Quirks & Quarks

'Gates of Hell' ritual sacrifices were a deadly geological magic trick

Ancient Roman sacrifices of animals attributed to the 'breath of the hellhound' were actually CO2 asphyxiation.
The hellhound Kerberos or Cerberus, painted by William Blake. The breath of Kerberos was thought to be what killed sacrificial animals at the "gates to hell"
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Passages to the underworld

In temples to Pluto, scattered throughout the ancient Roman world, uncanny sacrifices reinforced fear and respect for the god of the underworld. These "Plutoniums" were also known as "Gates to Hell" because they were thought to be passages from the underworld to the surface.

From these passages flowed the deadly "breath of Kerberos," the three-headed hellhound. During rituals, Roman priests would lead sacrificial animals to the gates, where the animals would mysteriously sicken and die, in view of the pilgrims in the temple above.

Part of the ruins of the Plutonium at Hierapolis in modern-day Turkey (Ömerulusoy, cc-by-sa-3.0)

Demystifying an ancient ritual

Now Professor Dr. Hardy Pfanz, a volcano biologist from the University of Duisburg-Essen in Germany, has built a case for just what the "magic trick" behind these rituals is. He's studied the ruins of a Plutonium at Hierapolis in Turkey, and found that vents at the site emit concentrated and toxic streams of carbon dioxide. Because of the arrangement of the temples around the sources of the gas, it killed the sacrificial animals, but not the priests or pilgrims in the audience.

CO2 can be produced underground by volcanic activity, or by seismic activity - friction and heat breaking down carbonate rock deep underground. This gas rises to the surface, but CO2 is heavier than air so it pools in low lying areas forming carbon dioxide lakes. At the Plutonium in Hierapolis, according to Dr. Pfanz, these lakes would have been up to a meter and a half deep. This meant sacrificial animals - sheep or cattle - led to the temple floor would have breathed the CO2 and asphyxiated. He surmises that priests would have stood with their heads above the level of the gas - perhaps on raised platforms or pedestals, and the audience would have been quite safe on seats well above.  

No longer deadly (mostly)

Most of the ruins of Plutoniums around the Roman world today don't have emissions of carbon dioxide. These vents open and close over geological time, and most are probably currently inactive. Dr. Pfanz says it's just fortunate that the temple at Hierapolis is still active and his team was able to study it.