The cave, considered by the ancient Romans to be a gateway to the underworld, was so deadly that it killed all the animals that entered it without harming the human priests who led them.
Millennia later, scientists believe they figured out why – a concentrated cloud of carbon dioxide that suffocated those who breathed it.
The cave dates back to 2,200 years and was rediscovered by archaeologists at the University of Salento in 2011.
Located in a city called Hierapolis in ancient Phrygia, now in Turkey, it was used to sacrifice bull animals led by castrated priests through Plutonium – or Pluto’s Gate, to the classical god of the underworld.
As the priests led the bulls into the arena, people could sit on elevated seats in the arena and watch the fumes coming from the gate bring the animals to death.
“This space is full of steam so foggy and dense that one can barely see the ground. Every animal that passes inside will meet instant death. I threw the sparrows and they immediately exhaled and fell, ” written by the Greek historian Strabo (64 BC NL – 24 AD).
It was this phenomenon that alerted the archaeological team to the location of the cave. Birds flying too close to the entrance to the cave suffocated and fell dead – indicating that thousands of years later it is still as deadly as ever.
The culprit is underground seismic activity, according to volcanologist Hardy Pfanz of the University of Duisburg in Essen, Germany, who conducted research into the cave’s leaking gas as early as 2018. The crack deep below the region emits large amounts of volcanic carbon dioxide.
The team measured carbon dioxide levels in the arena associated with the cave and found that the gas – slightly heavier than air – had created a “lake” that rose 40 centimeters (15.75 inches) above the arena floor.
They found that this gas is dispersed by the Sun during the day, but is the deadliest at dawn after a night of accumulation. The concentration reaches over 50 percent at the very bottom of the lake and rises to about 35 percent at 10 centimeters, which can even kill a person – but above 40 centimeters, the concentration decreases rapidly.
During the day, there is still some carbon dioxide in excess of about 5 centimeters, as evidenced by dead beetles found by a research team on the floor of the arena. And inside the cave, they estimated CO2 levels to be between 86 and 91 percent in all circumstances, because neither the sun nor the wind could enter.
In its contribution, the team notes that the characteristics of the cave had a strong element of tourism. Tourists could be sold small animals and birds that they could throw on the arena floor to be sacrificed, and on holidays larger animals were sacrificed by priests.
“While the bull stood in a gas lake with its mouth and nostrils between 60 and 90 cm high, the great adult galli priests always stood upright in the lake, making sure that their nose and mouth were high above the toxic level of the serpentine breath of death.” the team wrote in its 2018 paper.
“It is said that they sometimes used stones to make them bigger.”
Within minutes, spectators saw the large and strong bulls succumb to the fumes, while the priests remained strong and healthy — supposedly about the power of the gods or priests.
However, the researchers believe that the priests were well aware of the properties of the cave and its arena, and for maximum effect they probably made great sacrifices at dawn or dusk on quiet days.
They could also put their heads inside or enter the cave itself at noon ceremonies to show off their own strength and hold their breath to survive.
But the presence of oil lamps, according to the researcher who found it, Francesco D’Andria, also suggests that the priests approached the cave at night.
However, although they performed their ceremonies, the discovery may have helped to study the location of other plutonium by studying seismic activity.
The research was published in a journal Archaeological and anthropological sciences.
A version of this article was first published in February 2018.