A volcano's fiery red secret
Monte Amiata, an extinct volcano in southern Tuscany, contains toxic mercury within its mineral-rich subsoil. For centuries, this region was a global center for mercury production, thanks to vast deposits of a bright red mineral called cinnabar (HgS). Mining activity here dates back to Etruscan times, but it was the industrial-scale exploitation from the mid-19th century until the 1970s that left a lasting environmental scar. The Monte Amiata district became one of the world's most important mercury producers, yielding over 100,000 metric tons of the liquid metal between 1870 and 1980.
The main mining center was Abbadia San Salvatore, where operations continued until 1972. The process involved excavating the cinnabar ore and heating it in furnaces to release the mercury as a vapor, which was then condensed. This industrial activity dispersed enormous quantities of mercury into the local environment. Decades after the last mine closed, huge piles of roasted ore waste, known as calcines, and contaminated soil continue to leach mercury into the region's waterways, starting a long and complex toxic path.
From mine to methylmercury
The environmental problem begins when rainfall and runoff carry mercury from the old mine sites into the Paglia River. This river flows into the much larger Tiber River, carrying the contaminant for more than 100 kilometers downstream. Inorganic mercury from the mines is toxic, but a more dangerous transformation occurs in the river system.
In the oxygen-depleted sediments of reservoirs like the Alviano dam, anaerobic bacteria get to work. These microorganisms ingest the inorganic mercury and, through a metabolic process, convert it into methylmercury (CH₃Hg⁺), a highly toxic organic form. This neurotoxin is more easily absorbed by living organisms.
Methylmercury enters the aquatic food web, beginning with plankton and small invertebrates. As larger animals consume the smaller ones, the concentration of the toxin increases at each trophic level—a process called biomagnification. Fish, particularly long-lived predatory species, accumulate the highest concentrations in their tissues. This contamination extends far from the original mining source, posing a potential risk to wildlife and humans who might consume fish from the Tiber River basin. The pollution is widespread, with contaminated sediments found at least 200 km from the source.