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.

Sulfur dioxide (SO 2 ) enters the atmosphere through natural and anthropogenic processes. Mitigation of SO 2 emissions from many industrial activities has produced by-product sulfur and by-product synthetic gypsum essentially mineralogically identical to the primary materials extracted using mines and wells. Regulation to reduce anthropogenic SO 2 emissions was one of the first environmental protection efforts in the United States, which later became mandated under the Clean Air Act Amendments of 1990. The availability of by-product sulfur has increased over the years, and following the closure of the last domestic sulfur mine in 2000, it became the only domestic source of elemental sulfur in the United States. The most widely adopted means of reducing SO 2 emissions from coal-burning facilities has been to install flue gas desulfurization (FGD) equipment, which produces synthetic FGD gypsum. The decrease in SO 2 emissions since 1980 has significantly improved air quality in parts of the United States. By-products from these activities have replaced the supply of products, such as elemental sulfur, sulfuric acid (H 2 SO 4 ), and gypsum, through substitution of by-product for primary mining of these mineral commodities. The cascading effect of efforts in the United States to mitigate SO 2 emissions from multiple sources through the enactment of the Clean Air Act, and its amendments, has resulted in more than improved air quality alone. It has also, through the increasing availability of environmental products of SO 2 mitigation, such as by-product H 2 SO 4 , elemental sulfur, and by-product synthetic gypsum, reduced the environmental impacts of mining these materials from mineral deposits.