According to the ancient Romans, the Mediterranean is riddled with places where mortals can access the underworld. These “gates to hell” (or Plutoniums) are marked by stone structures, and some of them, like a cave in Hierapolis (now modern-day Turkey) seem to have supernatural powers. Ancient Romans would bring animals into the mysterious haze inside the cave, where they would swiftly die. Now, scientists have answered the mystery of what is killing these animals and how humans could escape seemingly unscathed.

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According to the ancient Romans, humans would enter the grotto as part of a ritualistic sacrifice and leave unharmed, while animals would quickly die. The Greek geographer Strabo once said, “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.” Some believed that the vapor was the breath of the hellhound Kerberos. Legend also has it that even birds flying by would drop out of the air.

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Scientists have found that the cause of this deadly mist is actually carbon dioxide from a volcanic fissure in the earth underneath the cave. Concentrations of carbon dioxide are stronger towards the ground, which helps explain why animals were impacted more than humans. The time of day also impacts its concentration, with wind and sunlight dispersing the vapor. That means that nighttime, and particularly right before dawn, are the deadliest times to enter the cave. At dawn, concentrations are strong enough to kill a human within a minute.

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Researchers believe that priests participating in the rituals understood that the higher you were from the ground, the longer you could stand in the cave, making them to appear to have supernatural powers. They may have also adjusted the time that they entered the cave to coincide with lower concentrations.

The cave was actually forgotten until just seven years ago, but the mystery around it has remained. Brave researchers, led by Hardy Pfanz at the University of Duisburg-Essen, wanted to understand the enigma, so they examined the grotto in detail. Pfanz’s method could be used to help solve the mysteries of other Plutoniums as well.

Via IFL Science

Images via Chris Parfitt and Carole Raddato