Devil's Sinkhole State Natural Area in the Texas Hill Country near Rocksprings protects a water-carved vertical cavern that has become one of the region's major bat habitats. The reserve covers 1,859.7 acres and is famed for the evening emergence of several million Mexican free-tailed bats from April through October. The sinkhole, with an opening about 50 by 140 feet and a cavern 350 feet deep, provides a sheltered roost. Its recorded discovery dates to 1876, when a scouting party returned with their spouses and Lucinde Katherine Billings named it Devil's Sinkhole. Recognized as a National Natural Landmark in 1968, it became state property in 1985 and opened to the public in 1992.