NATURAL BRIDGE CAVERNS. Natural Bridge Caverns, the largest known cavern in Texas, was discovered on March 27, 1960, by four spelunkers who were students at St. Mary's University in San Antonio. The cavern is located off Farm Road 1863 in the hill country of Comal County midway between New Braunfels and San Antonio (at 29°41' N, 98°21' W). The name was derived from the sixty-foot natural limestone slab bridge that spanned the amphitheater setting of the cavern's entrance. Commercial development of the cavern began on March 25, 1962, and it was opened to the public on July 3, 1964. During excavation of the entrance trail, arrowheads and spearheads dating from 5,000 B.C. were found; just inside the entrance were discovered jawbones of a species of grizzly bear that became extinct over 8,000 years ago. The cavern is almost 100 percent active and still growing, and the temperature is seventy degrees year-round. Because of constant drips and flowing water, the formations retain a luster that can be seen in few caverns. Under these growth conditions the formations appear to be made of wax, yet are as hard as limestone. The types of formation change from room to room. The largest room, named the Hall of the Mountain Kings, is 350 feet long, 100 feet wide, and 100 feet high. Among numerous formations found here the most distinct is a type called "fried eggs" (because that is what they look like), a rare cave formation. The developed portion of the cave, furnished with a half mile of paved trails and illuminated by 35,000 watts of indirect lighting, extends to as much as 260 feet below ground level. Some portions of the cave are still under exploration. Natural Bridge Caverns became a registered United States natural landmark in 1971.