Explorations in the dark continent: Did microplastics and microfibres get here before us?

Microplastic and microfibre pollution is a global concern, however, karst areas remain understudied. Because of their properties, these anthropogenic microparticles are particularly hazardous, and easily transportable, reaching also remote areas. The underground world, called also dark continent, is a treasure of information, and remained the last frontier of terrestrial exploration: many parts of the underground world have not yet been accessed. In the hypogeal environments, pollution is closely linked to the connections between surface and subterranean habitats, the hydrodynamics of the aquifer, geology, and local environmental conditions. This study aims to investigate, for the first time, the presence of microplastics and microfibres in unexplored caves, revealing how human activity could indirectly impact even the uncontaminated environments of the dark continent. Together with speleologists, we collected and investigated sediment samples from unexplored caves of the Abruzzo Region, Italy. Examined anthropogenic microparticles were counted and characterized by composition, size, shape, fluorescence, and colour, via microscopy and spectroscopy. Microplastic concentrations resulted low or absent, moreover, natural and regenerated microfibres ones were higher. Fibre-shape was the most common. Most of the microparticles were clear and fluorescent under UV light. Pollution sources in this area likely include atmospheric deposition, nearby human activities, roads, and garbage. These results highlight anthropogenic microparticle pollution exists in unexplored karst caves, which could impact subterranean habitats, species, and water resources. Given the link between surface and underground karst environments, more monitoring and protection are needed. This work encourages speleologists to collect samples during explorations too, as these rarely studied environments offer crucial insights into karst systems, potential threats, and conservation needs. Future long-term studies will clarify pollutant sources, transport, and effects on ecosystems.