Ploutonion

Its precinct encompassed a sacred grove, a temple dedicated to Plouton and Persephone, and an adjoining cave called the Charonion, after the ferryman of the dead.

[4] Pluto's Gate, the ploutonion at Phrygian Hierapolis (modern Pamukkale in Turkey), was connected to the local cult of Cybele.

Archaeological excavations in the 1960s showed that the ploutonion had been located within the sacred precinct of Apollo: "it consisted of a natural opening along a wall of travertine, leading to a grotto in which streams of hot water gushed forth to release a noxious exhalation".

[6] Strabo further records that Lake Avernus in Italy had been taken as a ploutonion because the gases it produced were so noxious that they overwhelmed birds flying overhead.

According to earlier sources, he says, this was the oracle of the dead (nekumanteion) sought by Odysseus in Book 11 of the Odyssey; Strabo, however, seems not to have himself regarded Avernus as a ploutonion.