been contaminated by leaking septic tank systems.

Oregon Caves National Monument, which is situated in the Sisiyou Mountains of southwestern Oregon, is one of our older and smaller parks (488 acres). The Oregon Caves were formed in marble rocks that were once a tropical reef. The original calcareous rocks of the reef were metamorphosed to marble when the reef got pushed about 12 miles under the earth surface before being uplifted to around 4,000 feet above sea level. The 3.5-mile, moderately strenuous cave tours take about 90 minutes. No reservations can be made, and waits of up to two hours are common in the summer. Scientists are keenly interested in the caves’ Pleistocene mammal fossils and numerous cave dwelling insects. The Oregon Caves were discovered in 1884, became a national monument in 1909, and were transferred to the Park Service in 1933.

Timpanogos Cave National Monument was established in 1922 in the Wasatch Mountains of Utah. This park shows that a federal landholding need not be very big to be very special. Just 250 acres in size, Timpanogos protects three spectacularly scenic, tunnel-connected caverns (Hansen, Middle, and Timpanogos) with some interesting fault-controlled passages, very colorful formations, and fragile helictites (plus some anthodites) in numbers seldom seen in public caves. The hour-long cave tours are very popular, but only available seasonally. The park is closed in the winter due to heavy alpine snows. Temperatures in the caves remain a chilly 45 degrees throughout the year.

Wind Cave National Park is located in the Badlands region of southwestern South Dakota not far from Mount Rushmore National Memorial. When President Theodore Roosevelt designated Wind Cave in 1903, he not only made it the seventh national park in America, but also the first to be officially called a National Park and the first created to preserve a cave. Wind Cave’s 116 miles of passageways make it the fifth-longest cave in the world. The “wind” in the cave’s name alludes to an interesting weather phenomenon that causes the wind direction at the cave’s mouth to shift 180 degrees. Changes in barometric pressure determine whether air is blown out of the cave’s mouth or sucked into it.

OTHER NATIONAL PARKS WITH CAVES

More than 70 national parks that were not established for the primary purpose of protecting cave resources nevertheless have caves within their borders. These caves exhibit great variety, are found in many areas of the country (most notably the desert Southwest), and in some cases contain important historical-cultural resources such as Native American artifacts or relics of old guano mining operations. Which park has the most caves? Actually, it’s Grand Canyon National Park, which contains over 400 known caves. Given the great size of the park and its abundance of bedded limestone, this should not be surprising. More surprising are the many cave-related anomalies such as the presence of caves in some cultural-historical units (such as the Chesapeake and Ohio Canal National Historic Park) or the existence of water-filled caves in Death Valley National Park.