The Natural Bridge Caverns are the largest known commercial caverns in the U.S. state of Texas. The name is derived from the 60 ft natural limestone slab bridge that spans the amphitheater setting of the cavern's entrance. The span was left suspended when a sinkhole collapsed below it. The caverns are located near the city of San Antonio, Texas in the Texas Hill Country next to the Natural Bridge Wildlife Ranch, a drive-through wildlife safari park. The caverns feature several unique speleothems and other geological formations. The temperature inside the cave is 21 degrees Celsius (70°F) year-round and the humidity rate is a constant 99 percent. The deepest part of the public tour is 180 feet below the surface, although undeveloped areas of the cavern reach depths of 230 feet. The caverns are still slowly developing. Due to the porosity of the limestone, rainwater travels downwards through the layers of rock, where it dissolves out calcite, a weak mineral that makes up all of the speleothems at Natural Bridge Caverns. After exiting the limestone, water enters the caverns where it flows and drips constantly throughout, causing the formations to retain a waxy luster that can be seen in a few caverns. History The caverns were discovered on March 27, 1960, by students Orion Knox Jr., Preston Knodell Jr., Al Brandt and Joe Cantu from St. Mary's University in nearby San Antonio. On their fourth trip into the caverns, the men discovered/explored just over a mile of passage. Subsequent explorations revealed 2 miles associated with what became known as the "North Cavern." After discovery, Orion Knox Jr. assisted the landowners in obtaining information and suggestions for development. Mrs. Clara Wuest (the landowner), wanted to show the world the cave under her property. Orion approached both the National Park Service and the Texas Park System. While both entities agreed that the cavern was substantial and merited development, both groups told the landowners that funds did not exist for their respective groups to undergo such an endeavor. Mrs. Wuest then decided that she would fund development. Orion dropped out of school to assist. He also approached Jack Burch, who had just finished work on developing the Caverns of Sonora near Sonora, Texas. Jack agreed to help and development began early in 1963. Mrs. Wuest remarried Harry Heidemann, a retired Texas Highway Patrolman, in the early 1960s. Together, they started work on developing the