Devil's Sinkhole State Natural Area, National Natural Landmark and state park in Edwards County, Texas.

Devil's Sinkhole is a 350-foot deep cavern in Edwards County, Texas, with a roughly 50-by-140-foot opening at the surface. It is Texas' largest single-chambered cave, formed in 150-million-year-old Edwards Limestone. The shaft walls are uneven and stepped in ledges, pale tan and gray where lit above and moss-coated below, with thin streams of water falling to a floor of broken dark rock.

Around one million years ago, slightly acidic water moving slowly through 150-million-year-old Edwards Limestone dissolved a cavern; when the water receded, the ceiling collapsed and formed the sinkhole. In 1876 rancher Ammon Billings led a scouting party west of Hackberry Creek and returned on May 21 with his wife Lucinde Katherine Billings and other wives, who named the opening. H. S. Barber carved his name inside in 1889, and in the 1920s workers mined bat guano there for fertilizer and potassium nitrate.

The name comes from Lucinde Katherine Billings and other wives in the party that returned to the sinkhole on May 21, 1876, who dubbed it "The Devil's Sinkhole." H. S. Barber carved his name inside the cave in 1889. In the 1920s workers mined bat guano there, sold as fertilizer or as a source of potassium nitrate.

Access is only through advance reservations; there is no walk-in entry. Evening bat flight tours run in summer only, and guided nature hikes are also offered. The site has a wheelchair-accessible viewing platform and picnic areas. Tours are led by the Devil's Sinkhole Society, a volunteer group working with Texas Parks and Wildlife and Bat Conservation International.

The sinkhole is Texas's largest single-chambered cave, and its bat colony is among the largest in the state. H. S. Barber carved his name inside the cave in 1889, a trace of early visitors that predates any formal protection.