Speleological studies in 1998, which followed a comprehensive research project, revealed the existence of over 70 m deep inverse arm of the cave siphon, which leads to the melting of limestone from the limestone massif of Beljanica. From the dam to the mill, the stream of water flows through the limestone, jumping from the steep sides and the waterfalls. The hydrological regime of the spring and its yield are not sufficiently tested. As the majority of karst springs, Krupaj spring has very changeable amount of water through the year. Close to the mill's construction, there is a thermal water spring, with the temperature of 26 °C and the flow of 6-10 litres per second. This water comes from a natural thermal spring that is sealed and escorted to the fountain. A dozen meters away from the fountain, another thermal phenomenon is represented by the spring in the form of a miniature waterfall that emerged from a self-extinguishing geological well.