Hierapolis - Temple of Apollo / Plutonium

The Plutonium, a cave beneath the Temple of Apollo, was a source of poisonous gas, probably carbon dioxide. The temple with a 20x15m/65x50ft podium and a flight of stairs to a pillared porch was built in the third century from plundered materials. An annex concealed the entrance to the underworld, the Plutonium. The priests of Cybele, who were venerated here long before Apollo, the city's principal god, were in charge of the cave and it was their practice to bring in birds and sometimes even bigger animals who were killed by the rising gas. The priests were unharmed, their heads being above the level of the gas. The cave is no longer in existence and the temple has collapsed.