Krubera Cave

Krubera Cave ( ab, Ӡоу Аҳаҧы, ka, კრუბერის გამოქვაბული or კრუბერის ღრმული, tr; also known as Voronya Cave, sometimes spelled Voronja Cave) is the second-deepest-known cave on Earth, after the Veryovkina Cave. It is located in the Arabika Massif of the Gagra Range of the Western Caucasus, in the Gagra District of Abkhazia, a breakaway region of Georgia.

The difference in elevation of the highest cave entrance (Arbaika) and its deepest explored point is . It became the deepest-known cave in the world in 2001 when the expedition of the Ukrainian Speleological Association reached a depth of which exceeded the depth of the previous deepest-known cave, Lamprechtsofen, in the Austrian Alps, by . In 2004, for the first time in the history of speleology, the Ukrainian Speleological Association expedition reached a depth greater than , and explored the cave to . Ukrainian diver Gennadiy Samokhin extended the cave by diving in the terminal sump to 46 metres' depth in 2007 and then to 52 m in 2012, setting successive world records of 2,191 m and 2,197 m, respectively. Krubera is one of the two known caves on Earth deeper than 2,000 metres.

Naming

Soviet speleologists who explored the cave in the 1960s named it for Russian geographer Alexander Kruber. The alternate name "Voronya Cave", "Crows' Cave" in Russian, was a name given to the cave by Kyiv speleologists during the 1980s, for the crows nesting in the entrance pit.

Location and background

The Arabika Massif, the home of Krubera (Voronya) Cave, is one of the largest high-mountain limestone karst massifs in the Western Caucasus. It is composed of Lower Cretaceous and Upper Jurassic limestones that dip continuously southwest to the Black Sea and plunge below the modern sea level.

To the northwest, north, northeast, and east, Arabika is bordered by the deeply incised canyons of Sandripsh, Kutushara, Gega and Bzyb rivers. The Bzyb River separates Arabika from the adjacent Bzybsky Massif, another outstanding karst area with many deep caves, including the Snezhnaja-Mezhonogo-Iljuzia System () and Pantjukhina Cave (). To the southwest, Arabika borders the Black Sea.

The Arabika Massif has a prominent high central sector with elevations above the tree line at ~. This is an area of classical glaciokarstic landscape, with numerous glacial trough valleys and cirques, with ridges and peaks between them. The bottoms of trough valleys and karst fields lie at elevations of , and ridges and peaks rise to . The highest peak is the Peak of Speleologists () but the dominant summit is a typical pyramidal horn of the Arabika Mount (). Some middle- to low-altitude ridges covered with forest lie between the central sector and the Black Sea. A plateau-like middle-altitude outlier of the massif in its south sector is Mamzdyshkha, with part of the plateau slightly emerging above the tree line.

Among several hundred caves known in the Arabika Massif, fifteen have been explored deeper than 400 m and five deeper than 1,000 m.

Krubera Cave is located at 2,256 m above sea level in the Ortobalagan Valley, a perfectly shaped, relatively shallow, glacial trough of the sub-Caucasian stretch, which holds the advanced position in the Arabika's central sector relative to the seashore. Since 1980, Ukrainian cavers have been undertaking systematic efforts in exploring deep caves in the Ortobalagan Valley, resulting in exploration of the Krubera Cave to its current depth and of the surrounding caves, now mostly connected to it. Another deep cave in the valley, located in its very upper part and explored by Moldavian and Ukrainian cavers is Berchilskaya Cave, deep. All large caves of the Ortobalagan Valley likely belong to a single hydrological system, connected to large submarine springs at the Black Sea shore.

Geology

The Ortobalagan Valley extends along the crest of the Berchil'sky anticline, which gently plunges northwest. The cave entrances are aligned along the anticlinal crest (Figure 2) but the caves are controlled by longitudinal, transverse, and oblique fractures and faults and exhibit complex winding patterns in the plan view, remaining largely within and near the anticlinal crest zone. The caves are predominantly combinations of vadose shafts and steep meandering passages, although in places they cut apparently old fossil passages at different levels (e.g., at in Kujbyshevskaja and Krubera caves, and in the non-Kujbyshevskaja branch of Krubera Cave, etc.). The deep parts of Krubera display a more pervasive conduit pattern with a mixture of phreatic morphology, characteristic of the zone of high-gradient floods, which can be up to 400 m above the low-flow water table, and vadose downcutting elements that are observed even below the water table.

The core part of the Arabika Massif is composed of the Upper Jurassic succession resting on the Bajocian Porphyritic Series, which includes sandstones, clays and conglomerates at the top, and tuff, tuffaceous sandstones, conglomerates and breccia, porphyry and lava. The Porphyritic series forms the non-karstic basement of Arabika, which is exposed only on the northern and eastern outskirts, locally in the bottoms of the Kutushara and Gega River valleys. In the central part of Arabika the Cretaceous cover (Valanginian and Hauterivian limestones, marls and sandstones) is retained only in a few ridges and peaks, but it lies intact through the low-altitude ridges to the south-west of the central part. There the Cretaceous succession includes Barremian and Aptian– Cenomanian limestones and marly limestones with abundant concretions of black chert.

The Upper Jurassic succession begins with thin-bedded Kimmeridgian– Oxfordian cherty limestones, marls, sandstones and clays, which are identified in the lower part of Krubera Cave. Above lies the thick Tithonian succession of thick-bedded limestones with marly and sandy varieties. Sandy limestones are particularly abundant through the upper 1,000 m sections of deep caves of the Ortobalagan Valley.

The tectonic structure of Arabika is dominated by the axis of the large sub-Caucasian anticline (oriented NW–SE), with the gently dipping southwestern mega-flank, complicated by several low-order folds, and steeply dipping northeastern flank (Figure 3). The axis of the anticline roughly coincides with the ridge bordering the Gelgeluk Valley to the north. Located on the southwestern flank of the major anticline is another large one (Berchil'sky), in which the crest is breached by the Ortobalagan Valley. There are several smaller sub-parallel anticlines and synclines farther southwest, between the Berchil' Ridge and the coast.

The plicative dislocation structure of the massif is severely complicated by faults, with the fault-block structure strongly controlling both cave development and groundwater flow. Major faults of the sub-Caucasian orientation delineate several large elongated blocks that experienced uplift with different rates during Pliocene. This had a pronounced effect on the development of deep groundwater circulation and of Krubera Cave in particular. Both longitudinal and transverse faults and related fracture zones play a role in guiding groundwater flow; the latter guide flow across the strike of major plicative dislocations, from the central sector toward the Black Sea.

Cave entrances

The cave has six entrances, in the gallery below they are arranged by altitude, highest to lowest, after the main one. All are situated above the tree line.

File:Outside View of the Main Entrance to Voronja Cave on Arabika, 2019.jpg, ''Voronja'', 2256 m
File:Outside View of the Arbaika Entrance to Voronja Cave on Arabika, 2019.jpg, ''Arbaika'', 2259 m
File:Outside View of the Gnomov Entrance to Voronja Cave on Arabika, 2019.jpg, ''Gnomov'', 2206 m
File:Outside View of the Kujbiševskaja Entrance to Voronja Cave on Arabika, 2019.jpg, ''Kujbyševskaja'', 2180 m
File:Outside View of the Detskaja Entrance to Voronja Cave on Arabika, 2019.jpg, ''Detskaja'', 2140 m
File:Outside View of the Genrihova Bezdna Entrance to Voronja Cave on Arabika, 2019.jpg, ''Genrihova bezdna'', 2110 m

Hydrogeology

Major on-shore karst springs with individual average discharges of are located at altitudes ranging from (Reprua Spring) to (Gega waterfall). Two of them are located in the shore area; these are Reprua (average discharge ; altitude above sea level) and Kholodnaja Rechka (; a.s.l.). Two more major springs are located in the river canyons bordering Arabika to the east: Goluboe Ozero in the Bzyb canyon (; a.s.l.) and Gega waterfall in the Gega canyon (; a.s.l.). There are also several smaller springs in the Gagra town. The Reprua River, one of the shortest rivers in the world, about long, starts in the cave and flows toward the Black Sea.

Some boreholes located along the shore of the Black Sea yield karstic groundwater from depths of 40–280 m below sea level. Other much deeper boreholes tapped low-salinity karstic waters at depths of 500 and 1,750 m in the Khashupse Valley near Gantiadi and 2,250 m near Gagra. This suggests the existence of a deep karst system and vigorous karst groundwater circulation at depth.

Submarine springs are known in the Arabika area, emerging from the floor of the Black Sea in front of the massif. Shallow springs at depths of 5–7 m can be reached by free dive near Gantiadi. Tamaz Kiknadze (1979) reported submarine springs near the eastern part of Gagra at depth of 25–30 m and Buachidze and Meliva (1967) revealed submarine discharge at depths up to −400 m by hydrochemical profiling. Recently an outstanding feature of the sea floor topography near Arabika has been revealed from a digital bathymetric map that combines depth soundings and high-resolution marine gravity data. This is a huge submarine depression in front of the Zhovekvara River mouth, which has dimensions of about 5 x 9 km and a maximum depth of about . The Arabika Submarine Depression is a closed feature with internal vertical relief of about (measured from its lowest rim) separated from the abyssal slope by the bar at a depth of about . It has steep northern and northeastern slopes (on the side of the massif) and gentle south and southwestern slopes. Its formation is apparently karstic. Presently this depression seems to be a focus of submarine discharge of the karst systems of Arabika.

The speleological explorations and a series of dye tracing experiments conducted during the 1980s under the coordination of Alexander Klimchouk have radically changed previous notions of the hydrogeology of Arabika, revealed its outstanding speleological perspectives and strongly stimulated further efforts for exploration of deep caves. Tracers injected in the Kujbyshevskaja Cave and the Iljukhina System were detected in the Kholodnaja Rechka and Reprua springs, proving groundwater flow to the south-southwest across major tectonic structures over a distance of 13–16 km as the crow flies (Figure 1). The tracer from Kujbyshevskaja Cave was also detected in a borehole located between these two springs, which yields groundwater from a depth of below sea level. This has been interpreted as an indication of the connection of the cave with the submarine discharge. The large "Central Karst Hydrologic System", which encompasses most of the southeastern flank of the Arabika anticline, had been identified in this way. The system became the deepest in the world with its overall vertical range of about (measuring to the borehole water-bearing horizon) or even (measuring to the deepest reported submarine discharge points).

Another tracer was injected in the Moskovskaja Cave (−970 m) and detected at the Gegsky Vodopad spring, indicating the presence of a karst hydrologic system comprising the northeastern flank of the Arabika anticline (the "Northern System"). No connections have been revealed with yet another major spring, Goluboje Ozero in the Bzyb River canyon, although it apparently drains a large area of the eastern sector of the massif (the hypothetical "Eastern Karst Hydrological System"). It is not clear where Sarma Cave (−1,550 m) drains to, Goluboje Ozero to the southeast or Reprua to the southwest, at the shore.

The results of the dye-tracing tests demonstrated that groundwater flow is not subordinate to the fold structure but is largely controlled by faults that cut across the strike of major folds, and that the large part of the central sector of Arabika is hydraulically connected to the springs along the seashore and with submarine discharge points.

Krubera Cave has an extremely steep profile and reveals a huge thickness of the vadose zone. The lower boundary of the vadose zone (the top of the phreatic zone) is at an elevation of about at low flow, which suggests a low overall hydraulic gradient of 0.007-0.008. Low- TDS groundwater is tapped by boreholes in the shore area at depths of 40–280, 500, 1,750, and 2,250 m below sea level, which suggests the existence of a deep flow system with vigorous flow. Submarine discharge along the Arabika coast is reported at depths up to ~400 m b.s.l.

It is difficult to interpret these facts in terms of the development of karst systems controlled by contemporary sea level, or within the range of its Pleistocene fluctuations (up to −150 m). In combination with the existence of the Arabika Submarine Depression, all these facts point to the possibility that karst systems in Arabika could have originated in response to the Messinian salinity crisis (5.96–5.33 Ma) when the Black Sea (Eastern Paratethys) could have almost dried up, as did the adjacent Mediterranean, where the dramatic sea level drop of ~1,500 m is well established.

Biology

The biocenosis of Krubera-Voronja is composed of more than 12 species of arthropods of several groups, such as pseudoscorpions, spiders, opiliones, crustaceans, springtails, beetles and dipterans. Krubera-Voronja cave is inhabited by endemic species, including four springtails discovered during the CAVEX Team expedition of 2010: ''Anurida stereoodorata