Germinario et al. Herit Sci (2020) 8:87 https://doi.org/10.1186/s40494-020-00433-9 RESEARCH ARTICLE Open Access Taya Caves, a Buddhist marvel hidden in underground Japan: stone properties, deterioration, and environmental setting Luigi Germinario1* , Chiaki T. Oguchi1, Yasuhiko Tamura2, Sohyun Ahn1 and Momoko Ogawa1 Abstract The Buddhist sacred site of Taya Caves is a gem hidden underground in Yokohama, Japan. The caves were excavated and sculpted into bare rock by Shingon Buddhist monks from the Kamakura until the Edo period (thirteenth–nine- teenth century), and dedicated to ascetic training, rituals, and pilgrimage. They are a maze of halls and galleries decorated with hundreds of rock-cut reliefs, picturing deities and masters of Buddhism, temples and shrines, real and fantastic animals, vegetal motifs, mandalas, zodiac signs, family crests, etc. The history and rock art of Taya Caves and the urge to preserve their cultural value led to this frst-ever scientifc investigation, dealing with the stone properties, deterioration, and environmental setting. Textural, mineralogical, geochemical, and petrophysical investigations were combined with a microclimate monitoring and chemical analyses of groundwater and rainwater. The caves are exca- vated into a clay-rich fossiliferous siltstone, extremely soft and porous and highly susceptible to water-driven weath- ering. Water represents a constant in Taya Caves, either fowing, dripping, and stagnant; or rising from the subsoil; or related to the extremely high relative humidity. Crusts and eforescences represent important indicators of mineral dissolution and mobilization. The crusts are made of gypsum, crystallized from the dissolution of calcareous bioclasts and oxidation of pyrite, with minor calcite. The eforescences are composed of chlorides, phosphates, sulfates, and carbonates, possibly deriving from agrochemicals and the surface vegetation cover. The salt weathering is strictly related to the microenvironmental variables and physico-chemical properties of the phases and waters involved. Rock-water interaction is particularly damaging even considering just the physical mechanisms. The stone is strongly sensitive to water absorption, hygroscopic adsorption, and slaking: the stresses generated by in-pore water and air movement and the swelling clay minerals may lead to rapid disintegration, especially during cyclic processes. This research is expected to raise concerns about the safeguard of Taya Caves and support future monitoring and conser- vation plans, and to foster a wider promotion and valorization of this heritage site. Keywords: Rock art, Salt weathering, Gypsum crust, Eforescence, Slaking, Microclimate monitoring, Groundwater chemistry, Siltstone decay, Pyrite, Swelling clay minerals Introduction and archaeological standing structures, whereas under- Caves in heritage science ground sites usually receive scarcer attention. Tis dis- Te research on properties, weathering, and conser- parity recurs in heritage protection at institutional level. vation of natural stone in cultural heritage is largely Among the 908 sites of cultural or mixed value inscribed devoted to open-air monuments, historical architecture, in the UNESCO’s list of World Heritage Sites, less than sixty (about 6%) include caves, hypogea, mines, rock- *Correspondence: [email protected] hewn architecture, and subterranean settlements. 1 Department of Civil and Environmental Engineering, Saitama University, Yet the underground cultural landscape has a relation- 255 Shimo‑Okubo, Sakura‑ku, Saitama‑shi, Saitama‑ken 338‑8570, Japan ship with human activities persisted for tens of thou- Full list of author information is available at the end of the article sands of years, involving religious and artistic practices, © The Author(s) 2020. 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The Creative Commons Public Domain Dedication waiver (http://creat​iveco​mmons​.org/publi​cdoma​in/ zero/1.0/) applies to the data made available in this article, unless otherwise stated in a credit line to the data. Germinario et al. Herit Sci (2020) 8:87 Page 2 of 20 sheltering and housing, burial, food production and stor- stable as possible. If the site or visitors are in danger, clos- age, exploitation of raw materials, etc. [1, 2]. From a sci- ing the show cave, temporarily or permanently, repre- entifc viewpoint, it represents a natural or semi-natural sents an arduous but recommended decision [7, 16–19]. setting entailing a variety of environment-material inter- actions, which are much diferent from those in built her- Aim itage. Te multidisciplinary approach is fundamental. 田谷の洞窟) Cave vulnerability can be evaluated by diferent risk Tis study addresses Taya Caves ( , a Bud- factors: geological-geomorphological (structural and dhist sacred site in Yokohama, Kanagawa prefecture, slope stability, seismic events, etc.); environmental (water central Japan. Te history and rock art enshrined in Taya interaction and weathering); biological (biomass growth); Caves, their allure, and the urge to comprehend, pre- and anthropic (human presence, site planning, local serve, and promote their cultural value led to this frst- urbanization and exploitation of natural resources) [3]. ever scientifc investigation, dealing with stone properties Te risk can be quantifed and mapped with the support and deterioration, set against the environmental back- of geographic information systems and numerical model- ground. Te cave vulnerability was evaluated through an ling [4, 5]. Anthropic factors are often the most critical, experimental approach involving the analysis of micro- and indirectly increase the environmental and biologi- environmental variables, i.e., air temperature, relative cal risks. Visitors alter the cave microclimate, introduce humidity (RH), and water chemistry; and textural, miner- new organic matter and microorganisms, and sometimes alogical, geochemical, and petrophysical properties of the cause vandalistic damage [6, 7]; the increased air temper- rock of the caves. Close attention is paid to the environ- mental constraints of stone deterioration, with emphasis ature and CO 2 level, associated with tourist fows, may accelerate condensation phenomena and cave wall corro- on salt weathering and rock-water interaction. Elements sion [8–12]. of interest lie in the studied lithology, relatively unusual Investigating those risk factors allows for a deeper in caves, and the surprising conservation of the rock art, comprehension of deterioration processes, which may be despite the extreme material weakness. extremely severe and afect both the cave structure and Being the frst research on this theme, the results are rock art, namely sculpted and painted surfaces. Weather- expected to support future conservation plans. Te com- ing patterns are widely discussed in the literature about plementary goal is to foster a wider promotion and val- monument decay [13–15] and their characteristics are orization of the site, little known presently. analogue in the cave environment, although their origin and time evolution are dissimilar. Generally, closer atten- Historical background tion needs to be reserved to water-related decay, e.g., salt Te Buddhist practice of retiring to isolated underground weathering, mineral dissolution, and biodeterioration. places for worship and meditation originated in India, What requires a diferent perspective is cave conser- where, from as early as the third century BCE, monks vation. First, the planning, documentation, and future had excavated grottoes into the side of clifs, adorning evaluation of any conservation campaign greatly ben- them with altars, sculptures, carvings, and murals. Te efts from a continuous environmental monitoring, of artistic outcomes were sometimes outstanding, like in air (temperature, humidity, CO 2 and Rn levels, airfows), the Deccan cave temples of Maharashtra. Compared to water (composition, physical parameters, hydrogeology), open-air timber architectures, cave temples ofered bet- and illumination; and graphic representation, often based ter durability and protection against weather adversities, on 3D digital reconstruction techniques nowadays [16, natural catastrophes, and human-made disasters. Tere, 17]. monks could pursue ascetic rituals in isolation and quiet- Conservation and restoration treatments may target ness while beneftting from a settled life [20, 21]. the cave and rock art directly. Some examples include Tis practice spread in East Asia as Buddhism gradu- surface cleaning, consolidation, structural reinforcement, ally made new proselytes, reaching Japan in the sixth biocide application, removal of debris, dust, mud nests, century CE through the Korean Peninsula. Many caves