Hell’s Gate mythhad an air of truth to it

For the Greeks and Romans, the cave in Hierapolis was a gateway to hell.

Castrated priests would lead sacrificial goats and bulls down through a cramped stairway into its depths, where they would die in minutes without so much as a drop of blood being shed.

Experiments have now shown the legends to be true, although the cause of death was not so much the deadly exhalations of Cerberus as shoulder-high ‘‘lakes’’ of

‘‘The fantastic thing is that the ancient writers did not exaggerate much when they described these ancient gates to hell.’’

poisonous carbon dioxide (CO2).

Perched on gleaming white travertine cliffs at Pamukkale, a tourist honeypot in modern Turkey, Hierapolis (the ‘‘Holy City’’) was home to a temple of the underworld in which worshippers would seek cures, prophecies or communion with the departed.

Ancient authorities such as Strabo and Pliny the Elder describe the grotto beneath the shrine as being filled with a spiritus letalis (deadly gas) or mantikon pneuma (breath of oracles) imbued with divine powers.For a donation, visitors were given birds and other animals to release and watch them die.

‘‘I threw in sparrows,’’ Strabo writes, ‘‘and they immediately breathed their last and fell.’’

A study published in the specialist journal, Archaeological and Anthropological Sciences, has pinned the blame on high concentrations of CO2 seeping up from the liquid mantle far below.

A team led by Hardy Pfanz, a volcano biologist, took a portable gas analyser into the temple and found two dead birds and more than 70 dead beetles.

CO2 levels were as high as 91 per cent. Animals suffer oxygen deprivation and acid accumulation at 5 per cent and 10 per cent is enough to kill a human.

Because CO2 is 50 per cent heavier than air it expands into pools known as ‘‘gas lakes’’.

The phenomenon has long been noted in wine cellars. In 1986 it killed 1700 people and 5000 cattle near Lake Nyos, Cameroon, after an earthquake.

Pfanz said: ‘‘The fantastic thing is that the ancient writers did not exaggerate much when they described these ancient gates to hell.’’