High Quality Content by WIKIPEDIA articles! Lake Bolsena (Italian: Lago di Bolsena) is a crater lake of central Italy, of volcanic origin, which was formed starting 370,000 years ago following the collapse of a caldera of the Vulsini volcanic complex into a deep aquifer. Roman historic records indicate activity of the Vulsini volcano occurred as recently as 104 BC, since when it has been dormant. The two islands in the southern part of the lake were formed by underwater eruptions following the initial collapse of the caldera.