For years any beast or bird that came in contact with the portal near Pamukkale, southern Turkey reportedly dropped dead amid claims they were killed by the deadly breath of Hades, the Greek god of the underworld.

During Ancient Greek and Roman times, people were also said to have been cut down if they dared to approach, The Sun reports.

But now it’s believed that deadly concentrations of C02 gas could be seeping from the Earth’s crust which could have caused the mystery deaths near the temples, tucked away in the ancient Phrygian city of Hierapolis. The new research was published in the journal Archaeological And Anthropological Sciences.

It was described by the Ancient Greek geographer Strabo as a deadly place to step foot in.

More than 2000 years ago, he wrote: “This space is full of a vapour so misty and dense that one can scarcely see the ground. Any animal that passes inside meets instant death. I threw in sparrows and they immediately breathed their last and fell.”

But while this may sound like something straight out of Indiana Jones, there appears to be some science behind it after all.

Indeed, birds have recently perished at the site.

Among the ruins, the archaeologists uncovered a cave with Ionic semi-columns. Upon them were inscriptions with dedications to other gods of the underworld — Pluto and Kore.

Italian archaeologist Professor Francesco D’Andria told Discovery News: “We could see the cave’s lethal properties during the excavation. Several birds died as they tried to get close to the warm opening, instantly killed by the carbon dioxide fumes.”

Prof D’Andria claims that pilgrims arriving at the site were given small birds to test the deadly effects of the cave.

Priests were said to have sacrificed bulls to Pluto while madly hallucinating from the toxic fumes.

Professor Hardy Pfanz, from the University of Duisburg-Essen, Germany, said the study detected highly concentrated levels of carbon dioxide.

He believes it is possible that the cave sits above the Babadag fault line which could release toxic gases from the Earth’s crust.

The study said: “In a grotto below the temple of Pluto, CO2 was found to be at deadly concentrations of up to 91 per cent. Astonishingly, these vapours are still emitted in concentrations that nowadays kill insects, birds and mammals.”

This article originally appeared on The Sun and is republished here with permission.