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 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 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 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.

Elevations on the north of the lake are the highest, with a maximum of 702 m (2,300 ft). As the lake is at 305 m (1,000 ft), no hill is more than 397 m (1,300 ft) higher than it. On the northern rim of the caldera is San Lorenzo Nuovo ("New Saint Lawrence"), which was moved from an older site (a hypothetical San Lorenzo Vecchio, "Old Saint Lawrence") further down the slope to avoid malaria. The northern shore of the lake once featured marshes, breeding grounds for the mosquitos that carry the disease. Currently it is agricultural. At the site of old San Lorenzo are Etruscan antiquities. To the north of San Lorenzo Nuovo and the caldera rim is Acquapendente.

The hills to the east are 600 m (2,000 ft) to 650 m (2,100 ft). Bolsena extends upward on the northeast shore, with Orvieto 14 km (8.7 mi) further to the northeast, at the edge of the volcanic region. On the southeast of the lake is Montefiascone at an elevation of 633 m (2,080 ft), up on the ridge of Montefiascone caldera. To the south of the lake is Marta, on the right bank of the Marta River, sole effluent of the lake. The shore there is straight and developed. Elevations are within 100 m (330 ft) of the lake. Next to Marta are Valentano and Capodimonte, the latter being built on and around nearly the only headland on the lake, which forms a protective harbor. About 15 km (9.3 mi) to the south are Tuscania and Viterbo, the latter being the regional capital.

From Valentano north is the Latera caldera, a shallow crater perhaps half the size of Lake Bolsena, with Lake Mezzano (usually too small for the map) at the western end. On its north rim is Latera. The floor of the caldera is mainly agricultural although the uncultivable rocky lava flows have been left forested. Although the hills on the west side of Lake Bolsena are only slightly higher than those on the south, the terrain is somewhat too rough for settlement. Fields extend as far as they can into v-shaped valleys and there is no flat shore.

The hills to the north loom over the lake. At their western edge are Grotte di Castro and Gradoli.