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The ancient city of Hierapolis, located in modern-day Turkey 

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This Roman ‘gate to hell’ killed its victims with a cloud of deadly carbon dioxide

Is it possible to walk through the gates of hell and live? The Romans thought so, and they staged elaborate sacrifices at what they believed were entrances to the underworld scattered across the ancient Mediterranean. The sacrifices—healthy bulls led down to the gates of hell—died quickly without human intervention, but the castrated priests who accompanied them returned unharmed. Now, a new study of one ancient site suggests that these “miracles” may have a simple geological explanation.

Rediscovered just 7 years ago, the gate to hell at the ancient city of Hierapolis, in modern-day Turkey, is a stone doorway leading to a small cavelike grotto. The gate was built into one wall of a rectangular, open-aired arena, topped by a temple and surrounded by raised stone seating for visitors. The city itself sits in one of the region’s most geologically active areas; 2200 years ago, its thermal springs were believed to have great healing powers. But a deep fissure running beneath Hierapolis constantly emits volcanic carbon dioxide (CO2), which pours forth as a visible mist. The gate—also known as the Plutonium, for Pluto, the god of the underworld—is built directly above it. In 2011, archaeologists showed that the gate is still deadly: Birds that fly too close suffocate and die.

Now, a research team led by volcano biologist Hardy Pfanz at the University of Duisburg-Essen in Germany has studied the shrine’s killing potential in greater detail. Pfanz and his colleagues measured the CO2 concentration in the arena over time. During the day, the sun’s warmth dissipates the gas. But at night the gas—slightly heavier than air—billows out and forms a CO2 “lake” on the sheltered arena floor. It is particularly deadly at dawn, when the CO2 concentration 40 centimeters above the arena floor reaches 35%, enough to asphyxiate and kill animals or even people within a few minutes, Pfanz says. But concentrations fall rapidly with height.

The eunuch priests likely made their sacrifices in only the morning or evening hours, when the concentration of the gas was highest, Pfanz says. Sacrificial animals were not tall enough to keep their heads fully clear of the CO2 lake, and as they became dizzy, their heads would have dropped even lower, exposing them to higher CO2 concentrations and leading to death by asphyxiation. The priests, however, were tall enough to keep their heads above the dangerous gasses, and may have even stood on stones to add to their height. “They … knew that the deadly breath of [the mythical hellhound] Kerberos only reached a certain maximum height,” Pfanz says.

A 3D reconstruction of the Plutonium at Hierapolis. The Gate to Hell is located in the wall below the stone seating.

Massimo Limoncelli

Strabo, an ancient Greek historian who visited the Plutonium at Hierapolis about 2000 years ago, recorded that the priests could even place their heads within the gate to hell and suffer no ill effects. Strabo thought this immunity may have been due to their castration. Pfanz believes they were aware of the local chemical environment. For instance, he thinks the priests would have been careful to avoid getting too close to the gate other than at noon, when the shrine was relatively safe. Archaeologist Francesco D’Andria of the University of Salento in Lecce, Italy, who led the team that discovered the Plutonium at Hierapolis in 2011, isn’t so sure. His team has found many ancient oil lamps immediately around the gate to hell, suggesting priests may have ventured near it at night despite the dangerous CO2 levels.

The new findings are “tremendously exciting,” says Gil Renberg, a classicist who researches Greek and Roman religious beliefs at the University of Nebraska in Lincoln. “This scientific information proves the veracity of ancient sources and helps explain not only why people could enter, but also why animals would die.”

It’s likely that at least some of the other Plutoniums worked in the same way. Renberg thinks the chemical survey methods used by Pfanz and his team could help provide a firmer idea of the exact location of the gate to hell at a site called Akaraka, also in modern-day Turkey.