ABSTRACT Discoveries in the 1980s greatly expanded speleologists’ understanding of the role that hypogenic groundwater flow can play in developing caves at depth. Ascending groundwater charged with carbon dioxide and, especially, hydrogen sulfide can readily dissolve carbonate bedrock just below and above the water table. Sulfuric acid speleogenesis, in which anoxic, rising, sulfidic groundwater mixes with oxygenated cave atmosphere to form aggressive sulfuric acid (H 2 SO 4) formed spectacular caves in Carlsbad Caverns National Park, USA. Cueva de Villa Luz in Mexico provides an aggressively active example of sulfuric acid speleogenesis processes, and the Frasassi Caves in Italy preserve the results of sulfuric acid speleogenesis in its upper levels while sulfidic groundwater currently enlarges cave passages in the lower levels. Many caves in east-central Nevada and western Utah (USA) are products of hypogenic speleogenesis and formed before the current topography fully developed. Wet climate during the late Neogene and Pleistocene brought extensive meteoric infiltration into the caves, and calcite speleothems (e.g., stalactites, stalagmites, shields) coat the walls and floors of the caves, concealing evidence of the earlier hypogenic stage. However, by studying the speleogenetic features in well-established sulfuric acid speleogenesis caves, evidence of hypogenic, probably sulfidic, speleogenesis in many Great Basin caves can be teased out. Compelling evidence of hypogenic speleogenesis in these caves include folia, mammillaries, bubble trails, cupolas, and metatyuyamunite. Sulfuric acid speleogenesis signs include hollow coralloid stalagmites, trays, gypsum crust, pseudoscallops, rills, and acid pool notches. Lehman Caves in Great Basin National Park is particularly informative because a low-permeability capstone protected about half of the cave from significant meteoric infiltration, preserving early speleogenetic features.

Abstract Part of the Permian Capitan Reef Complex is exposed in the Guadalupe Mountains of southeastern New Mexico and western Texas (Fig. 1). The reef complex includes the Capitan Limestone and the carbonate backreef beds of the Artesia Group that comprise a lithosome called the Capitan aquifer (Hiss, 1976). This lithosome contains well-developed solution openings that range from microscopic to voids the size of Carlsbad Cavern. This solution porosity was once thought to be caused by weak carbonic acid in the phreatic zone within the Capitan aquifer (Bretz, 1949). During the last 15 years, however, workers have obtained evidence indicating that sulfuric acid may be a major cause of carbonate dissolution (Egemeier, 1973; Jagnow, 1977; Palmer et al., 1977; Maslyn, 1979; Davis, 1980; Kirkland, 1982; and Hill, 1987). Sulfuric acid is generated when oxygen (O 2 ) is introduced into solutions containing dissolved hydrogen sulfide (H 2 S) gas (Hill, 1987). Hydrogen sulfide is common in subsurface formations in southeastern New Mexico (Bjorklund and Motts, 1959; Hinds and Cunningham, 1970, pp. 4 and 7). In southeastern New Mexico and elsewhere along the subsurface trend of the Capitan reef, H 2 S is present in accumulations of oil and gas and in associated saline water (Schram, 1956a, p. 103, and 1956b, p. 307; Wilson, 1956, p. 179; and Roswell Geological Society Symposium Committee, 1956a, p. 181, and 1956b, p. 291).