The Ploutonion or Plutonium is a small Roman shrine built into a natural cleft in the rock from which carbon dioxide seeps in lethal quantities — a phenomenon known to the Greeks and Romans, who interpreted the cave as an entrance to the underworld. Bulls and small animals would die within minutes of entering; priests, taller and walking upright, could go in briefly without harm and built up a profitable temple business on the back of it.
The shrine was excavated in 2013 and a section of its travertine altar, marble facing, and dedicatory inscription to Pluto are visible behind a low railing. A safety fence keeps visitors well back from the cave mouth itself — the CO2 is still there. Birds occasionally drop dead inside the enclosure, which the guide will point out if you stand long enough.