In this study, a systematic review of the scientific literature was carried out to summarize the emerging evidence on microplastic pollution in natural caves. After the screening of 655 papers on the topic from a combined search on the Web of Knowledge and the Scopus databases, we found only 14 studies reporting quantitative data on microplastics from a total of 27 natural caves. Most of the assessments focused on water and sediment, with very limited investigations concerning the cave biota. Overall, the most common types of particles found in caves were small (<1 mm) fibers (~70–90% of items), transparent or light-colored, mostly made of polyethylene and polyethylene terephthalate. Anthropogenic cellulosic materials, however, represented a non-negligible portion of particles (i.e., ~20–30%). Microplastic concentrations in caves varied between 0.017 and 911 items/L for water and 7.9 and 4777 items/kg for sediment, thus falling within the levels of microplastic pollution found in other terrestrial, freshwater, and marine environments. Levels of microplastic pollution appear largely variable among caves, stressing the need to extend the geographic and environmental ranges of the assessments, which are currently concentrated on Italian caves on land, with very few case studies from other regions of the world and from marine caves. Despite their putative isolation, natural caves have a high vulnerability to microplastic contamination, requiring much more research effort to understand the potential risk that plastics pose to these fragile ecosystems.
