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.

• Understand more about the relationship between acids and bases in Riverdeep's Chemistry Gateways activity, Properties of Acids and Bases.
• Review the products of the photosynthetic process in Riverdeep's Biology Gateways activity, An Overview of Photosynthesis. (These activities require Logal Express. Get a free trial subscription.)

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.

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.

• 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.