In the ancient city of Hierapolis there was a cave said to kill any sacrifice led towards it. So deadly was this cave, that the Romans thought it to be the entrance to the underworld. It’s still lethal today: if you travel to this site, now part of Turkey, you may see birds fall dead from the sky. Scientists finally think they know why.

It turns out the cave emits a toxic cloud of carbon dioxide, suffocating those that breathe the air near it. According to a study published in Archaeological and Anthropological Sciences, seismic activity beneath the ground of the cave means large amounts of volcanic carbon dioxide seeps into the surface.

Measurements taken from the archeological site found a “lake” of the poisonous gas rising 40cm above the arena floor – caused because CO2 is heavier than air. At the mouth of the deadly grotto the scientists found concentrations of 4-53% CO2, growing more intense at night. Those that stand there are likely to be killed within a minute.

In ancient Greco-Roman Hierapolis, the cave was used for animal sacrifices, led through the Plutonium (Pluto’s gate) in reverence of the Roman god of the underworld. Priests would lead bulls into the arena, with spectators lining raised seats, for the animals to be killed in the haze.

“This space is full of a vapour so misty and dense that one can scarcely see the ground. Any animal that passes inside meets instant death. I threw in sparrows and they immediately breathed their last and fell,” wrote Greek historian Strabo when he visited the Plutonium.

For a long time, these accounts were the only evidence of such a “gateway to the underworld”. It was only a few years ago that the Plutonium was rediscovered, partly thanks to dead birds close to its opening.

(Above: The site of the Plutonium as it stands today. Credit: Francesco D’Andria/University of Salento)

If sacrificial bulls were killed by the carbon dioxide, why were the priests left unharmed? Strabo hypothesised in his ancient account that the priests were not killed because they were castrated, although he also noted that they held their breath and didn’t go in as far as the animals. In the new study, the researchers think it may also have something to do with the height of humans compared to bulls.

“While the bull was standing within the gas lake with its mouth and nostrils at a height between 60 and 90 cm, the large grown priests (galli) always stood upright within the lake caring that their nose and mouth were way above the toxic level of the Hadean breath of death,” the paper explains, adding that there are also reports of priests standing on stones to make them taller.

It’s also possible that the priests had a better idea of the cave’s properties, and how the gas would be weaker during the day than the night. It’s thought that they put on quite a show for ancient tourists, who would be sold birds and other small animals to throw for themselves into the arena.

After all, what’s a hellish portal without a bit of audience participation?

(Lead image: A reconstruction of the Hierapolis Plutonium. Credit: Francesco D’Andria/University of Salento)