## 1 Introduction  
Subterranean cultural heritage sites, such as catacombs, ancient tombs, and other hypogean, sites are distributed underground worldwide and exhibit various forms and sizes [1, 2]. Hypogean sites are characterized by a stable temperature, high relative humidity, and limited air circulation year-round [3, 4, 5], and they can thus be conserved over the long term in a state unaffected by the external environment. However, human factors, such as tomb robbery, excavation surveys, and tourist activities, have altered the unique environment of hypogreal sites [6, 7] and have promoted the emergence and growth of microbes [8, 9]. Hypogean sites are threatened by the growth of a variety of biological species, including cyanobacteria, green algae, diatoms, and various other types of microorganisms [3, 10, 11]. Notably, microorganisms cause discoloration and stains and form biofilms in ancient tombs, which are irreversible changes that induce aesthetic and structural damages, posing a major challenge for conservation [8, 12, 13, 14]. The black or dark spots detected at the Takamatsuzuka and Kitora tumuli in Japan and the Etruscan hypogreal tombs in Italy were caused by _Accmonium_ colonization [7, 15]. The _Streptomyces_ strain isolated from ancient Egyptian tombs and Buyeo Royal Tomb No. 1 in South Korea promoted discoloration and physical damage to murals [16, 17, 18, 19]. Biocides based on alcohol, phenol, quaternary ammonium, and triazole compounds have mainly been applied in the past several decades to effectively remove microorganisms from cultural heritage sites [20; 21]; however, these agents pose additional risks related to their toxicity, lack of degradability, and environmental persistence [22; 23]. Certain biocides have been prohibited, as they were determined to be harmful to humans and the environment [24; 25]. Moreover, biocides could serve as a source of nutrition for some microorganisms and actually promote their growth; the continued use of a biocide could result in the emergence of resistant strains and, by reducing competition, could provide an opportunity for colonization by a new microbial species [26; 27; 28]. To resolve these issues, it is essential to investigate environmentally friendly biocides [24; 29; 30]. Preconditions for an appropriate biocide application at a cultural heritage site include being harmless to the user, having minimal impact on the environment, and--most importantly--not interfering with the substrate [31; 32; 33]. Natural biocides have attracted substantial attention as an alternative to conventional chemical biocides in the conservation of cultural heritage sites [13; 34; 35]. Plant-based essential oils (EOs) are obtained through a variety of extraction methods from the flower, stem, leaf, and fruit [36]; they contain volatile aromatic compounds characterized by high volatility and lipid solubility [37; 38]. Different EOs exert diverse effects, including antibacterial, antifungal, insecticideidal, antioxidant, and anti-inflammatory effects [36; 38]. Consequently, the application of plant-based EOs as green biocides in cultural heritage conservation has been investigated in recent years [22; 39]. Several EOs have been shown to exert potent antibacterial and antifungal effects on microbes isolated from various materials, including stone, resin, and wood [24; 29; 34; 40; 41]. To date, the applicability of EOs in the conservation and restoration of hypogean sites has been tested using Greek aromatic herb, shrub, and tree extracts on microorganisms isolated from caves [42], and the effect of EOs has been tested on microorganisms isolated from the mural paintings in ancient tombs [43]. However, no EO has shown 100% efficacy across microbial species [4]. Therefore, research is required to develop essential oils that do not harm murals created with various techniques and in enclosed environments such as ancient tombs.