This Roman ‘gate to hell’ killed its victims with a cloud of deadly carbon dioxide. By Colin Barras. sciencemag.org. February 16, 2018. 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.

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