## 1 Introduction  
Cave ecosystems display unique environmental conditions that are unfavorable for the life and development of most living organisms [1]. They appear as highly zonal environments with characteristics defined by the ground waters, surrounding rocks, and karst morphology [2; 3; 4]. The conditions of subterranean facilities significantly differ from the surface and include, i.e., a constant temperature through the year (oscillating between 6 to 10 \({}^{\circ}\)C), high humidity (often close to saturation) [5], minimal to no light, restricted or absent air flow with the outside environment, oligrophy, and sometimes elevated levels of CO\({}_{2}\)[6; 7]. All of the above occur especially in the deeper parts of the underground sites [8; 9; 10; 11]. As a result, only a few microorganisms possess specific adaptations to cope with such living conditions [9; 12]. Thus, the extreme microclimate within the underground makes it a unique source of extremophilic bacterial or fungal isolates [7]. Fungi in the cave environments play important roles--mainly as parasites/pathogens, saprotrophs (decomposers), or mutualistic fungi (e.g., mycorrhizae), but also participants in geological processes [4; 13; 14]. Some fungal species are known to parasitize insects [15; 16; 17], while others may contribute to the inorganic nutrient pool replenishment by dissolving the bedrock [18]. Another unique feature of underground fungi is their ability to take part in the formation of speleothems [19]. The majority of fungal taxa frequently found in caves are cosmopolitan species and might inhabit various environments associated with soils, plant material, or insects [14]. In general, cave environments are nutrient-poor [13; 20; 21], and hence, they are extreme habitats providing specific ecological niches only for highly specialized microorganisms. However, not every underground site is oligotrophic, as some caves receive a significant input of soil and organic matter [22]. They originate mainly from dripping waters and animal feces [22; 23; 24]. Soil and organic substrates can be also brought inside due to external air currents, as well as accidentally by tourists [25; 26]. Ascomycota is the dominant fungal phylum in underground ecosystems, representing \(\sim\)69% of all cultivated fungi [27]. The Basidiomycota phylum makes up 20%, Zygomycota 6.6%, Mycetozoa 2.6%, Oomycota 1%, and other phyla (namely, Amoebozoa, Phytridomycota, Microsporidiomycota, and Percolozoa) 0.8% [14]. Some studies have suggested that the most commonly found taxa might be a result of the specific study methodology rather than biological patterns in cave mycology. For instance, the use of nutrient-rich media and incubation at room temperature (above 20 \({}^{\circ}\)C) support the growth of the commonly reported fungi that belong to Ascomycota [14; 25]. In general, fungi have a high plasticity and their capacity to adopt various forms (filaments, yeasts and yeast-like fungi) in response to adverse conditions allows them to inhabit various environments, including soil [28]. In addition, fungi are capable of producing a wide variety of extracellular enzymes which decompose the underground organic residues (such as soil components) [29; 30; 31]. Fungi usually dominate in highly or slightly acidic soils [32]. However, as shown by spelemoyological research by [PERSON] et al., fungi can also inhabit alkaline calcareous soils [33]. Additionally, many fungal species are able to act as an effective biosorbent of toxic metals such as cadmium, copper, mercury, lead, and zinc [34]. It should be noted that assessing fungal diversity within underground sites is a relatively new discipline, as in the past, only the emergence of selected fungal species was evaluated [14]. Among them, _Pseudogymoacus destructans (Pd)_, causing a bat disease called white-nose syndrome, brought growing attention to this matter [35]. _Pd_ is able to persist between hibernation cycles of bats within cave soil before infecting bat roosts.