Abstract Karst landscapes and karst aquifers, which are composed of a variety of soluble rocks such as salt, gypsum, anhydrite, limestone, dolomite and quartzite, are fascinating areas of study. As karst rocks are abundant on the Earth’s surface, the fast evolution of karst landscapes and the rapid flow of water through karst aquifers present challenges from a number of different perspectives. This collection of 25 papers deals with different aspects of these challenges, including karst geology, geomorphology and speleogenesis, karst hydrogeology, karst modelling, and karst hazards and management. Together these papers provide a state-of-the-art review of the current challenges and solutions in describing karst from a scientific perspective.

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 Ashalim maze cave, and neighbouring caves in the NW Negev Desert, Israel demonstrate hypogene karst features. These features are shown to have developed as a result of the mixing of two types of groundwater flowing in opposite directions within two tiers of Cretaceous rock aquifers. The stable isotope composition indicates that the lower Kurnub sandstone aquifer was recharged over far-field Nubian Sandstone outcrops in the vicinity of the Precambrian basement outcrops of the Sinai Desert, which belongs to the Afro-Arabian dome. The water flows northward and rises into the Judea carbonate aquifer through deep faults. A similar hydrogeological system is inferred for the speleogenetic period of Ashalim Cave. Dewatering of the cave occurred in the Pliocene due to regional uplift. This is indicated by the first vadose speleothems, dated to the late Pliocene (3.1 Ma). This was followed by surface denudation, which breached the cave and formed the present entrance.

Abstract Speleothems from Ashalim Cave, located in the arid central Negev Desert, Israel, were used in a reconstruction of the palaeoclimate of the northern Saharan–Arabian desert margin. The sequence of speleothems is composed of three stratigraphic members: the yellow Pliocene Basal Member, the brown Early Pleistocene Intermediate Member and the thin Middle–Late Pleistocene Young Member. The age of the Basal Member is c. 3.1 Ma and the base of the Intermediate Member is 1.272 ± 0.018 Ma. Two last deposition periods of the Young Member occurred at 221–190 ka (Negev Humid Period (NHP) 2) and 134–114 ka (NHP-1), associated with interglacial marine isotopic stages 7.3–7.1 and 5.5, respectively. NHP-1 and -2 occurred when the African monsoon index was highest in the last 221 ka. The δ 18 O values of the speleothems range between −6.9 and −11.2‰, 2–4‰ less than in the speleothems of central and northern Israel. This may indicate a remote southern tropical source of precipitation, although during NHP-1 and -2 the thickness of the Negev Desert speleothems decreases from north to south, showing a stronger northern Mediterranean source of moisture. The δ 13 C values of the speleothems (3.5 to −8.5‰) show steppe to semi-desert C4 type vegetation. The 87 Sr/ 86 Sr ratios of the speleothems increased from c. 0.7078 in the Pliocene to 0.7082–0.7085 in the Pleistocene, indicating an increasing supply of desert dust and a decrease in host rock weathering.