Why animals near the ancient Roman "gate to hell" really dropped dead

Thousands of years ago in Hierapolis (now Turkey), tourists visited a temple named Plutonium built at a cave thought to be a gateway to the underworld. Magically, large and small animals would drop dead at the entrance to the cave while priest somehow survived. This isn't legend, it's reality. And now scientists have determined why. From CNN:

Research published by the journal of Archaeological and Anthropological Sciences in February shows that a fissure in the earth's surface, deep beneath the site, emits carbon dioxide at concentrations so high it can be deadly.

Using a portable gas analyzer, Hardy Pfanz and his team of volcanologists found CO2 at levels ranging from 4-53% at the mouth of the cave, and as high as 91% inside -- more than enough to kill living organisms...

Pfanz's research adds another possibility: the fact that the animals and priests are different heights. CO2 is a heavier than oxygen, therefore it settles lower, forming a toxic gas lake above the ground. "The nostrils of the animals were way in the gas lake," he says, whereas the priests stood taller, above the gas lake.

Above: digital rendering of the temple