Underground Wonders

December 10, 2001

Rock and Water
More than 335 miles of accessible passages make Mammoth Cave in Kentucky the world's longest cave. Lechuguilla Cave, part of Carlsbad Caverns in New Mexico, is the nation's deepest at 1,567 feet. Together, these two caves attract more than 2,000,000 visitors annually. What natural processes make caves so fascinating?

The most common caves are solution caves. They form when soluble carbonate rock, such as limestone, dolomite, marble, or gypsum, is dissolved by groundwater. Limestone makes up the majority of caves in the United States.

In addition to soluble carbonate rock, two additional factors must be present for caves to form. First, rainwater seeps through cracks in the bedrock, absorbing carbon dioxide (CO2) from the atmosphere, decaying leaves, and soil on the way. The CO2-laden water changes into a weak form of carbonic acid (the same acid in carbonated soft drinks). The acidified water trickles down into the bedrock moving horizontally between layers of rock and vertically into joints in the rock. All the while, it is very slowly dissolving the calcite, the main mineral in limestone.

It also takes time for caves to form. Over many thousands of years, water-filled passages (or conduits) begin to appear. Water that collects above ground gets funneled down into the passages. This early period in a cave's geologic lifetime may take from 3,000 to 10,000 years. Over the next 10,000 to 100,000 years, the passages enlarge and the accumulated water is drained off by the natural underground springs or flowing streams. The "holes" that remain after the water drains are caves.

As the bedrock dissolves and "fills" with holes (think of how a piece of Swiss cheese looks), it forms a type of terrain called karst. Karst usually consists of rolling hills and sinkholes above the ground and a large network of underground cave systems, springs, and streams. (A sinkhole is a depression in the land where water can quickly enter the limestone bedrock lying beneath the surface.)

In the United States, significant regions of karst exist throughout the Appalachian and Ozark mountain ranges, Kentucky, Missouri, Florida, and parts of Texas and New Mexico.

  • What do you know about the terrain in your area? Do you live in a region with a lot of caves? If you don't know, look at this map from the U.S. Geological Survey.

Underground Glossary
column: formation that occurs when a stalactite and stalagmite meet

dripstone: general name for cave formations caused by dripping water (e.g., soda straws, stalagmites)

flowstone: general name for sheetlike cave formations caused by flowing water (e.g., rimstone dams)

rimstone dam: raised deposits of calcite that form around pools of water on the cave floor

soda straw: long hollow tube hanging from cave ceiling; often the start of stalactites

speleology (spee lee AH lo gee): scientific study of caves; from the Greek word for cave, spelaion

spelunking: practice or hobby of cave exploration

Meet the Troglobites
Caves teem with unique forms of animal life. Animals that live exclusively in caves and cannot survive outside them are known as troglobites. Almost 1,000 known species of crustaceans, insects, and spiders live exclusively in caves. Even though troglobites live underground, they still depend on life above ground for their food supply. Water filled with CO2 released through the process of photosynthesis, percolates down into the cave.

Bats, raccoons, and other animals that regularly visit but do not reside underground permanently, feed on plants or other vegetation growing near the cave. Waste products from these animals become food for some troglobites. Decaying plants, leaves or sticks washed into the cave by occasional flooding also break down into simple nutrients for some cave-dwelling species.

Troglobites have made some physical changes in order to survive without light. Some possess highly developed sensory organs, such as antennae, to detect predators or prey. Some are eyeless or blind, since a life underground does not require the energy to maintain sight. Many lack coloration; they are pink or gray or almost white. They also tend to be small physically, due to limited food supplies in caves. Generally, troglobites also live longer than their counterparts above the surface.

However, life above ground can also have a destructive effect on the life beneath it. In a 1998 proposal, the U.S. Fish & Wildlife Service stated that "...construction; filling of caves... contamination from... sewer leaks, run-off, and pesticides; predation by and competition with non-native fire ants; and vandalism" threaten the cave habitat. While the proposal concerned specific caves in Texas, caves across the United States face similar threats.

Deforestation decreases bat populations, leading to a reduction in bat waste, which some troglobites require for food. Some karst areas do not retain water, making them especially susceptible to groundwater pollution. In regions around the country, sinkholes used as garbage dumps and cave passages used for sewer lines contaminate and pollute the groundwater supply.

Agricultural pesticides and activities related to construction and urban development, such as grading and filling, also pose a serious threat to underground habitats. For these reasons, numerous species of troglobites have been placed on the endangered species list.

Interior Decorating
Water that has seeped through the soluble carbonate rock becomes filled with calcite. As this calcite-filled water drips or flows, in some cases, into the cave, it deposits minute amounts of the calcite on the cave interior. Over time, speleothems, or cave formations, take shape from the calcite deposits.

Two of the most common speleothems are stalactites, icicle-shaped formations clinging to the ceiling of the cave, and stalagmites, pillar-shaped formations rooted to the floor of the cave.

How to remember which is which? Stalactites hold "tight" to the cave ceiling, and stalagmites "might" one day reach the top.

More Links
Check out the Mammoth Cave National Park site. There's an online cave guide, maps, images, and interesting facts about the largest cave system in the world.

Refer to the National Speleological Society site for additional links to science topics related to caves, including cave biology and ecology, cave archaeology and geology.

Nothing but Footprints
More than 100 caves in the United States operate commercial tours, but human visitors also pose a threat to cave life.

  • Bacteria from skin and lint from clothes remain in the cave long after the human visitors leave.
  • Candy wrappers, food crumbs, or used batteries can be toxic to troglobites.
  • Cave formations are damaged instantly if touched, drawn on, or scratched.
  • The cave's delicate ecosystem can be harmed by overturning a stone or brushing up against a cave wall.
On most commercial cave tours, access is confined to certain areas, and there are rules posted about good cave behavior. But not all human visitors heed the warnings. If you tour a cave, remember to follow the National Speleological Society's Golden Rule of Caving: "Kill nothing but time. Take nothing but pictures. Leave nothing but footprints."

Related Activities
Food Chains and Webs
Understand more about relationships in an ecosystem in this Biology Gateways activity.
Cave Rescue and Math
Solve a Destination Math fractions problem on a rescue effort from a cave.
Reefs in Danger
Read about another biodiverse habitat, coral reefs, in this Riverdeep archive article.