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 Around the second century BCE, the kings of Pergamon built a thermal space at Hierapolis, in what is now modern-day Turkey. Seven years ago, a “gate to the underworld” – also known as Plutonium, named after the Roman god of the underworld – was discovered there, emitting a curious haze that proved to be lethal to anything that stood inside the cave for too long. Initially spotted by ScienceMag, the paper – in the journal Archaeological and Anthropological Sciences – explains that the cave sits over a fissure that continuously releases pent-up geologic carbon dioxide in considerable quantities, even today. to 24 A.D., makes a great deal more sense given what we know today: “This space is full of a vapor so misty and dense that one can scarcely see the ground … bulls that are led into it fall and are dragged out dead,” he wrote. Should you visit, you might have the plutonium all to yourself. “The Galli [Eunuch priests] stood on stones around the poor bull or goat and demonstrated their supernatural powers,” lead author Prof. Hardy Pfanz, a researcher in volcanism and biology at UDE, told IFLScience. “At this height they could stand for 20-40 minutes without being endangered.” 
 
 Independent of these stones, “nobody could enter the gate to hell without getting asphyxiated,” but “if the Galli kept their breath for a while they could crawl into the gate up to their waist.”
– Centuries ago, bulls, rams, and other animals led into an ancient cave for religious ceremonies died of seemingly mystical causes, while the priests accompanying them suffered no such fate. Scientists say they've now figured out the secret behind this "Gate to Hell" in the ancient city of Hierapolis, per Bloomberg, and it has nothing to do with gods of the underworld. Instead: carbon dioxide. A study in Archaeological and Anthropological Sciences reveals that the cave located in modern Turkey, which was rediscovered in 2013, sits over a fissure that releases the gas. Volcanologist Hardy Pfanz and his team used a portable gas analyzer to figure out the phenomenon. So how did the priests survive? Because carbon dioxide is heavier than oxygen, it sinks—meaning animals closer to the ground succumbed, while the humans were OK. Or mostly OK: Ancient accounts depict the priests sometimes emerging and struggling for breath as spectators watched from a nearby site. IFLScience explains that the old-time priests likely understood that concentrations were diluted during the daytime by sunlight and wind—and so they held sacrifices before dawn for a quicker kill—and were also smart enough to climb up on stones around the doomed animals to show off their supposed powers. "At this height they could stand for 20 [to] 40 minutes without being endangered," Pfanz says. Without the stones, he adds, "nobody could enter the Gate to Hell without getting asphyxiated." (An incredibly old human fossil was recently found in a cave in Israel.)