Lake Bolsena is one of the largest basins in Italy and has an elliptical shape with a perimeter of about 43 km. It was formed over 300,000 years ago following an intense seismic activity of the Vulsinio volcanic complex. The waters of the lake are fed mainly by rain, to a lesser extent by modest springs and small ditches included within the catchment area.

On the banks, the natural vegetation is dominated by oak woods which alternate with chestnut trees at the highest altitudes, large cultivations of vines and olive trees. The shores of the lake are mainly made up of plants with submerged roots and thick reeds. The particular microclimate favors a luxuriant vegetation, in particular on the Bisentina and Martana islands, allowing the growth of plants such as plane trees, eucalyptus, horse chestnuts, magnolias, cedars and seventeenth-century imported palms. Poplars, willows, alders and ash trees are also very frequent.

In the waters, the most common species of algae are those of gray-green color, belonging to the genus Chara; due to the considerable consistency of its colonies, Lake Bolsena has been defined as a “caraceous lake”.