We visited this very interesting site today,
“History: Hueco Tanks State Historic Site, is located 32 miles northeast of El Paso in El Paso County. It was obtained from the county by special deed on June 12, 1969, and by purchase of 121 additional acres on August 10, 1970. This site was opened to the public in May of 1970. This 860.3-acre park is named for the large natural rock basins or "huecos" that have furnished a supply of trapped rain water to dwellers and travelers in this arid region of west Texas for millennia.
A unique legacy of lively and fantastic rock paintings greets the visitor at the "tanks." From Archaic hunters and foragers of thousands of years ago to relatively recent Mescalero Apaches, Native Americans have drawn strange mythological designs and human and animal figures on the rocks of the area. The site's notable pictographs also include more than 200 face designs or "masks" left by the prehistoric Jornada Mogollon culture. Hueco Tanks was the site of the last Indian battle in the county. Apaches, Kiowas, and earlier Indian groups camped here and left behind pictographs telling of their adventures. These tanks served as watering places for the Butterfield Overland Mail Route”