The lake is the deepest, about 170 m (560 ft), in Lazio. It is about 3.5 km (2.2 mi) long by 2.3 km (1.4 mi) wide, formed by the overlapping union of two volcanic craters, an origin indicated by the ridge in its center, which rises to a depth of 70 m (230 ft). Plutarch reports that in 406 BC the lake surged over the surrounding hills, despite there being no rain nor tributaries into the lake to explain it (Life of Camillus). The ensuing flood destroyed fields and vineyards, eventually pouring into the sea.
Around 395 BC, during the wars between Rome and Veii, a discharge tunnel was built crossing the crater walls. It served as an emissary whenever the water level overflowed its mouth. According to Titus Livius, this feat of engineering was incited by the Oracle of Delphi: the Roman victory against Veii would be possible only when the lake waters were channeled and used for irrigation. The emissary is at 293 meters over the sea level. The tunnel (1200 m long, 1,20 m wide and 2 m high) ends at a spot called Le Mole, below Castel Gandolfo.