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Gourgouthakas cave (LOC1 code name) is located at an altitude of 1550m, in the area of Atzines or Atzinolakas, on the northern side of Lefka Ori (White Mountains) of Crete, Greece. It is among the two deepest caves in Greece and the 50 deepest in the world, reaching a depth of 1100m. Gourgouthakas cave was found during the French caving expedition “Lefka Ori 1981” of the Speleological Society of Plantaurel (SSP) and its explorations began in the 1990s by the Speleo Group Catamaran. The cave was explored and mapped to its current depth during an international expedition that was held in 1998, while the last expedition was held in 2001. Herein, the “Gourgouthakas 2022” expedition, a joined effort from 40 speleologists from 12 speleological clubs of Greece, which took place 21 years since the last one (SELAS-SPOK expedition, 2001), is reported. Through this expedition we aimed at mapping the cave with advanced contemporary tools to create a more detailed mapping, scuba cave diving at the first siphon at the depth of 678m, scientific sampling for geological and biological research and gathering the speleological community of Greece. The expedition was held in two phases. During the first phase (9 days), the speleologists set the basecamp, rigged the cave with new materials until the second bivouac at -649m, and installed the telecommunications in the cave. At the second phase (16 days), the cave was rigged, photographed, mapped and lithostratigraphically studied until the second siphon at the depth of 1100m.Macroinvertebrates and microbes were sampled from 7 and 9 different stations respectively, while water samples were collected from 5 different stations for eDNA metagenome analysis. Several faults were recorded from the entrance to the deepest part of the cave with the stratigraphic column constituted, from top to bottom, by Triassic to Jurassic Trypali unit with mainly recrystallized marble (entrance to -522m), Plattenkalk unit with a thin layer of Oligocene metaflysch (-522m to -850m), partly marbled limestone (-850m to -900m) and Plattenkalk unit with several siliceous concretions (-900m to -1100m). In total, 52 macroinvertebrate individuals belonging to more than 7 different troglobiont taxa were sampled, with a new species of niphargid amphipod being described, while more than 200 stains of microbes were cultivated from the scraped collected samples. This expedition provided the opportunity to many speleologists to experience the acclimatization and demands of deep cave exploration and set the basis for subsequent expeditions in the area in many years to come.

As part of the International Geoscience IGCP-715 project, we present the core objective and preliminary analyses on the karst development of the study areas. Our aim is to further characterize the geomorphologic features of the extended karst of Koiliaris Critical Zone Observatory (KCZO), Crete, Greece. Simultaneously to better understand the main drivers of karst development we compare the CKZO karst system with other areas in different tectonic contexts such as Oman (Salma Plateau), the northern UAE and southern China (Guilin karst area).Hydrological studies and previous geomorphologic analysis of KCZO suggest that 27% of the total water budget is coming from the adjacent watershed in the east where an extensive karst system with two explored super-deep caves is situated (Liontari Cave - 1100 m, Gourgouthakas Cave - 1200 m). The area is build up by a continuous carbonate succession exceeding 5 km in depth, lying on top of the Hellenic subduction zone. Field work and Google Earth mapping show two dominantly striking directions of failures (fault, fracture surfaces), trending E-W to ESE-WNW (90-120°) and N-S to NNE-SSE (0-22.5°). The N-S surfaces are mainly fractures while the E-W ones are mainly thrusts and/or strike-slip faults with obvious large displacements of hundreds of meters. The karst development in a subduction zone with dramatic thrusting on the overriding plate has created super-deep caves which are controlled by the vertical bedding and a series of faults and fractures. The area exhibits two layers with different hydraulic properties, a fast water-transferring zone and a slower one which is consistent and supports the hypothesis of the hydrologic model. At the Salma Plateau, in Oman, the karstic system is related to rapid uplifted Eocene limestones that overlay the Semail Ophiolite. There is a large cave (Majilis Al Jinn) at an area of interconnected fractures (and/or faults?). It is the only karstic system presented inhere which has similarities with the karstic system in the KCZO.At the UAE and northern Oman (Musandam) is an active collision zone between Arabia and Eurasia with 2000-m-thick allochthonous Mesozoic limestones. The area lacks a subsurface karst system, and the only karst has developed in steep wadis.Finally, the Guilin area in China represents a former passive margin with a Devonian limestone. It features a spectacular karst of conical peaks (fengcong) and tower peaks (fenglin). Caves exhibit mainly a horizontal development and there is no similarities to the KCZO.