Here is the oldest place worshipped by the local people in Hierapolis ancient city. Local people call here as a demons hole. The temple of Apollo was built beside of it because plutonium had religious importance. 

This hole is large enough to be entered by a person. The poisonous gas (Carbon Dioxide) comes out from plutonium and it was believed to be sent by Pluto, the god of under world. Birds and oxen that approached to plutonium died immediately. 

The religious men mentioned to the local people that they were created in a superlative position because they entered to hole by a breath control and came out of it alive. So all locals believed and obeyed them.