[...] First, near the Mesogis, opposite Laodiceia, to Hierapolis, where there are the hot springs and the Plutonium [τὸ Πλουτώνιον], both of which have something marvellous about them; for the water of the springs so easily congeals and changes into stone that people conduct streams of it through ditches and thus make stone fences consisting of single stones, while the Plutonium, below a small brow of the mountainous country that lies above it, is an opening of only moderate size, large enough to admit a man, but it reaches a considerable depth, and it is enclosed by a quadrilateral handrail, about a half a plethrum in circumference, and this space is full of a vapour [ὁμιχλώδης] so misty and dense that one can scarcely see the ground.
Now to those who approach the handrail anywhere round the enclosure the air is harmless, since the outside is free from that vapour in calm weather, for the vapour then stays inside the enclosure, but any animal that passes inside meets instant death [τῷ δ’ εἵσω παριόντι ζώῳ θάνατος παραχρῆμα ἀπαντᾷ]. At any rate bulls that are led into it fall and are dragged out dead; and I threw in sparrows and they immediately breathed their last and fell.
But the Galli [Γάλλοι], who are eunuchs, pass inside with such impunity that they even approach the opening, bend over it, and descend into it to a certain depth, though they hold their breath as much as they can (for I could see in their countenances an indication of a kind of suffocating attack, as it were), - whether this immunity belongs to all who are maimed in this way or only to those round the temple, or whether it is because of divine providence, as would be likely in the case of divine obsessions, or whether it is the result of certain physical powers that are antidotes against the vapour. [...]

(Geographica XIII 4, 14]

Das Plutonion von Hierapolis (in Phrygien) galt als einer der Eingänge zur Unterwelt.

Strabon zeigt sich ja sehr skeptisch ... und experimentiert sogar mit Spatzen.
Seine Erklärungshypothesen erscheinen freilich nicht so gelungen wie die moderne, auf einem Experiment beruhende: Durch eine Kluft steigt CO₂ aus dem Erdinneren, das sich, weil schwerer als die normale Luft, unten lagert, so daß die Tiere daran erstickt sind, während die - hochgewachsenen oder auf Steinen stehenden - Priester unbehelligt blieben.
Woher wußten die Priester davon? Sie können es mit Kerzen erprobt haben.

Ein Artikel dazu ist u.d.T. "Am Jenseits" in der FAZ vom 26. Februar 2018 erschienen.