Ancient city of Hierapolis, located in modern-day Turkey,
Ancient city of Hierapolis, located in modern-day Turkey,Creative Commons

A cave in the ancient Roman city of Hierapolis, which is now in modern-day Turkey, is known as the "gate to hell" or the "portal to the underworld". The reason behind the deadly name is the mysterious death of all animals taken inside it. However, the cause of the deaths was unknown so far.

Scientists now believe that they have figured out the reason behind the deaths, and blame a concentrated cloud of carbon dioxide for it. Though most of the deaths happened centuries ago, the scientists warn that the place is still "deadly", as the cave is still full of the dangerous gas.

Romans used the "gate to hell" to stage sacrificial rituals in which castrated priests would lead healthy bulls through the entrance, and the animals died quickly, but the priests returned unharmed, Science Alert reported.

Greek historian Strabo, who lived from 64 BCE to 24 CE, wrote: "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. I threw in sparrows and they immediately breathed their last and fell."

However, researchers at the University of Duisburg-Essen in Germany in a new study have explained the reason behind the deaths. 

The city is in one of the region's most geologically active areas; in fact, around 2,200 years ago, the thermal springs of the city were believed to have great healing powers. However, they also had a deep fissure running beneath Hierapolis that constantly emitted volcanic carbon dioxide, and the gate, which is also known as the Plutonium — for Pluto, the god of the underworld — is built directly above it, Science Magazine reported.

The researchers believe the priests survived because they were aware of the concentrated levels of carbon dioxide. They would possibly hold their breath or stand above the "lake" of carbon dioxide.

The team wrote in the paper: "While the bull was standing within the gas lake with its mouth and nostrils at a height between 60 and 90 cm, the largely grown priests (galli) always stood upright within the lake caring that their nose and mouth were way above the toxic level of the Hadean breath of death."

The researchers measured carbon dioxide concentrations in the arena connected to the cave used for animal sacrifices and found that the gas, which is slightly heavier than air, formed a "lake" that rose 40 cm (15.75 inches) above the arena floor.

"The concentrations of CO2 escaping from the mouth of the grotto to the outside atmosphere are still in the range of 4-53 percent CO2 depending on the height above ground level. They reach concentrations during the night that would easily kill even a human being within a minute," the researchers wrote in the paper.

Inside the cave, they estimated that the levels of carbon dioxide range between 86 and 91 percent at all times, as neither sun nor wind can enter the place.

The study was published in the journal Archaeological and Anthropological Sciences.