Averno (ävĕr´nō), anc. Avernus (əvûr´nəs) [Gr.,=without bird], small crater lake, .6 mi (.9 km) wide, Campania, S Italy, between Cuma and Puteoli, near the Tyrrhenian Sea. Its intense sulfuric vapors, caused by volcanic activity (now extinguished), supposedly killed the birds flying over it, hence its name. The ancient Romans, impressed by its vapors and its gloomy aspect, regarded it as the entrance to hell; later the name was used for hell itself. Near the lake its personification, the deus Avernus, was worshiped.