Around the second century BCE, the kings of Pergamon built a thermal space at Hierapolis, in what is now modern-day Turkey. Now, it lies in ruins, but something within still stirs – something far more ancient than those that once lived atop it.

Seven years ago, a “gate to the underworld” – also known as Plutonium, named after the Roman god of the underworld – was discovered there, emitting a curious haze that proved to be lethal to anything that stood inside the cave for too long. It’s known that humans that entered the cave fared better than the bulls, rams, and songbirds they often sacrificed, and a new paper has finally revealed why.

Initially spotted by ScienceMag, the paper – in the journal Archaeological and Anthropological Sciences – explains that the cave sits over a fissure that continuously releases pent-up geologic carbon dioxide in considerable quantities, even today. The concentration of carbon dioxide drops off with distance from the ground, though, which is why animals in the cave died but the taller priests who brought them into it often survived.

The team, led by the University of Duisburg-Essen (UDE), explain that stories of ritualistic sacrifices in the cave appear in the writings of several long-gone scribes. Strabo, a Greek geographer, once recalled: “This space is full of a vapor so misty and dense that one can scarcely see the ground. Any animal that passes inside meets instant death.”

Rather nonchalantly, he added: “I threw in sparrows and they immediately breathed their last and fell.”

Although the emergence of volcanic gases was known by this point, the team wanted to better understand the fine details of this sinister portal. Taking careful measurements over time, they also found out that the sunlight and wind during the day helped disperse the deadly mist.

This makes the hours before dawn the riskiest time to enter the cave. At night, concentrations were so high that they “would easily kill even a human being within a minute.”

“These emissions are thought to reflect the Hadean breath and/or the breath of the hellhound Kerberos guarding the entrance to hell,” they add.

The Eunuch priests were savvy enough to know how the concentrations changed throughout the day, which is why the team surmises that sacrifices took place when they were highest. Emerging from the cave seemingly unharmed, their “godlike” powers became the stuff of legends.

“The Galli [Eunuch priests] stood on stones around the poor bull or goat and demonstrated their supernatural powers,” lead author Prof. Hardy Pfanz, a researcher in volcanism and biology at UDE, told IFLScience. “At this height they could stand for 20-40 minutes without being endangered.”

Independent of these stones, “nobody could enter the gate to hell without getting asphyxiated,” but “if the Galli kept their breath for a while they could crawl into the gate up to their waist.”

There are global variations on this asphyxiation-by-carbon-dioxide phenomenon. Rather infamously, back in 1986, Cameroon’s volcanic Lake Nyos – which had accumulated and trapped centuries’ worth of dissolved carbon dioxide gas within its waters – unleashed its colorless, odorless payload all at once. The cascading gas wiped out every living thing in its path, including more than 1,700 people.

Although it’s not yet certain what triggered the sudden overturning of the lake and the subsequent release of gas, it could easily happen again – which is why authorities are artificially degassing it to make sure that it can’t.

A similar, but less deadly event took place a couple of years earlier at Cameroon's Lake Monoun. Although these so-called "limnic eruptions" are far more sudden and catastrophic compared to the perpetual emissions at Plutonium, both speak to the lethal power of geologic CO₂.

Rather curiously, traditional villagers living near Lake Monoun claimed that evil spirits periodically escaped the lake in order to kill people.

“More than 2,000 years ago, these phenomena could not be explained scientifically but only by the imagination of supernatural forces from Hadean depths or well-meaning gods,” the team’s study concludes. Times may have changed, but the power of (geological) hell clearly still inspires fear in the hearts of many.