In the collective memory of communities from various villages in the Western and Northern Bulgaria the idea about the chthonic entity called the water bull is kept alive. The most developed and preserved beliefs about this creature have been found among the inhabitants of the villages nearby the Pastrina Mountain, in the NorthWest administrative regions of Montana and Vratsa.
The analysis of the texts on the water bull and its water source shows that local communities have adapted the facts from the natural environment to their models of knowledge by using the established thesaurus of traditional concepts and ideas. In this way, the actual hydrological processes have been encoded through the use of folkloric and mythological models, and the knowledge about different stages of local and regional karst history is preserved.
Keywords: water bull, folklore thesaurus, demonology, chthonic creature, hydrology, karst history