From Archaeology (May/June 2018):
According to some ancient sources, the Greco-Roman priests of Hierapolis would lead sacrificial animals toward the city’s purported gate to the underworld, where the beasts would suddenly collapse and die. A recent study has finally revealed the truth behind this 2,000-year-old mystery. “Pluto’s Gate” is actually built atop a fault that emits carbon dioxide. Because it is heavier than air, the gas collects in higher concentrations closer to the ground. The priests were tall enough to avoid toxic levels of the gas, but the animals were not so fortunate.
