In the heart of the spa is the thermal spring. We find it in the central part of the spa centre. Superficially, it is captured in a stone many-angled dome, covered with translucent panes that connect at the top of the cut-off pyramid. Through the panes, we can notice the slightly foaming water.
The water is without colour scent or taste. If disturbed it attains a milky appearance. In each season of the year, it has a temperature of 36°C that holds very well. It is conjectured that the water arrives from a depth of 1200 metres from the calcareous level on which in the later geological eras, younger surface formations sunk and formed. Supposedly, the water seethes out from the interior in five veins. One is stronger, four weaker.
Fro the collecting spring the water is immediately directed to the pool and marble cabins. Just under the outflow into the pool there is a gate on the pipe, which enables the water to be directed into twelve marble bathing baths. Several steps in front of the outflow into the pool, there is a shaft from which with the help of a pump drinking water is obtained. The water flows into the pool from four other springs, one medium and three smaller ones.
The thermal sap spring has extraordinary physiochemical properties for which we can use it for treating many illnesses. The thermal water in Dobrna, which is carbon acidulous (3.4 % gaseous CO2), acrothermal (35 to 4oC) with a slight radioactive action, and contains a variety of inorganic substances, is used for thermal baths (cabin, pool) for pine and salt baths (2 to 3% NaCL), for marsh bathes and for marsh and fango compresses.
(From the book by Janko Orožen, ‘Dobrna, preteklost in sedanjost zdravilišča, kraja in okolice, 1975’)
