The Plutonion (Pluto's Gate)
A cave that still kills birds. Priests led bulls in and walked out alive; the CO₂ pooling at ankle height did the rest.
Enough context, no spoilers
Why it matters
A sanctuary of Pluto over a fissure venting volcanic carbon dioxide, described by Strabo and Cassius Dio as a place where sacrificial animals dropped dead. Christians walled up the entrance in the 4th century and it was lost until Francesco D'Andria's team relocated and cleared it in 2013. Researchers later measured up to 91 percent CO₂ inside the grotto.
What to know before you go
Strip away the "gateway to hell" branding and what is left is better than the legend. A fissure under Hierapolis vents volcanic carbon dioxide. Because CO₂ is heavier than air it settles into a layer near the ground, thickest in the cool still hours before the sun stirs it. Ancient priests understood the practical consequence perfectly: a bull led into the enclosure at dawn had its head in the lethal layer and died; a man standing upright beside it had his head above it and did not. That was the miracle, performed for a paying audience seated on stepped benches above the gas. Modern measurement has confirmed the mechanism — up to 91 percent CO₂ inside the grotto, enough to kill in minutes. Which is why the visitor experience today is entirely from behind a barrier. The cave mouth and the seating are fenced; there is no way in and no reason to want one. Birds and insects still die at the mouth. Treat the ground level with respect, especially first thing on a cold clear morning, and do not put a child down there for a photograph.