Underground

Prague underground consists of some two hundreds of different, mostly minor structures which can be divided into three principal categories: karst, pseudo-karst and historical underground. Most historical underground structures are cellars, vaults, drainage tunnels, etc. These structures are subjected more or less to the care of the Monument Conservation authorities which concerns mostly environmental protection (drainage of city centres, sewerage, environmental contamination) rather than classic nature protection. On the other hand, some outstanding structures of historical underground, such as the Rudolph Gallery, represent unique geological exposures or provide access to the springs feeding minor water reservoirs (Prague Castle water supply system).
If the historical underground does not form part of some building, it is usually ignored by the Monument Conservation authorities. Therefore, it should be registered at least by the nature conservation agencies as a locality of interest.

Karst Phenomena
Altogether 32 caves are registered in the territory of Prague. Most of them are remodelled fragments of paleokarst cavities the length of which does not exceed a few metres, as a rule. However, oven these small cavities often provide archaeological finds or important Quaternary exposures.
The most important Prague cave was the no longer existing Prokop Cave situated within the quarry boundaries in the central part of the Prokop Valley. Its total length attained 120 m. Narrow parts alternated with dome-like spaces. The cave was believed to have been the abode of St. Procopius the Hermit and used be the target of pilgrimiges in the 17th - 19th centuries. At the end of last century J. Kořenský salvaged skeletal finds from the remainders of the cave fill which belonged both to the Holocene and to big Pleistocene mammals (hairy rhinoceros). The finds include also a human lower jaw of uncertain age which could belong both to a Paleolithic hunter (in this case the cave would be an important European site of the fossil man) and to the substantially more recent prehistorical man.
The last remains of the cave were destroyed after the Second World War. At present the quarry is used by the army. It comprises a number of minor further cavities giving the possibility of cave continuation and further paleontological finds. It is still the most important karst site in the territory of Prague. Historical descriptions by J. Arbes and K. V. Zap mention the existence of a fracture with an underground watercourse. It is probable that there is a flooded karst system below the erosion base between the Prokop quarry and the Hlubočepy lake.
Below the Butovice hillfort, in a conspicuous erosion trench above the former swimming pool, there is a well preserved system of five minor cavities called Korálové Jeskyně (Corral Caves). They have yielded sporadic archaeological finds dating from several different cultures (Eneolithic - the Slavs) as well as middle and younger Holocene profiles with paleontological finds. The Hlubočepy cave in the Prokop Valley below the Bašta hill has yielded ample paleontological material which enables a reconstruction of natural conditions of the environs of Prague in the Holocene. Like elsewhere in the Bohemian Karst this site was influenced by the significant wet phase of the Middle Holocene (Atlantic - Epiatlantic). After the end of the Bronze Age (1st millenium B. C.) the area of Prague and its environs was intensively deforested and converted into a cultural steppe.
Further minor cavities can be found in the Podolí and the Braník quarries, in the rock parade of the Vltava canyon and in the Radotín Valley. Particular attention should be afforded to the conspicuous rock formation with a cave, called Zubák. It is situated below Zadní Kopanina in the valley of the Mlýnský Potok brook, fed by one of the big springs of the Bohemian Karst. The whole valley is an area of exceptional concentration of Quaternary sites characterized by valley calcareous tufa, slope series and cave sediments. Another outstanding Prague karst spring with a deposit of calcareous tufa is situated in the Čertova Strouha (Devil's Ditch) near Malá Chuchle. In the upper part of the "Ditch" the Chuchle cave, 14 m long, can be found in an abandoned quarry. The number of major Prague caves includes also the Věž (Tower) cave on the upper edge of the Radotín Valley above the Rutice Mill and the Radotín cave in disharmonically folded rocks above the Špaček mill west of the Cikánka.
Prague caves are not big, but their fills provide an irreplaceable testimony to the development of Prague landscape in prehistoric times. Only a small part of existing cavities is open to direct observation. Some of them reach even below the valley floor level, a fact testified to by an unexpected discovery of a cave during the construction of the Barrandov bridge, or are filled with older sediments. On the Dívčí Hrady hill, for instance, an almost 100 m deep abyss was drilled through, filled with Cretaceous sediments.

Pseudokarst
Prague pseudokarst is bound with Proterozoic and Cretaceous rocks. In 1883, during the operation of the quarry in the Šestákova Skála, a cavity was discovered in the Šárka lydites. It was 6 m long, 2 m high and 2 m wide, and was covered with quartz crystals weighing as many as 3.5 kg. This unique cavity was destroyed in the course of further quaarry operation. Nevertheless there are futher five cavities up to 5 m long in the Šárka valley which are of partly corrosive character.
The edges of Cretaceous plateaux are affected by block slides in the course of which fracture cavities 20 - 60 cm wide and several metres long have originated. After heavy rains a more than 10 m deep cleft with a short gallery filled with sediments opened near the Hunger Wall on the Petřín hill on March 29, 1961. Similar cleft on the Loreta Square was mentioned already by Přibík of Radotín, the chronicler of Charles IV (1346 - 1378). Further narrow and deep fractures are known from Střešovice and Prosek rocks. From the viewpoint of nature protection they are not very important; however, their impact on the life of the city may be considerable: the unstable, some 60 m wide zone along the edges of Cretaceous plateaux should not be loaded with buildings, which may be threatened by damage and even collapse.

Historical Underground
From the many types of historical underground phenomena two groups of underground structures are important for nature protection. They are water supply galleries and old mines giving access to geological formations which are difficult to reach otherwise (coal seams, ore layers, lower faces of strata with fossils and traces of life).
The most important underground work in Prague, which is a technical monument of European importance, is the Rudolph Gallery connecting the left hand Vltava embankment above Šverma Bridge with the gallery administrator' house (so-called Havírna) with a house near Šlechta restaurant in the Stromovka park. It conveys water to the ponds in the King's Game Preserve. It is 1 102 m long, 0.7 - 1.0 m wide and 2 - 3 m high and is provided with four shafts 22 - 49 m high. The gallery crosses the Letná formation and represents one of its best exposures. Locally it is decorated with rich stalactites. Emperor Rudolph had ordered its construction in 1581 and the miners of Kutná Hora silver mines completed the work in 1593. Since that time it has been repaired almost continuously, as a result of which it has been operating until the present day.
The Prosek underground between the streets Na Stráži and Hutě in Hloubětín represents a unique group of sites important for both natural science and history. A considerable part of the already accessible underground consists of sandstone quarries. Their crushed sandstone was used for floor sprinkling, manufacture of sand soap, fine stucco plaster or in foundries. Apart from that there are also water captation galleries (e.g. the gallery near the pond below the Vysočany elevated highway, not accessible at present) and former coal mines.
The Močálka in the Prosek rocks is a characteristic mining work. It is a fragment of a considerably longer underground labyrinth. The length of the galleries accessible at present is about 500 m, but the overall length of the labyrinth could attain 5 - 7 km. The mines served the extraction of white, kaolinitic sandstone of Peruce strata. The mineral was extracted manually, as the sandstone could be dug with picks.
Numerous margins of Cretaceous plateaux in Prague are actually the edges of customary open-cast quarries extracting the rusty Koryčany sandstone for construction purposes. The quarries stopped where fields began or where further quarrying was prevented by thick overburden. In some of these cases the works proceeded underground.
The beginnings of mining were recorded in writing only at the end of the 18th century, and the extraction continued until the first half of the 20th century. The unique character of the Močálka and, to a lesser extent, some other underground workings in Prosek consist in the fact that the miners during their work selected the softest parts of the rock, i.e. chiefly those parts where water leaked along tectonic faults. In this way they actually exhumed the natural state. The final form of the galeries recalls natural caves rather than man-made mine galleries and we can consider them as a certain germinal form of sandstone rock cities. While e.g. in the Kokořín Valley the softest sandstone parts have been eroded by natural factors, in the Prosek underground such "block" disintegration was due to human activities.
There are other galleries in the neighbourhood of the Močálka which, however, have mostly collapsed. They are called America I and America II. Also the three-level underground maze discovered in 1966 during the subsidence in the street Nad Krocínkou was of similar character. Its total length was about 300 m. The broken galleries were filled with concrete.
Another important underground phenomenon is the Bilý Kůň (White Horse) above the Hutě in Hloubětín. A 10 m deep well leads to a maze of galleries as many as 5 m high and some 350 m long. Until the beginning of the 60s these underground galleries were used for the storage of fruits and vegetables.
The Zlatý Kůň (Golden Horse) hill between the Kbely airfield, the Bažantnice wood and the Mladá Boleslav highway witnessed the repeated and often unsuccessful attempts at coal mining for almost a century. Every year as many as ten applications for coal mining permission were filed, especially between 1820 and 1850. Geological conditions are similar to those of the Petřín hill. Shallow coastal bog had created a coal seam of variable thickness which was exploited as fuel in case of good quality coal, or used for the manufacture of suplhuric acid if the sulphide content was high.
The historical underground works fall into the fuzzy zone between technical monuments and natural formations and generally suffer from unclear protection conditions. If the artificial underground work contains some valuable natural elements, it should be protected - similarly as e.g. an abandoned quarry - by the nature conservation agencies.

View from the cave above Rutice Mill in Radotín Valley (PR).

Prokop Cave - section (from V. Cílek, 1995).

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