Lake Bolsena is a lake of volcanic origin in the northern part of the province of Viterbo called Alto Lazio (“Upper Latium”) or Tuscia in central Italy. It is the largest volcanic lake in Europe. Roman historic records indicate activity of the Vulsini volcano occurred as recently as 104 BC; it has been dormant since then. The two islands in the southern part of the lake were formed by underwater eruptions following the collapse that created the depression.

The lake has a long historic tradition. The Romans called it Lacus Volsinii, adapting the Etruscan name, Velzna, of the last Etruscan city to hold out against Rome, which was translocated after 264 BC, and its original location today has not been securely identified. The lake is bordered on one side by a modernized version of the Roman consular road Via Cassia.The lake has an oval shape typical of crater lakes. The long axis of the ellipse is aligned in a north–south direction. The bottom is roughly conical, reaching a maximum depth at a point in the middle. The entire lake is surrounded by hills and small summits.