For years, any beast or bird that came in contact with the portal in the ancient Hellenic archaeological site of Hierapolis, or Pammukale, in modern-day Turkey reportedly dropped dead. During Ancient Hellenic and Roman times, people were also said to have been cut down if they dared to approach. In ancient Hellenic times, it was said they were killed by the deadly breath of Hades. Scientists have now discovered the cause of the phenomenon, and it's not Hades--not directly, at least.

"When one crosses over the Mesogis, between the Carians and the territory of Nysa, which latter is a country on the far side of the Maeander extending to Cibyratis and Cabalis, one comes to certain cities.

First, near the Mesogis, opposite Laodiceia, to Hierapolis, where are the hot springs and the Plutonium, both of which have something marvelous about them; for the water of the springs so easily congeals and changes into stone that people conduct streams of it through ditches and thus make stone fences consisting of single stones, while the Plutonium, below a small brow of the mountainous country that lies above it, is an opening of only moderate size, large enough to admit a man, but it reaches a considerable depth, and it is enclosed by a quadrilateral handrail, about half a plethrum in circumference, and this space is full of a vapor so misty and dense that one can scarcely see the ground.

Now to those who approach the handrail anywhere round the enclosure the air is harmless, since the outside is free from that vapor in calm weather, for the vapor then stays inside the enclosure, but any animal that passes inside meets instant death. At any rate, bulls that are led into it fall and are dragged out dead; and I threw in sparrows and they immediately breathed their last and fell.

But the Galli, who are eunuchs, pass inside with such impunity that they even approach the opening, bend over it, and descend into it to a certain depth, though they hold their breath as much as they can (for I could see in their countenances an indication of a kind of suffocating attack, as it were),—whether this immunity belongs to all who are maimed in this way or only to those round the temple, or whether it is because of divine providence, as would be likely in the case of divine obsessions, or whether it is, the result of certain physical powers that are antidotes against the vapor.

The changing of water into stone is said also to be the case with the rivers in Laodiceia, although their water is potable. The water at Hierapolis is remarkably adapted also to the dyeing of wool, so that wool dyed with the roots342 rival those dyed with the coccus343 or with the marine purple.344 And the supply of water is so abundant that the city is full of natural baths.
[Strabo, Geography, 13.15]

According, to a Daily Sabah report, a team of German researchers from the University of Duisburg-Essen found what was actually “Hades’ breath”. They discovered that the grotto sits directly above the Babadag fault line from which carbon dioxide escapes the Earth’s crust and fills the cave, at levels that may have been lethal to humans in ancient times. A study published in the journal Archaeological and Anthropological Sciences reported:

​"In a grotto below the temple of Pluto, CO2 was found to be at deadly concentrations of up to 91 percent. Astonishingly, these vapors are still emitted in concentrations that nowadays kill insects, birds and mammals."

The Hierapolis site retains other powerful places as well. Mineral hot springs cascade down the hill near the ruins of the ancient city. The site has been used as a healing spa since the second century BC under the Greek Seleucid Empire.

Visitors can still admire the ancient city and the museum, while at the same time they can swim in its therapeutic hot springs.