The foundations of the Temple of Apollo at Hierapolis remain today. Once the heart of the city, this was where people came to consult the temple's oracle. The adjoining spring, known as the Plutonium, was where people gave sacrifices to the gods. Apparently only the temple's eunuch priests understood the secret of holding one's breath around the toxic fumes that billowed up from Hades, immediately killing the small animals and birds they sacrificed.
Temple of Apollo