Lago di Bolsena is the largest volcanic lake in Europe, formed in the crater of an ancient volcano about 370,000 years ago. The water is clean, the shore is largely undeveloped, and in summer the lake is the main reason people come to Bolsena. Swimming is excellent from the rocky beach below the town and from several other access points around the perimeter.
Boat rentals are available from the lakefront, and it is possible to row out to the two small islands. The lake also has a reputation for eels, which appear in local restaurants prepared alla brace or in umido; the eel of Bolsena was famously served to Pope Martin IV in 1281, and the mediaeval poet Dante placed the pope in Purgatory specifically for his gluttony in eating too many of them.