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 is supplied entirely from the aquifer, rainfall and runoff, with one outlet at the southern end. A sewage treatment plant filters most of the raw sewage from the surrounding communities. Constructed in 1996, it features pipelines transporting the sewage from every major community around the lake to the treatment plant on the Marta River; that is, no effluents enter the lake. Fertilizers are a second source of contamination. However, the chemical content of the lake is monitored at several stations around it. 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. In addition to the historic sites of all periods, Lake Bolsena is currently surrounded by numerous tourist establishments, largely for camping, agrotourism and bed and breakfasts. One third of the lake was donated to the Catholic church by the noble family Alberici of Orvieto. In recognition of the donation the Alberici family was honored with a ceremony three times a year performed by the Bishop of Orvieto. Geography 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 on the flanks and summits of which are the comuni. The watershed was home to 22,000 permanent residents in 2004, and 35,000 in the summer season. Elevations on the north of the lake are the highest, with a maximum of . As the lake is at , no hill is more than higher than it. On the northern rim of the caldera is