Located in modern day Denizli, Turkey, "Hades' Gate" — named after the Roman god of the underworld — was believed to be a passage into the underworld by the ancient Greeks. Tales claimed people would drop down dead when they entered the mysterious Roman-Greco stone grotto in the ruins of Hierapolis.
The new study, published in the journal Archaeological and Anthropological Sciences, explains that the cave sits above the Badadag fault line, meaning toxic gases would have escaped the Earth's crust and filled the grotto. Even today the ancient ruins still emit highly concentrated levels of carbon dioxide that can be lethal to small animals.
"In a grotto below the temple of Pluto, CO2 was found to be at deadly concentrations of up to 91%," the study said, "Astonishingly, these vapuors are still emitted in concentrations that nowadays kill insects, birds and mammals."
Ancient Greeks believed the grotto was filled with the deadly breath of Hades, who was the brother of Zeus, the god of the sky, and was given the duty of looking after the dead and ruling the underworld.