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 Based on existing classifications of caves that often involve descriptive terms, a classification is presented that is based purely on genetic processes. An attribute key is developed that allows the classification of caves by means of cave maps, photographs and reports. This method is applied to a dataset of 6007 caves in a study area in eastern Austria. The area comprises diverse geological units of the Eastern Alps and the southern Bohemian Massif. A total of 94% of the caves could be classified with the surprising result that mechanical weathering and erosion caves are almost as common as solution caves even though the vast majority of caves are developed in carbonate rocks. Field checks confirmed the result and showed that the error is acceptable. The classified caves can also be used as indicator of natural phenomena like gravitational mass movements or vulnerable karst areas by decision-makers non-specialized in cave genesis.

Abstract Small-size karst landforms may potentially provide very useful information to fully understand the behaviour of karst systems and their dynamics. In this chapter we demonstrate the need to pay attention to such features. ‘Inghiottitoio della Masseria Rotolo’, located in a remarkable karst area of southern Italy, has in recent years become the most controversial and discussed speleological site in Apulia. Even though it has been known for several decades, recently excavation work has allowed cavers to enter a huge karst system, eventually reaching the water table. The total depth of the cave is now 324 m, making it the deepest in the region. This chapter summarizes the history of discoveries at the site, starting from the description of the polje, also including information about the link between toponymy and karst. The works carried out at the swallet site are then described to emphasize the importance of the often neglected small-size karst features. In fact, when carefully observed and studied, these might be able to shed new light and greatly increase our knowledge about karst. The final part of the chapter deals with the cave system and provides an outline of the ongoing research.

Abstract The Puerto Princesa Underground River, amongst the largest caves of the Philippine Islands, is the most visited show cave in the country, even though it has undergone no tourism adaptation at all. Its scientific importance primarily relies on the fact that it is one of the largest known underground estuaries in the world, and the effect of tides is visible along more than 7 km of the cave length. The complex relationships between sea and freshwater influence not only the hydrodynamics of the system and the speleogenetic processes presently active, but also its climate and its ecosystem. The systematic exploration and research of this coastal karst system started some 40 years ago and have shown that the Puerto Princesa Underground River is one of the most important caves in the world with regard to many different scientific fields. Speleogenesis concerns the initial phreatic solution followed by vadose erosion with periodical marine invasion, and subsequent saline/freshwater-mixing processes during sea-level highstands. The hydrodynamic behaviour of the water flowing inside the cave is rather complex, being simultaneously controlled by allogenic recharge and tides. Speleothems abundantly occur with several forms, some of which have never been described before. Several minerals, some of which are very rare, are present, together with palaeontological remains exposed by differential corrosion on rock walls. Last but not least, two large populations of bats and swiftlets sustain a complex subterranean ecosystem.

Abstract Three major problems in cave micrometeorology are analysed: the concept of the temperature of a cave and its phenomenology; the internal energy flows and consequent local entropy production; and a non-hydrostatic physical model of the underground convective air circulation. A cave’s temperature is rich in information, but it is often difficult to obtain because it requires experimental accuracies to be pushed beyond the reach of common external meteorological instruments to detect a variety of factors, such as thermal sedimentation, seasonal variations and the effects of external morphology. Energy flow has an essential role in estimating the sensitivity of a cave to external inputs. A model for evaluating the local entropy production is developed. Entropy seems to be the most basic parameter, explaining both the sensitivity of the environment and its tendency to form complex structures. Analysis of the second-order terms of convective air circulation is more complex than expected; nevertheless, only such an analysis is able to explain the behaviours (e.g. continuity in the airflow) that cannot be predicted by the first-order models.

Abstract The Škocjan Caves are included in UNESCO’s World Heritage List due to their outstanding natural features. The caves include a large underground canyon containing the Reka River, collapse dolines with vegetation in rock fissures and impressive archaeological sites with a rich history of speleological and scientific research. They are also included in the Ramsar Directory of Wetlands of International Importance. Together with their broader surface area, the site is known as the UNESCO Karst Biosphere Reserve. The aim of the management of the reserve is to protect the World Heritage Site and to preserve its outstanding universal value for future generations. The protection activities are regulated by the provisions of international documents, the Škocjan Caves Regional Park Act and the park’s management plan. These activities include monitoring of the water quality in the Reka River and meteorological surveys on the surface. Monitoring of the microclimate of the caves focuses on measuring the effects of tourism and monitoring the levels of radon, with the aim of the ensuring the safety of the park’s employees. Ensuring a favourable status for the underground habitats and species is laid down in the Natura 2000 management programme. Particular attention is paid to ensuring high-quality, safe visits to the caves and providing educational and awareness-raising activities on the surface of the park.

Abstract The mine caves of Naica (Chihuahua, Mexico) are famous because they host large gypsum crystals. Mine works intersected new caves hosting the largest crystals in the world in the year 2000. From 2006 these caves became the object of a multidisciplinary research project with the goal of inferring their ages, the boundary conditions for their formation and the mechanisms inducing their development. Several other scientific aspects were also considered, including palynology, mineralogy, microbiology, physiology, hydrogeology and astrobiology. From 2006 to 2009, scientists and explorers tried to ensure the complete documentation of these natural wonders because they were expected to be accessible for only a few years. As a result of their location c. 160 m below the natural groundwater level, they were predicted to be flooded with thermal water as soon as dewatering of the mine ceased. This occurred at the end of 2015, so that the lower part of the mine is already submerged and in the near future the giant crystal caves will also disappear. Theoretically, it is still possible to maintain these incredible wonders for future generations, but this seems highly unlikely. Soon the crystals will be submerged below c. 150 m of hot water, restarting their incredible slow growth.

Modern scientific study of karst phenomena came into being during the 90 years before the Geological Society of America was founded in 1888. It began with broad acceptance of the uniformitarian principle (1800s), basic understanding of processes of carbonate and sulfate rock dissolution and precipitation (1820s), and the equations of Hagen, Poiseuille, and Darcy for groundwater flow in porous, fractured, and soluble media (1840–1856). The Dalmation descriptive name “karst” (meaning “stony ground”), adopted by regional surveyors and travelers, came into general use in the 1850s also. The first U.S. Geological Survey (USGS) report on hydrogeology by Chamberlin in 1885 was one of many early texts that stressed the importance of conduit flow in limestone areas. The 50 years following 1888 were dominated by studies in the “classical” karst region of western Slovenia, including definition of the principal types of surface landforms and proposals for their development within cycles of erosion, two sharply contrasted models for storage and flow in limestone aquifers, and promotion of a theory that accessible caves formed chiefly in the vadose zone. Following publication of a USGS report on the major springs in the nation in 1927, American scientists entered the debates in force, proposing that caves should develop primarily below the water table, along it, or create it; they also emphasized the importance of soil CO 2 in boosting rates of solution in carbonate rocks. Russian investigators established the principles of mixing corrosion. The pace of development throughout karst studies accelerated after the Second World War. In the later 1940s and 1950s, the formative studies of solution kinetics began, while improvements in methods of measuring solute concentrations set the stage for global rate models to be developed in succeeding decades. Spatial quantitative analysis came to dominate study of surface landforms, particularly sinkhole distribution patterns. The confusion that had arisen regarding the development of meteoric water (epigene) caves was resolved with a general model emphasizing the controlling roles of lithology and geologic structure: Increasingly, it was recognized that these two variables also explained many of the differences observed between karst aquifers and landform assemblages in different geographical areas. Opening of China to western scholars after 1980 gave access to the astonishing karst lands in the south of that country.