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.)