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From Alexander Klimchouk, Krubera (Voronja) . In: William B. White and David C. Culver, editors, Encyclopedia of Caves. Chennai: Academic Press, 2012, pp. 443-450. ISBN: 978-0-12-383832-2 Copyright © 2012 Elsevier Inc. Academic Press. Author’s personal copy

KRUBERA (VORONJA) CAVE 443 KRUBERA (VORONJA) CAVE B1800À1900 m. This is an arena of classical Alpine- Alexander Klimchouk type , a glaciokarstic landscape with numerous glacial trough valleys and cirques, with ridges and Ukrainian Institute of and Karstology, Simferopol, peaks between them. The bottoms of the trough val- leys and karst fields lie at elevations of 2000À2350 m, and ridges and peaks rise to 2500À2700 m. The high- est peak is the Peak of Speleologists (2705 m) but INTRODUCTION the dominant summit is a typical pyramidal horn of the Arabika Mount (2695 m). Some middle- to low- altitude ridges covered with forest lie between the At the dawn of the new millennium, in January 2001, central sector and the . A plateau-like Krubera (Voronja) Cave in the Arabika Massif, middle-altitude outlier of the massif in its south sec- Western , became the deepest known cave in 2 tor is Mamzdyshkha, with part of the plateau slightly the world, with a depth of 1710 m (Klimchouk and emerging above the tree line. Kasjan, 2001). For the first time, the deepest known cave was found to be outside Western Europe. In the article on Krubera Cave in the previous edition of this KRUBERA CAVE AND OTHER DEEP encyclopedia (written in 2003), when the explored CAVES IN THE ARABIKA MASSIF depth of the cave was still at 21710 m, the present author wrote: “The future possibility of locating a 1 Among several hundred caves known in the Arabika 2000 m system in the area is exceptionally good.” Massif, 15 have been explored below 400 m and five Discovering the first cave on the planet deeper than below 1000 m (shown in Fig. 1C). Several are located 2000 m had been a long-standing dream of cavers within the Ortobalagan Valley, a perfectly shaped, rela- around the world, and this was set in 2001 as an official tively shallow, perched glacial trough of the sub- goal of the Call of the Abyss project of the Ukrainian Caucasian stretch, which holds the advanced position in Speleological Association (Ukr.S.A.), one of the most the central sector toward the seashore (Fig. 2). Since 1980, ambitious and successful exploration projects in the his- Ukrainian cavers have been undertaking systematic tory of speleology. In October 2004 this goal was efforts in exploring deep caves in the Ortobalagan Valley reached, when Krubera Cave was pushed to depth of including Krubera (Voronja) Cave (number 1 on Fig. 1C; 2080 m. In subsequent years, the Ukr.S.A. expeditions 22191 m), and the Arabikskaja System (number 4 on have explored the cave to depth of 2191 m in the main Fig. 1C), which consists of Kujbyshevskaja Cave branch, and also explored the second branch in this (21110 m) and Genrikhova Bezdna Cave (2965 m to the cave, called Nekujbyshevskaja, to a depth of 1697 m. junction with Kujbyshevskaja). Another deep cave in the valley, located in its very upper part, is Berchilskaja Cave (2500 m; number 11 on Fig. 1C)exploredby THE ARABIKA MASSIF: LOCATION Moldavian and Ukrainian cavers. The Ortobalagan AND PHYSIOGRAPHY Valley extends along the crest of the Berchilsky , which dips gently northwest. The Arabika Massif is one of the largest An open-mouthed 60-m shaft, the Krubera entrance, massifs of the (Fig. 1A,B). It is was first documented in the early 1960s by Georgian located in , a republic that officially belongs researchers, who named it after Alexander Kruber, a to but since 1992 holds claim to being an founder of karst science in . The early exploration independent state. was stalled by an impassable squeeze in a meandering To the northwest, north, northeast, and east, passage at 295 m which led off from the foot of the Arabika is bordered by the deeply incised canyons of entrance shaft. During the 1980s, the main focus of Sandripsh, Kuturusha, Gega, and rivers (Fig. 1C). the Ukrainian expeditions to the Ortobalagan Valley The Bzyb River separates Arabika from the adjacent were Kujbyshevskaja Cave (21110 m) and Genrikhova Bzybsky Massif, another outstanding karst area Bezdna Cave (2956 m) connected into a single system in with many deep caves, including the Snezhnaja- 1987 (the Arabikskaya System). At the same time, these Mezhonogo-Iljuzia system (21,753 m) and Pantjukhina expeditions had pushed Krubera Cave from 295 to Cave (21508 m). To the southwest, Arabika is bor- 2340 m by breaking through a series of critically nar- dered by the Black Sea, with dipping con- row meanders between successive vertical shafts. tinuously below the sea level. During this period the cave received its secondary name The Arabika Massif has a prominent high central Voronja (Crow’s Cave), owing to the number of crows sector with elevations above the tree line at nesting in the entrance shaft.

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444 K

FIGURE 1 (A) and (B): Location and shaded maps of the Arabika and Bzubsky massifs. The sea-floor topography is based on the SRTM30plus data (NASA). (C) Deep caves are indicated by dots and numbers (explained in the text) and major springs are specified by two- character indexes (explained in the legend). Solid black lines show the crests of major anticline folds. Red arrows reflect previous ideas on groundwater basins and flow directions controlled by major fold structures. White and yellow arrows indicate the actual hydrologic connec- tions established by dye-tracing experiments in 1984À1985.

From 1992À1999 the explorations in Arabika were major breakthrough by discovering and exploring two suspended due to the Georgian-Abkhazian ethnic con- branches that stretched from the old series in different flict and subsequent turmoil. In 1999, the Ukr.S.A. expe- directions: the Main Branch to 2740 m and the dition recommenced work in Krubera Cave and made a Nekujbyshevskaja Branch to 2500 m. The Main Branch

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KRUBERA (VORONJA) CAVE 445

FIGURE 2 Major caves in the Ortobalagan Valley. Red dots indicate dolines. was quickly pushed farther in 2000: in August to combinations of vadose shafts and steep meandering 21200 m and September to 21410 m. In January 2001, passages, although in places they cut apparently old fos- the Ukr.S.A. expedition explored the cave to 21710 m, sil passages at different levels (e.g., at altitudes of establishing it as the new deepest cave in the world. 2070À2040 m in Kujbyshevskaja Cave, 1200À1240 m in In 2004, the cave was explored to depth of 1840 m in the Main Branch of Krubera, and 980À1150 m in the August, and to 22080 m in October. Further efforts in Nekujbyshevskaja Branch of Krubera Cave, etc.). The the Main Branch led to reaching the terminal sump antiquity of these passages is supported by the ages of at 22145 m in 2006. The current deepest point at speleothems falling beyond the 230Th dating limit 22191 m was reached by the Ukr.S.A. expedition in (.500 ka). Both branches of Krubera Cave are extremely 2007 through diving to 246 m in the terminal sump. In vertical up to the depths of about 1600 to 1700 m (alti- the Nekujbyshevskaja Branch, systematic digging efforts tudes of about 750À650 m). The Main Branch below this in boulder chokes since 2004 resulted in a series of level becomes more inclined, largely following the strata breakthroughs and the eventual exploration to the depth dip, but then it goes steeply down again to the depths of of 1697 m in 2010. Most of the Ukr.S.A. explorations in about 2050 to 2150 m (altitudes of about 200À100 m). Krubera Cave since 1999 have been led by Yury Kasyan. Cave development is strongly controlled by the block- All the large caves of the Ortobalagan Valley belong structure. The cave entrances are aligned along the to a single hydrologic system, developed in and near the anticlinal crest (Figs. 2 and 3) but the cave passages and crest zone of the Berchil’sky anticline. The direct connec- shafts are controlled by diagonal and orthogonal frac- tion of Krubera Cave with the Arabikskaja System, tures and faults, and comprise complex winding pat- although not established yet, is a sound speleological terns in the plan view, remaining largely within and possibility. The Main Branch and the Nekujbyshevskaja near the anticlinal crest zone. Two dominant diagonal, Branch in Krubera are largely independent, predomi- SEÀNW-stretching lines are recognizable in the plan pat- nantly vertical, parts of the system which deviate from tern of Krubera Cave. The other diagonal direction, of the Krubera old series at 2220 to 2240 m (Figs. 2 and 3). SWÀNE orientation, is also used by many smaller frag- Their upper sections (to altitudes of about 900À800 m) ments of Krubera Cave and by the most part of cut through the thickly bedded and massive, often sandy Genrikhova Bezdna Cave. The south À north orthogonal Upper Jurassic limestones. The caves are predominantly direction is expressed in a large fossil passage in the

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FIGURE 3 Combined cave profile projected on the axis of the Ortobalagan Valley. Krubera Cave is shown in white, other caves in light brown.

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KRUBERA (VORONJA) CAVE 447

Nekujbyshevskaja Branch at altitudes of 980À1150 m, in The cave is very cold, with water temperatures of the terminal sump passage in the Main Branch, and also 1.0Cat2100 m depth, rising slowly up to 7.2Cat in some parts of the nearby Kujbushevskaja Cave. The 2000 m depth. Air temperature is only a few tenths of deep portion of Krubera display a more pervasive con- a degree higher. The phreatic zone effectively inter- duit pattern with a mixture of phreatic morphology, cepts the geothermal heat flux and drains it through characteristic of the zone of high-gradient floods, and springs. This leaves the vadose zone above under vadose downcutting elements that are observed even strong influence of cold percolation that mainly comes below the water table (in the terminal sump). from snow melt at altitudes of about 2000À2300 m. The cave hydrology is very variable, depending on This results in an expressed geothermal depression season. During winter, flow through the cave is at a characteristic for alpine-type karst massifs. minimum as recharge from the surface is virtually Other deep caves in Arabika include the Iljukhina absent. Late May and July are marked by the maximum System (21273 m; number 3 in Fig. 1C) located in the flux due to massive snow melting. During this period center of the massif, Dzou Cave (21090 m; number 5 of the highest flow, conduits in many parts of the sys- in Fig. 1C) and Moskovskaja Cave (21125 m; number 6 tem at altitudes below 1100À1000 m get filled with in Fig. 1C) in the northeastern part, and Sarma Cave water above local obstructions up to local levels of (21760 m; number 13 in Fig. 1C) in the southeastern 10À20 m (the “bottleneck” effect). In the summer and part of the high sector of Arabika. Sarma Cave, now fall seasons, flow varies with the precipitation regime. the second deepest cave in the world, is located along During low-flow periods, a small permanent stream (up the same anticline as Krubera but on its southern 2 to 1 L s 1) first appears in the Krubera Main Branch at a slope, at the similar advanced position relative to the depth of about 340 m. The flow disappears and reap- seashore. pears at various levels, but never increases significantly, even in the deepest sections. There are only small per- manent water flows in the Nekujbyshevskaya Branch, THE ARABIKA MASSIF: GEOLOGY but at the depth of 1660 m a large influent stream with 2 discharge of about 10 L s 1 comes from a tight side pas- The Arabika Massif is composed of Lower sage. This water most likely arrives from the nearby and Upper Jurassic limestones that dip continuously Kujbyshevskaya Cave, where the only stream of com- southwest to the Black Sea and extend below the patible discharge is known, disappearing in a boulder modern sea level (Fig. 4). In the central part of Arabika choke at the bottom at 21110 m. the Cretaceous cover is retained only in a few ridges and

FIGURE 4 Schematic geological and speleohydrological section of the Arabika Massif. See text for explanations.

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448 K peaks, as well as in small patches within the trough val- ranging from 1 m (Reproa Spring) to 540 m (Gegsky leys ( and Hauterivan limestones, marls, and Vodopad). Two of them are located in the shore area: 2 ). In the south and southeast, on the coastwise these are Reproa (average discharge 2.5 m3 s 1,alt. ridges, the Cretaceous succession is more continuous and 1 m asl; RE in Fig. 1C) and Kholodnaja Rechka 2 includes and -Senomanian limestones (1.2 m3 s 1, 50 m asl; KR in Fig. 1C). Other two major and marly limestones with abundant of black springs are located in the river canyons bordering chert. The core part of the massif is composed of the Arabika to the east: Goluboe Ozero in the Bzyb canyon 2 Upper Jurassic succession resting on the (2.5 m3 s 1, 90 m asl; GO in Fig. 1C) and Gegsky 2 Porphyritic Series, which includes sandstones, clays, and Vodopad in the Gega canyon (1 m3 s 1,540masl;GVin conglomerates at the top, as well as tuff, tuff sandstones, Fig. 1C). There are also several smaller springs in the conglomerates and , porphyry, and . The town of . Porphyritic Series forms the nonkarstic basement of Some boreholes located along the shore of the Arabika, which is exposed only on the northern and east- Black Sea yield karstic groundwater from depths of ern outskirts, locally in the bottoms of the Kutushara and 40À280 m below sea level. Georgian hydrogeologists Gega River valleys. reported that other much deeper boreholes tapped The Upper Jurassic succession begins from the bot- karstic waters low in total dissolved solids at depths of tom with thin-bedded - cherty 500 and 1750 m in the Khashupse Valley near limestones, marls, sandstones, and clays, which are iden- Tsandripsh and 2250 m near Gagra. This suggests the tified in the lowermost part of Krubera Cave. Above lies existence of a deep karst system and vigorous karst the thick Titonian succession of thick-bedded limestones groundwater circulation at depth. Submarine springs with marly and sandy varieties. Sandy limestones are are known in the Arabika area, emerging from the particularly abundant through the upper 1000-m sec- floor of the Black Sea in front of the massif. Shallow tions of deep caves of the Ortobalagan Valley. springs at depths of 5À7 m below sea level can be The tectonic structure of Arabika is dominated by the reached by free dive near Tsandripsh. Other subma- large sub-Caucasian (NWÀSE oriented) anticline, with rine springs are known near the eastern part of Gagra the gently dipping southwestern megaflank, complicated at depths of 25À30 m. Offshore hydrochemical profil- by several low-order folds, and steeply dipping north- ing revealed submarine discharge at depths up to eastern flank. The axis of the anticline roughly coincides 400 m. Recently, an outstanding feature of the sea-floor with the ridge bordering the Gelgeluk Valley to the north. topography near Arabika has been recognized using Located on the southwestern flank of the major anticline the DEM based on the SRTM30Plus dataset (NASA). is another large one (Berchil), in which the crest is brea- This is a huge submarine depression on the shelf in ched by the Ortobalagan Valley. There are several smaller front of the Zhoekvara River mouth, which has dimen- subparallel and synclines farther southwest, sions of about 5 3 9 km and a maximum depth of between the Berchil anticline and the coast (Fig. 1C). about 380 m bsl. The Arabika Submarine Depression The plicative dislocation structure of the massif is (ASD; shown on Figs. 1C and 4) is a closed feature severely complicated by faults, with the fault-block with internal vertical relief of about 120 m (measured structure strongly controlling both cave development from its lowest rim) separated from the abyssal slope and groundwater flow. Major faults of the sub- by the bar at a depth of about 260 m. It has steep Caucasian orientation delineate several large elongated northern and northeastern slopes (on the side of the blocks that experienced uplift with different rates during massif) and gentle south and southwestern slopes. Its and Pleistocene. This had a pronounced effect formation is apparently karstic. Presently ASD seems on the development of deep groundwater circulation to be a focus of submarine discharge of the karst sys- and the karst system. Both longitudinal and transverse tems of Arabika. The existence of ASD, along with faults and related zones play a role in guiding other lines of evidences, points to the possibility of groundwaterflow;thefracturezonesguideflowacross much lower sea level positions in the past than is sug- the strike of major plicative dislocations, from the central gested by Pleistocene glacioeustatic oscillations. sector toward the Black Sea. The hydrogeologic model for the Arabika Massif that dominated before the 1980s did not allow a pos- sibility of hydrologic connection between the central THE ARABIKA MASSIF: high sector of the massif and the coastal springs. The HYDROGEOLOGY model implied the existence of elongated groundwa- ter basins corresponding to synclines and separated Major onshore karst springs with individual average by anticlines of the sub-Caucasian trend, with sev- 2 discharges of 1À2.5 m3 s 1 are located at altitudes eral superimposed aquifers in each basin vertically

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KRUBERA (VORONJA) CAVE 449 separated by minor “nonkarstifiable” beds within EVOLUTION OF THE DEEP CAVE the carbonate succession (Kiknadze, 1972; 1979). SYSTEM IN ARABIKA According to this notion, groundwater recharged within the central part of the massif would flow Krubera Cave has an extremely steep profile and northeast, beneath the nonkarstic cover, and south- reveals a huge thickness of the vadose zone. The lower west to the Goluboje Ozero spring, as shown by red boundary of the vadose zone (the top of the phreatic arrows on Figure 1C. The recharge areas for the zone) is at an elevation of about 110 m at low flow, coastal and submarine springs were assumed to be which suggests a low overall hydraulic gradient of only the proximal low-altitude ridges. 0.007À0.008. Groundwater low in dissolved solids is This model was disproved by speleological explora- tapped by boreholes in the shore area at depths of tions and dye-tracing studies during the 1980s under the 40À280, 500, 1750, and 2250 m below sea level, which coordination of the Institute of Geological Sciences of the suggests the existence of a deep flow system with vigor- Ukrainian Academy of Science. A series of large-scale ous flow. Submarine discharge along the Arabika coast dye-tracing experiments was conducted in Arabika in is reported at depths up to B400 m bsl. A huge closed 1984 and 1985. Tracers injected in the Kujbyshevskaja submarine depression is revealed at the sea floor next to and Iljukhina caves were detected in the Kholodnaja Arabika, with the deepest point of B400 m bsl. It is dif- Rechka and Reproa springs, proving groundwater flow ficult to interpret these facts in terms of the development to the south-southwest across major tectonic structures of karst systems controlled by contemporary sea level, over a distance of 13À16 km as the crow flies (shown by or even within the range of its Pleistocene fluctuations white and yellow arrows on Fig. 1C). The tracer from (up to 2150 m). Kujbyshevskaja was also detected in a borehole located A hypothesis is suggested that early karst systems in between these two springs (BH on Fig. 1C), which yields Arabika could have originated in response to the groundwaterfromadepthof200mbelowsealevel.This Messinian salinity crisis (5.96À5.33 Ma BP), when the has been interpreted as an indication of the connection of Black Sea (Eastern ) could have almost dried the cave with the submarine discharge. The large Central up. The dramatic sea level drop of B1500 m is well Karst Hydrologic System, which encompasses most of established for the Messinian time in the adjacent the southeastern flank of the Arabika anticline, had been Mediterranean. French karstologists have recently dem- identified in this way (Klimchouk, 1990). These experi- onstrated (e.g., Mocochain et al.,2006) that the Messinian ments have revealed the deepest karst hydrologic system crisis played a great role in karst development in the in the world with its overall vertical range of about Mediterranean region, where deep conduits had formed 2500 m (measuring to the borehole water-bearing hori- in response to the Messinian lowering of the base level zon) or even 2700 m (measuring to the deepest reported and imposed a strong influence on subsequent karst submarine discharge points). Another tracer was injected evolution. in the Moskovskaja Cave (2970 m) and detected at the The hypothesis that the dramatic sea level drop Gegsky Vodopad spring, indicating the presence of a could have taken place in the Black Sea basin during karst hydrologic system comprising the northeastern the Messinian time had been put forward long ago flank of the Arabika anticline (the Northern System). No (Hsu and Giovanoli, 1979) mainly based on deep-sea connections have been proven of any cave with yet drilling data (DSDP Sites 379, 380, 381). It has been another major spring, Goluboje Ozero in the Bzyb River strongly corroborated by recent studies of regional canyon, although it apparently drains a large area of the geology, including data from bio- and magnetostrati- eastern sector of the massif (the hypothetical Eastern graphy of the key sedimentary sequences, seismic pro- Karst Hydrological System). It is not clear where Sarma filing (establishing the Messinian erosional surface in Cave (21760 m) drains to, Goluboje Ozero to the south- the Eastern Paratethys), studies of deep-water delta east or Reproa to the southwest, at the shore. complexes, and so on. The results of the dye-tracing tests have radically Before the Late Miocene, the present coastal Western changed notions of the hydrogeology of Arabika and Caucasus was a low- to middle-altitude mountain ter- revealed its outstanding speleological perspectives, rain. Temporary desiccation of the Black Sea in the strongly stimulating further efforts for exploration of Messinian time established the base level at many hun- deep caves. They demonstrated that groundwater flow dreds of meters below the present level and caused con- is not subordinate to the fold structure but is largely duit development deep in the Arabika Massif. These controlled by faults that cut across the strike of major early systems were flooded after the Pliocene transgres- folds, and that the large part of the central sector of sion. Uplifts of the Arabika area during Pliocene and Arabika is hydraulically connected to the springs along especially Pleistocene were highly differentiated by elon- the seashore and with submarine discharge points. gate zones (blocks) of the sub-Caucasian stretch (parallel

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450 K to the coast). The total uplift amounted to 2À2.5 km in exceptionally high vertical range of almost 2200 m has the central sector of Arabika, whereas it was minimal been discovered in the Arabika Massif. There were (0.1À0.2 km) in the zone proximal to the coast. unique geological and paleogeographic preconditions Hydraulic continuity was always maintained across the for that. zones, between the main recharge area in the rising cen- tral sector of Arabika and the coastal zone and subma- rine springs. The presence of high conduit porosity of Acknowledgment the Messinian origin in the coastal/submarine sector allowed the zone of high hydraulic gradient during This article is an outcome of enormous efforts of several generations uplift to be pushed far inland, beneath the rising central of cavers who have explored the deep caves in Arabika over the past sector (Fig. 4). This created favorable conditions for the 30 years. enhanced conduit development at depth in the central sector, quick adjustment of the water table to new uplift pulses, and eventual development of a huge vadose Bibliography zone and extremely steep and deep cave systems such as Krubera Cave. This was further favored by recurring Hsu, K. J., & Giovanoli, F. (1979). Messinian event in the Black Sea. 2 Palaeogeography, Palaeoclimatology, Palaeoecology, 29(1À2), 75À94. sea level drops up to 150 m during the Pleistocene, Kiknadze, T. Z. (1972). Karst of the Arabika Massif. Tbilisi, Russia: which caused the steepening of hydraulic gradients Metzniereba. beneath the central sector of Arabika and enhancement Kiknadze, T. Z. (1979). Geology, hydrogeology and activity of limestone of conduit porosity in the upper part of the present karst. Tbilisi, Russia: Metzniereba. phreatic zone. Klimchouk, A. B. (1990). Karst circulation systems of the Arabika Massif. Peschery (Caves). Perm: Perm University. This evolution scenario is indirectly supported by 230 Klimchouk, A., & Kasjan, J. (2001). Krubera (Voronja): In a search Th dating of speleothems from the deep parts of for the route to 2000 meters depth: The deepest cave in the world Krubera Cave. Stalagmites from depths of 1640 m and in the Arabika Massif, Western Caucasus. NSS News, 59(9), 1820 m (elevations 640 m and 436 m asl) have yielded 252À257. ages older than 200 ka (max. 276 ka), which suggests Mocochain, L., Clauzon, G., Bigot, J. Y., & Brunet, P. (2006). Geodynamic evolution of the perimediterranean karst during the that the deep vadose zone already existed in the Messinian and the Pliocene: Evidence from the Ardeche˙ and the Middle Pleistocene at the latest. Rhoˆne Valley systems canyons, southern . Sedimentary The above discussion demonstrates that it is not by Geology, 188À189, 219À233. chance that the deepest cave in the world with the

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