In the 1780s the honorary member of St. Petersburg Academy of Sciences, natural scientist Eric (Kirill) Laksman became interested in the Baikal area caves, which are created in karst rocks. He had probably collected the first scientific data on the enigmatic and many-sided world in the very center of Eurasia. Here is what he wrote about the Nizhneudinskaya cave, well-known today: "It took me 24 hours to measure and describe it, to gather stalactites, the enormous resources which nature had hidden in the remote, most terrible and surprising country."

Today two hundred similar objects are well known, many of which are mapped, in some of them tourist itineraries are paved, while experts know incomparably more about the origin and development of different underground caves of Irkutsk amphitheater, as the southeastern part of Irkutsk-Cheremkhovsk piedmont plane is called, which is surrounded by the stepped bottoms of Eastern Sayan Mountains, Maritime and Baikal crests from the south-west, south and south-east. In the journal Priroda (Nature), No. 5, 2007, Yelena Trofimova, Cand. Sc. (Geography), told about the modern state of these caves.

The western coast of Baikal, the lower flow of the river Kitoya and the Uda basin - these are the places where speleologists set forth to descend into underground systems. The entrance can be located both at the top of an inaccessible rock and at waterline, while its dimensions vary from 0.3 - 0.5 m to 15 - 7 m. Though these caves are weakly watered, in some of them one can see "bottomless" lakes with siphons.

In 50 percent of the examined area there are carbonate, sulphate and salt-bearing rocks, "threaded" with seismogenic cracks and exposed to intense physical weathering. Underground labyrinths are formed in them. Here karst processes are developed at a high rate - from 50 mm/1,000 years showing themselves even at a depth of 1 kilometer. Absolute altitudes on plains are 250 - 500, while in mountains - 1,000 - 2,500 m, and slightly mineralized waters coming from above and, therefore, having a strong dissolving capacity, easily penetrate even at such distances.

Mechanisms of forming of Irkutsk amphitheater caves are different. Forty-six percent of their total number are corrosion-rupture cavities emerging with displacement of blocks of rocks along tectonic disturbances (their characteristic feature - vertical wells or cracks). Fifty-three percent are corrosion-erosion cavities, emerging in the area of horizontal circulation of underground waters in side edges of valleys (most of horizontal caves belong to them). Other genetic classes are comparatively small: for example, corrosion caves are formed at water-dividing areas of the ridges of Eastern Sayan Mountains under the influence of water from melted snow on limestone. Only one cave-labyrinth located in the upper reaches of the Lena river owes its "birth" to changes of pressure of underground waters. Finally, on the western coast of Lake Baikal there are small corrosion-abrasive cavities, results of destruction of rocks by disturbances, streams, etc.

A number of caves (21 percent) are galleries, grottos, wells and cracks. However, so called binary underground caves in combination with the above-mentioned simple forms are more numerous (47 percent).

The most extensive intricate caves of several floors have a cold "entrance" part at constantly positive air temperatures (1.2 - 4.0 °C) in deep zones and relative humidity of 70 - 100 percent. Horizontal caves, vertical cavities, as well as all complex mines, located at a depth

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of 150 m from the surface have a warm microclimate with positive air temperatures at considerable gradients of the latter according to the length (depth) - 1.5 - 2.5 °C per 1 m. In grottos, on the contrary, a negative yearly average air temperature of deep premises is preserved, and from season to season the characteristics of "the entrance" part vary, while humidity varies from 55 to 90 percent. It is typical that in all cases the air exchange with the surface is extremely weak.

The author classifies and describes different deposits of caves: residual, aqueous hemogenic, organic, anthropogenic, as well as ice and snow. Thus, russet clays lying low in a rather powerful layer (sometimes up to 8 m) are formed in the course of dissolving limestone: 400 kg are formed from 1 m³. Speleologists give exotic names to picturesque stalactites, stalagmites, hangings from derived calcite: "Proud Inca", "Sea Jellyfish", "Dinosaur's mouth", and dripstones - corallites - of the so-called Musical Hall of one of the well-known caves of Irkutsk amphitheater "Dream" melodically sound when somebody hardly touches them. As Trofimova points out, a real "cave pearl" was found at the bottom of a small lake in Razdolinskaya shaft.

However, in the researcher's opinion, the underground ice which twinkles with "all colors of the rainbow creating a unique picture in eternal darkness and silence" is the most attractive. Its several varieties have been described. Congelation ice is formed at freezing of

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water leaking through cracks; it covers bottom, walls and ceilings with sparkling blankets, stalactites and stalagmites. Sublimation ice forming hexahedron (up to 12 cm) and needle crystals is created at condensation of moisture on chilled surface. Sedimentary and metamorphic ice appears due to recrystallization of snow getting into underground cavity through the cave mouth. Small one-year old snowfields, often located near entrances complete new ice formations. Unfortunately, since 1976 these fabulous "treasures" have been gradually disappearing, and in the last decade at a rate of 3.2 - 11.7 cm/year. The contemplated reason is global climate warming, which is confirmed by meteorologists' observations, according to which since 1967 the stable positive trend of average annual air temperature has set in in the region.

The prominent scientist and traveler Ivan Chersky, who made up the first geological map of Baikal in 1875, gathered many well-preserved bones of 19 species of extinct animals in the Nizhneudinskaya cave - the Siberian long-haired rhinoceros, cave bear and others. These bleak shelters are inhabited even today: bats of the Vespertilionidae family, Myotis (owl-moths) and Plecotus (long-eared bats), insects from the subclass of cryptomandibulars of the Entognatba class chose them. In the caves of the western coast of Lake Baikal in the new Stone and Iron Ages there lived our forefathers, which is testified by a lot of data, including human skulls and bones (the geographer Pyotr Kropotkin found them in the Bolshaya Kadilnaya cave in 1865). Sometimes animals were buried in underground cavities. Today Buddhist Buryats organize chapels and carry out rituals there.

Trofimova published the data showing a degree of disturbance of underground surroundings under the influence of anthropogenic factor. The caves which are difficult of access due to surrounding impassable hogs or mountains and have been unknown till recently (about 15 years) proved to be the least changed. On the contrary, the most visited objects suffered most of all. It is not accidental that in 1981, 1985 and 1987, Irkutsk executive authorities made an attempt to protect 7 caves by giving them the status of the state monument of nature. Twenty-seven underground cavities are in the list of the ones which need to be protected in the near future. It is necessary to protect the caves, where objects of collector or marketable value are discovered (rare specimens of paleontological remains, or mummies). Only specialists can be admitted here, which means that there is a need to prevent dissemination of information about location of these underground cavities. Expeditions and student practice must take place in the second class cavities. Finally, there should be chosen less important objects for mass visits.

Trofimova Ye., "The Caves of Irkutsk Amphitheater", "Priroda" (Nature), No. 5, 2007

Prepared by Yevgeniya SIDOROVA