Hidden in a corner of Hierapolis above Pamukkale, the Ploutonion — known to Greeks and Romans as the Gate of Pluto — is a small ruined sanctuary built over a natural vent in the rock. The vent releases volcanic carbon dioxide which, because it is heavier than air, settles in a shallow layer that suffocates anything that crouches near it. Roman priests used the effect to spectacular religious advantage: animals brought to the gate by pilgrims died on the spot while the priests, standing tall on the steps above the cloud, lived.
The site lay forgotten until 2013, when Italian archaeologists pieced together the description in Strabo with a marble inscription found at Hierapolis and confirmed the vent. You cannot enter the inner chamber — the gas is still lethal — but a viewing platform lets you see the steps, the broken ionic columns, and the small pool where the cloud forms. It is a few minutes' walk from the Antique Pool.