Krubera (Voronja) Cave: -2.158 m

In the period between August 19 - September 20, 2006 the Ukrainian Speleological Association (Ukr.S.A.) and the Ukrainian Institute of Speleology and Karstology (UISK) have conducted a regular expedition of the CA Project to Krubera (Voronja) Cave in Arabika Massif, Abkhazia.
The expedition was led by Yury Kasjan and consisted of 16 speleologists from Ukraine (from Kiev, Kharkov, Simferopol, Sevestopol, Poltava, Dnepropetrovsk and Kamenetz-Podilsky) and Russia (Moscow and Irkutsk). New explorations have been made in different parts of the cave, including the lowermost part, as well as various scientific observations.

A major progress has been achieved in the Nekujbyshevskaja Branch (a distinct branch diverging from the main one at -250m and stretching to the north-west, the direction opposite to the overall trend of the main branch) by a team led by Kirylo Markovskoj. They continued exploring a new part, discovered and pushed to a boulder choke at -643m a year ago after breaking through the upper boulder choke at -490m. During this expedition the choke at -643m has been also broken and the branch pushed to -1004m, a point where the team run out of rope. The work in the lower part of the main branch can be summarized as follows:

1) The lowermost part below the Kvitochka siphon at -1980m has been resurveyed. The depth
of the post-siphon section (to the spit at the table of the Two Captains siphon, the one at the bottom) is measured to be 164m.

2) The Two Captains siphon has been dove by Gennady Samokhin for 40m in length and 14m in
depth, to the point where he was stopped by a squeeze. So, the new overall depth of Krubera Cave is now - 2158m (see notes on survey data below). The absolute elevation of the lowermost reached point in the cave is about 100m a.s.l. but it still far (more than 12km) from the Black Sea coast where the discharge from the system occurs.
3) Some new explorations have been made in the lower part of the cave: an ascending side passage at -1710m has been explored for 150m/+20m; a dry bypass of the siphon Unitaz at -2068m has been found; a siphon at -1775m (first tested in August 2004) has been dove by G.Samokhin, who passed a narrow place at -10m but stopped at an underwater boulder choke in the ascending leg. The scientific program in this expedition included geological observations, various sampling (clastic sediments for mineralogic analysis, microbiologic materials, speleothems for U-series dating), temperature measurements (over 200 measurements through the depth profile in both the main branch and Nekujbyshevskaja) and observations of the groundwater level fluctuations from past flood evidences. The latter suggest that the water level may arise during a year up to approximately -1700m, the fluctuation for more than 450m (during spring snowmelt floods or major rainfalls).

The updated map of the whole Krubera Cave will be published soon.
Krubera total depth: -2,158m
Krubera total length: 10,870m

Notes on the survey data and depth measurements:

The complete integrated Krubera survey dataset, maintained by the Ukr.S.A. through many years, consists of more than 1850 vectors to the date. Survey is performed using Suunto compasses KB-14, clinometers PM-5 (or Tandem instruments) and Leica-Disto meters or fibron tapes (older surveys in the upper parts have been re-done during last several years).
During the October 2005 Ukr.S.A. expedition to Krubera Cave a team from the Moscow Branch of the Russian Geographic Society (T.Nemchenko and A.Degtjarev), supported by the Bulgarian cavers, has conducted an independent thorough hydro-levelling work from the entrance to √1194m in the main branch, in order to verify the actual precision of depth figures obtained from the standard survey. A comprehensive methodological substantiation for the technique, and the analysis of errors involved, has been performed by A.Degtjarev, E.Snetkov and A.Gurjanov of Moscow (available in Russian in Svet n.29, 2005 and at: http://www.rgo-speleo.ru/biblio/hydroniv.htm).
They demonstrated that the correct application of the hydro-levelling technique allows measuring depth with the 0.2% accuracy, i.e. with an error of 4m for a 2000m depth. At the same time, they estimated that the combined error in determining depth due to some commonly practiced improper procedures could be up to 2%. The roundtripping closure (up and down measurements) for the section 0-916m (80 measurements), performed in October 2005, gave an error of 0.05m. i.e. less than 0.01%.
Therefore, the results of this survey can be reasonably taken as an etalon dataset. Comparison of the standard Ukr.S.A. survey with the etalon data for particular points in the 0-1194m interval has revealed actual errors varying between +0.97 and -1.37 %, with an average error being 0.9%. Based on this, the overall accuracy of the Ukr.S.A. standard survey in depth measurements is assumed to be 1.0%.

The results of another hydro-leveling work, performed by the Moscow-based Cavex team in July-August 2005 and claimed to correct the Ukr.S.A. figure for the old bottom from -2080m to -2064m (to -2047m in other sources), have been shown to suffer from improper procedures involved. Comparison of their results with the etalon data for particular points shows the actual error of the Cavex hydro-levelling measurements varying within -0.64 and -2.2% (average 1.2%), which is even greater than the errors of the standard Ukr.S.A. survey as compared to the same etalon. Therefore, there is no reason for correction for the figure previously reported by the Ukr.S.A. for the old bottom.

CONCLUSION: The overall error of the standard Ukr.S.A. survey is proven to be within 1%.
The current depth of Krubera Cave, reported here as -2158m, has the margins of possible error between +22m and - 22m.
Text from:
Alexander Klimchouk, the CA Project coordinator,
based on the report by Yury Kasjan, the expedition leader.

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