A Caldera Born of Steam
Lago di Bolsena occupies a massive depression in the Monti Vulsini volcanic district, a region of intense volcanic activity that shaped central Italy. The lake basin is a volcano-tectonic depression, formed by a series of collapses that began around 600,000 years ago. The most significant event occurred approximately 300,000 years ago when rising magma encountered a massive underground aquifer. The resulting phreatomagmatic eruption was exceptionally violent. The instantaneous conversion of groundwater to high-pressure steam produced a massive explosion that blasted away the overlying rock and caused the surface to collapse inward, creating the caldera.
This process differs from a typical eruption where a volcano's peak blows off. Instead, the ground subsided over a vast area, leaving a roughly circular depression that would eventually fill with water. The lake today measures 13 km by 11 km and has a maximum depth of 151 meters. It is the largest lake of volcanic origin in Europe. The entire Vulsini volcanic complex covers an area of about 2,200 square kilometers and is composed primarily of pyroclastic products like tuff and ash, evidence of its explosive history. The towns of Orvieto and Civita di Bagnoregio are built upon thick ignimbrite sheets—rock formed from pyroclastic flows produced by these ancient eruptions.
The Hidden Funnel and Volcanic Islands
The lake's placid surface hides the violence of its formation. High-resolution seismic reflection surveys, which use sound waves to map subsurface geology, have revealed the original explosion crater hidden beneath the lake floor and subsequent sediments. This data shows a distinct funnel-shaped structure, known as a diatreme, extending 150 meters below the lakebed. This shape is the classic geological signature of a maar-type eruption, where magma and groundwater interact explosively. The lake's current depth of 151 meters, combined with the buried crater, points to an immense initial cavity.
The volcanic story did not end with the main caldera collapse. Subsequent underwater eruptions continued to shape the lake. These later events formed the two islands that rise from the southern part of the lake: Isola Bisentina and Isola Martana. Geologically, they are tuff cones, formed by less intense phreatomagmatic eruptions that built up layers of consolidated volcanic ash on the lakebed until they emerged from the water. Isola Martana, the smaller of the two, is the exposed portion of a submerged volcanic crater. The area still shows signs of geothermal activity, with gas emissions from the lakebed and numerous thermal springs in the surrounding region.