The cave Eisensteinhöhle is a 2-km long crevice cave that is significantly overprinted by hydrothermal karst processes. It was opened during quarrying in the Fischau Hills, on the western margin of the Vienna Basin. This pull-apart basin cuts the eastern foothills of the Alps and is formed by a major NE-SW striking, sinistral transform fault. The western margin consists of NNE-SSW striking normal faults that create paths for thermal water to rise from the central basin. The deepest part of the

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... ave, 73 m below the entrance, hosts a pond with 15.5 +/- 0.4 °C warm water, that sometimes acts as a spring. The cave and in particular the spring have attracted attention since their discovery in 1855. The water level and temperature fluctuate and at a certain level, water visibly discharges into a nearby narrow fissure. A record of sporadically taken data on water level and discharge exists from the late 1980s on. As there was no obvious connection to precipitation events or periods, it was impossible to determine how the spring is connected to an aquifer and where the water comes from. Gerhard Winkler, a local cave guide, began systematic observations with the installation of a stick gauge in 1992 and took measurements whenever he visited the cave. In October 2015, a data logger (SEBA Dipper PT) was installed for automated measurements of the water level and temperature of the spring. A pumping test, conducted on 13/7/2016, showed the geometry and the volume of the spring of about 2.8 m³ and that it is fed by a very small inlet out of the sandy bottom. At the time of the pumping test, the discharge was only 4.5 l/h but during previous overflow events, discharges of up to 289 l/h were recorded. Water temperature and hydrochemistry hint towards a mixture of an old thermal component and a young meteoric component. Within the first two years of the continuous monitoring, the water level was almost stable with few periods of high level (almost at overflow) that lasted for about 3 to 4 weeks each. The water temperature increa [...]