Origin and Environmental Role of Negative Air Ions in the Ravne Underground Complex, Bosnia-Herzegovina: A Multidisciplinary Geological, Hydrological, and Microclimatic Analysis

Abstract

The Ravne Underground Complex near Visoko, Bosnia-Herzegovina, has attracted scientific attention due to unusually high concentrations of NAIs (negative air ions) measured within its tunnel system over a fifteen-year period. Measurements conducted between 2010 and 2025 frequently recorded NAI concentrations exceeding 20,000 ions/cm3, with peak values substantially higher than those commonly reported in forests, caves, mountain environments, or urban settings. To investigate the possible origin of these elevated concentrations, environmental measurements were evaluated together with geological, petrographic, hydrological, and microclimatic observations collected throughout the tunnel network. The underground system is developed within quartz-bearing conglomerate formations overlying marl and sandstone deposits. Petrographic analyses identified quartz, quartzite, calcite, chert, sandstone, shale, schist, and basalt within the conglomerate matrix. Permanent underground water circulation, stable humidity, relatively constant temperatures, and limited atmospheric disturbance characterize the subterranean environment. These conditions may favor ion generation through water-air interactions and contribute to the long-term persistence of ionized air within confined underground spaces. The combined evidence suggests that elevated NAI concentrations in the Ravne Underground Complex result from the interaction of geological composition, hydrological activity, and stable microclimatic conditions. The site represents a valuable natural laboratory for future research in environmental science, hydrogeology, cave atmospheres, geophysics, and atmospheric ionization processes.