From the early 19th century through the Civil War, caves in Kentucky, Tennessee, Virginia, West Virginia, Alabama, Georgia, Arkansas, and Missouri were important sources of nitrates, an essential ingredient of gun powder. Surface or near-surface accumulations of nitrate salts form coatings on rock walls, fill cracks and crevices, and mingle with cave earth. The origin of the nitrate salts is not clearly understood, but the salts are believed to result from the action of nitrifying bacteria on organic matter or humus. Although no accurate records of production were kept, it has been estimated that over 15,000 tons of niter earth (producing 200 tons of potassium nitrate) were removed from Mammoth Cave in Kentucky between 1811 and 1814.
Caves have also been a source of bat guano, a material mined as a phosphate fertilizer in the Southern United States and Mexico. In general, the largest deposits have occurred in limestone caves within the flight range of the Mexican free-tailed bat.
Scientists value caves as natural underground laboratories. Of paramount importance is the fact that caves and other solution cavities in limestone have a direct bearing on the underground water system. Cavernous limestone strata are among the most productive aquifers (water-bearing beds) in the United States and are therefore important sources of water. Because of this, U.S. Geological Survey research programs concerned with limestone regions commonly include studies of the path, rate of flow, amount, and quality of water circulating through caves and hidden passageways.
Geological engineers and others concerned with ground stability are aware that regions underlain by cavernous limestone present special construction problems. Studies of the subsurface conditions are especially important in areas of limestone and gypsum because of the danger of ground failure and subsidence.
There are about 17,000 known caves in the United States. They occur in every State except Rhode Island and Louisiana. About 125 caves have been opened to the public for study and enjoyment. Of these, 15 are in national parks or monuments, and 30 are in State parks. The remainder are privately owned and operated. Most of these caves are in the Appalachian Mountains, the Ozark Mountains, the Black Hills, and the limestone regions of Kentucky, Tennessee, and Indiana.
Caves are natural features and should be protected, but many have been vandalized by careless visitors or damaged by poorly planned commercial development. Some caves have been stripped of speleothems which took thousands of years to form and in many places will not form again. All should try to prevent this random destruction of these natural wonderlands. Follow the footprints of others; look but don't touch; bring away only photographs; leave no evidence of your visit.