Hierapolis means “sacred city”, because of the temples located at the site, was believed by ancients to be founded by the god Apollo. People in Hierapolis worshipped Apollo as their main God. Beside Apollo, Artemis-the goddess- had been very important too. Local people in Hierapolis had worshipped many other gods and goddesses such as Dionysus, Leto who was the mother of Apollo, Poseidon who was the reason of earthquakes, Pluto who was the god of underworld, Heracles etc. It is also thought the name ‘Hierapolis’ derives from the mythological figure Hiera, who was the wife of Telephus (son of Hercules and founder of Pergamum).
The Greek foundations of the Temple of Apollo still remain, although the rest is Roman, dated to the 3rd century AD. Similar to Delphi, the temple had an oracle tended by eunuch priests. It was believed in ancient times that the gases emitted from an adjoining spring called the Plutonium (dedicated to Pluto, the god of the underworld) were lethal to all but the priests who had a direct line to Hades. To demonstrate this point the priests would toss in birds and small animals and watch them die. The Plutonium is sealed off today because it continues to give off toxic vapours.
The Temple of Apollo Lairbenos was built to honor the cities principal god. Inside the temple was the Ploutonion (Pluto's Gate); it is a religious site dedicated to the god Pluto. The structure sits atop a cave that was used as a ritual passage to the underworld.