The Devil's Sinkhole, northeast of Rocksprings in Edwards County, is thought to be the largest single-chamber cavern in Texas. It was named in 1876 by the wives of Ammon Billings and other men who had discovered the entrance after an encounter with Indians. The earliest known explorer of the cave was H. S. Barber, whose name is carved in the cave with the date October 19, 1889. The pit entrance is approximately fifty feet wide and expands downward into an oval room, roughly 320 feet in diameter, that is partly filled with fallen rock. The cave is 350 feet deep. Cave explorers from all over the United States have been drawn to it because of its impressive size and rumors of lost bat rooms. Two water-filled lake rooms have also been explored by divers to depths of up to forty feet. The cave is home to one of the largest freetail bat colonies in the state, and guano has been removed sporadically from the cave for use as fertilizer. The cave was added to the National Registry of Natural Landmarks in the early 1970s. It and the surrounding 1,860 acres were purchased by the state of Texas in 1985 and opened to limited public access as a state natural area in 1992. Guided viewing trips are organized by a local volunteer group, but the interior of the cave is accessible only to researchers and experienced vertical cavers. See also CAVES.