In Hierapolis, within the sacred area, is the oldest local sanctuary, called the Plutonium or Chronion, or Charonion a shrine to the god of the underworld. The Plutonium was described by several ancient writers including Strabo, Cassius Dio, and Damascius. It is a small cave, just large enough for one person to enter through a fenced entrance, beyond which stairs go down, and from which emerges suffocating carbon dioxide gas caused by underground geologic activity. Behind the 3 square metres (32 sq ft) roofed chamber is a deep cleft in the rock, through which fast flowing hot water passes releasing a sharp smelling gas. Because people died in the gas, people thought that the gas was sent by Pluto, god of the underworld.
During the early years of the town, castrated priests of Cybele descended into the Plutonium, crawled over the floor to pockets of oxygen or held their breath. Carbon dioxide is heavier than air and so tends to settle in hollows. They then came up to show that they were immune to the gas. People believed a miracle had happened and that therefore the priests were infused with superior powers and had divine protection.
An enclosed area of 2,000 square metres (22,000 sq ft) stood in front of the entrance. It was covered by a thick layer of suffocating gas, killing everyone who dared to enter this area. The priests sold birds and other animals to the visitors, so that they could try out how deadly this enclosed area was. Visitors could (for a fee) ask questions of the oracle of Pluto. This provided a considerable source of income for the temple. The entrance to the Plutonium was closed off during the Christian times.