At least 3,000 caves and sinkholes are known in Texas, distributed in karst areas covering about 20 percent of the state. Karst is terrain formed by the dissolution of bedrock, and generally is characterized by sinkholes and caves that channel water underground. Texas caves and karst aquifers are important economic, scientific, and recreational resources. Karst requires soluble rocks. The majority of Texas caves occur in the Cretaceous limestones of the Edwards Group, Glen Rose, and Austin Chalk, distributed in the Balcones Fault Zone, the Edwards Plateau, the Stockton Plateau, and the Cibolo Creek and Guadalupe River basins. In the Llano region the Ellenburger Group carbonates (Ordovician age) are intensely cavernous. Permian reef limestones in West Texas contain important caves. Two gypsum karst areas (Permian age) occur north of Van Horn, Culberson County, and in fourteen counties in Northwest Texas. Some unusual caves occur in granite (Enchanted Rock Cave, Llano County), volcanic tuff and conglomerate (Big Bend), sandstone, travertine, selenite, shale, caliche, and other materials. Many caves are being degraded, filled, or quarried before their contents can be adequately studied. Honey Creek Cave is the state's longest at thirty-two kilometers (twenty miles) and is still being explored. The cave, a tributary to the Guadalupe River, extends under Comal and Kendall counties. Powell's Cave System, a complex of three caves in Menard County, is at least twenty-one kilometers (13 miles) long and "growing" as cavers continue to map it. There are at least 100 caves longer than 1,000 feet, and there are at least thirteen caves deeper than ninety-one meters (300 feet). Sorcerer's Cave (Terrell County) is the deepest at 170 meters (558 feet). The largest cave in terms of volume may be Fern Cave (Val Verde County), estimated at about 300,000 cubic meters (ten million cubic feet).
The scientific resources of Texas caves are many. Hundreds of ancient species, specially adapted to an energy-efficient life in permanent darkness, are scattered through the karst of Central Texas. Cave-adapted salamanders, catfishes, shrimps, isopods, amphipods, snails, spiders, harvestmen, pseudoscorpions, beetles, millipedes, centipedes, and other life forms have been described. Most of these eyeless troglodytes occur in the Balcones Fault Zone, where geologic isolation in faulted, river-dissected karst blocks has resulted in an evolutionary history like that of an archipelago. Some of these species are endangered by land development, overuse of groundwater, pollution, and such pests as the imported fire ant. About two dozen Texas caverns harbor about 100 million Mexican free-tailed bats from April to November every year. These migratory bats consume 6,000 to 18,000 metric tons of insects annually in Texas. The largest known mammal colony in the world is the colony of twenty million or more Mexican freetails in Bracken Bat Cave, Comal County. Bats are recognized as important but are feared by many nevertheless. A 1917 state law protecting them was rescinded during a rabies scare in 1957. Several other insectivorous bat species inhabit hundreds of Texas caves, but have been killed or driven out of some caves by vandals. Bat Conservation International moved its headquarters to Austin in 1986 and has been educating the public on the ecological importance of bats.
About twenty-five Texas caves have yielded important fossils of vertebrate animals. Extinct species, such as the scimitar cat, dire wolf, Columbian mammoth, ground sloth, glyptodon, spectacled bear, and flat-headed peccary, denned in, fell in, or were eaten in Texas caves. Radiocarbon dates up to 23,000 years before the present have been recorded. Bats have utilized Texas caves for many millenia. The remains of small mammals found in cave soil and flowstone strata have chronicled the climatic shifts in Texas since the ice ages ended about 11,000 years ago. Central Texas was a cool, moist environment until about 3,000 years ago. Such burrowing mammals as moles and gophers were common. With the increasing aridity there was a massive loss of soil. A second episode of soil loss was caused by fire and overgrazing by domestic animals that continues to this day. Paleo Indians utilized Texas cliffs and rockshelters for "animal kills." As long ago as 12,400 years, Bonfire Shelter near the Rio Grande received animals driven off a cliff. People processed the carcasses in the shelter. Kills of mammoth, bison, and horse occurred several times. In the Archaic Period (9,000–1,000 years ago) many shelters in the lower Pecos River and Devils River area were inhabited by hunter-gatherers. Fine pictographs may still be seen in Fate Bell Shelter at Seminole Canyon State Historical Park near Comstock. Pit burials, where the dead were dropped into deep sinkholes, also have been documented. Important archeological materials no doubt remain to be found in caves and are protected by law.
Early scientific work in Texas caves began in 1896 with the description of the Texas blind salamander, Typhlomolge rathbuni, from an artesian well at San Marcos. Important bat guano caves were documented in 1901; the caves had been sources of nitrates for gunpowder but became fertilizer mines for citrus and vegetable farms. Serious speleology in Texas began with the 1948 publication of The Caves of Texas by the National Speleological Society. Caving groups (grottos) formed in the 1950s and systematic documentation of the state's caves began, first by the grottos and the Texas Cave Survey, then by the Texas Speleological Survey, founded in 1961. NSS conventions were held in Texas in 1964, 1978, and 1994. In 1994 the Texas Speleological Association included eleven grottos in major cities. Caves are conserved and managed by the Texas Cave Management Association, the Nature Conservancy, the Texas Parks and Wildlife Department, Texas Cave Conservancy, and Bat Conservation International, and many private landowners. The State Caverns Protection Act protects caves from vandalism and destruction. Another statute protects landowners from liability for injuries to cave visitors, unless they have paid for access to the cave.
The Edwards Aquifer, which extends from Del Rio to north of Austin along the margin of the Edwards Plateau, is a karst aquifer that supplies drinking water to 1.5 million people in the San Antonio area. As pumping began to exceed natural recharge and water levels declined, several rare species and the human economy were threatened. The Comal and San Marcos rivers, which originate from large karst springs, are important in maintaining the Guadalupe River ecosystem all the way to the San Antonio Bay estuary on the Texas coast.
Texas caves abound with natural delights. Eight show caves are open to the public: Cascade Caverns and Cave Without A Name (both at Boerne), Caverns of Sonora (Sonora), Inner Space Cavern (Georgetown), Longhorn Cavern (Burnet), Natural Bridge Caverns (New Braunfels), Wonder Cave (San Marcos), and West Cave (a botanical preserve and travertine cave near Austin). The Caverns of Sonora is considered by many to be the most beautiful cave in the world. The other caves offer an amazing variety of beautiful speleothems (rock formations), fossils, and history. Wild caving tours are now offered at Colorado Bend State Park and Kickapoo Caverns State Natural Area.