'Gateway to Hell' Still Deadly After Thousands of Years

A portal leading to an underground cave in Türkiye emits CO2 at concentrations so strong that it can kill animals and humans at night or early in the morning.

The Gates of Hades in the ancient city of Hierapolis. Photo: Arkeonews — The Gates of Hades in the ancient city of Hierapolis. Photo: *Arkeonews*

Rediscovered in 2013 by Italian archaeologists following a hot spring, the Gate of Hell in the ancient city of Hierapolis, in modern-day Türkiye, is a stone entrance to a small cave. The entrance is located on the wall of a rectangular amphitheatre, topped with a shrine and surrounded by raised stone benches for spectators.

The city itself sits in one of the most geologically active areas in the region. 2,200 years ago, the hot springs here were believed to have healing properties. But a deep fissure beneath Hierapolis regularly spews out carbon dioxide (CO2), which seeps out as a visible mist. The Gate of Hades was built directly over it. In 2011, archaeologists proved that the gate was still deadly. Birds that flew too close to it were suffocated.

In 2018, a team led by volcanologist Hardy Pfanz at the University of Duisburg-Essen in Germany studied the threat posed by the portal in more detail. Pfanz and his colleagues measured CO2 concentrations in the stands over time. During the day, the warmth of the sun dissipates the gas. But at night, the slightly heavier-than-air gas swirls and forms a CO2 “lake” above the arena floor. The phenomenon is particularly dangerous at dawn, when CO2 concentrations 40 centimeters above the arena floor reach 35 percent, enough to suffocate and kill animals or even humans within minutes, according to Pfanz. But CO2 concentrations drop rapidly at higher altitudes.

Temple priests were more likely to sacrifice in the early morning or late afternoon, when CO2 concentrations were highest. The sacrificial animals were not tall enough to extend their heads above the CO2 pools. When they became dizzy, their heads would drop even lower, exposing them to higher concentrations of CO2, leading to death by asphyxiation. However, the priests were tall enough that their heads were above the toxic gas, and they could even stand on stone steps.

Strabo, an ancient Greek historian who visited the Gates of Hades in Hierapolis 2,000 years ago, recorded that monks even stuck their heads inside the gates and were unaffected. Pfanz suggests that the monks were aware of the local chemical environment. For example, they were careful to avoid getting too close to the gates at times other than midday, when the temple was relatively safe. Archaeologist Francesco D'Andria of the University of Salento in Lecce, Italy, who led the team that discovered the Gates of Hades in Hierapolis in 2011, is less certain. His team found several oil lamps around the gates of hell, suggesting that monks would still come near them at night despite dangerous CO2 levels.

An Khang (According to IFL Science/Science )

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