(from Wikipedia):
Lake Bolsena (Italian: Lago di Bolsena) is a crater lake of central Italy, of volcanic origin, which began to collect 370,000 years ago following the formation of a caldera in the Vulsini volcanic complex. 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 caldera.
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.[4] Fertilizers are a second source of contamination. However, the chemical content of the lake is monitored at several stations around it.
Lying within the northern part of the province of Viterbo that is called Alto Lazio ("Upper Latium") or Tuscia, the lake has a long historic tradition. The Romans called itLacus 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 updated forms 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.