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The site represents a specific type of geyser, which is different from typical geysers by its situation in a volcanic mountain which is now inactive, and by its artificial activation through a well which was 404,5 meters deep and its low temperature thermal water. The site is situated near the village of Herl'any in the middle part of the Kosická basin on west uphill of the mountains Slanske vrchy, which are a component of the volcanic neogene territory of the Carphatian arch. The sea covered the territory during the Tertiary period and detritic and pelitic sediments were deposited there. During the period of marine sedimentation, in the Sarmat period, volcanic activity was activated and conditioned the formation of the volcanic Slanske vrchy mountains. The complex of volcanic rocks of the Slanske vrchy covered the sediment. These rocks were created by a line of andesite volcans, which, as e. result of intensive post-volcanic erosion activity, retain just relicts of their original construction. In their construction we can see a central crater, transitional (stratovolcanic jacket;, and peripheral volcanic zones. The geological construction of the territory was cut by tectonic faults of a retrogressional character. Faults broke the complex of vulcanites, neogene sediments, and substrate paleogene rocks and were important factors in establishing the regime and circulation of underground water. Herl'any geyser activity is maintained mostly by a system of rain water infiltration, the circulation of underground water and by a system of carbon oxide admission. The carbon oxide acts as a main energy source causing fluctuating water eruptions. The water is filled with carbon oxide and after that the whole system works on the siphon principle. A spa existed in herl'any from the 18th century but its development required a necessary and sufficient supply of curative mineral water. Thus, well work was started in 1870 and the first horizon of underground water was struck at a depth of 111 meters. When the well reached a depth of 172 meters the first eruption occurred. The next eruption occurred when a depth of 275 meters was achieved. When the well achieved a depth of 330 meters water erupted for 10 days reaching to a height of 112 meters. The well was finished after reaching a depth of 404.5 meters. Eruption intervals in the beginning were 8 - 9 hours and the intensity of the water was 21 - 36 litres per second. The current eruption intervals are between 32 - 34 hours, with the water reaching a height of 15 meters. The eruption lasts approximately 26 minutes and has an average intensity from 25 to 30 litres per second. The interval between eruptions is shortened in the case of efficient rainfalls. Although time between individual eruptions varies and the intensity decreases, it is assumed, if geyser will not be mechanically damaged, that it will work for a long time. In 125 years of existence, the geyser has erupted more than 40000 times and sprinkled on the surface around 20 million of partially mineralised water. Water erupted by the geyser is not extremely well-suited for curative purposes, because the pharmacologically effectivesubstances are shaken from the water during the eruption.