Writers of the time, including Pliny the Elder and the Greek geographer Strabo, described these sacrifices as a chilling spectacle. A priest would lead an anima

Writers of the time, including Pliny the Elder and the Greek geographer Strabo, described these sacrifices as a chilling spectacle. A priest would lead an animal, perhaps a sheep or a bull, into the shrine. As if by the hand of the god, the animal would instantly drop dead, while the priest would walk out alive. "I threw in sparrows, and they immediately breathed their last and fell," wrote Strabo.. .."I wondered, could this Gate to Hell be a volcanic vent?".. "We saw dozens of dead creatures around the entrance: mice, sparrows, blackbirds, many beetles, wasps and other insects. So, we knew right away that the stories were true..." .."When Pfanz tested the air around the vent with a portable gas analyser he discovered the reason: toxic levels of carbon dioxide. Normal air contains just 0.04% CO2 but Pfanz was shocked to discover that the concentration around the shrine reached a staggering 80%. "Just a few minutes exposure to 10% carbon dioxide can kill you," he said, "so the levels here are really deadly." "These ultra-high levels of carbon dioxide are caused by the same geological system that created the area's hot springs and dramatic travertine terraces. Hierapolis is built on the Pamukkale fault, a 35km-long active tectonic fault zone where cracks in the Earth's crust allow mineral-rich water and deadly gases to escape to the surface. One of them runs directly under the city centre and into the Ploutonion." "Almost certainly the choice of the Ploutonion's location was directly related to the seismic gas vents that exist here," said Yeomans. "Given that the underworld and the deities and myths associated with it were a significant part of their religious ethos, it makes sense that they would construct temples and shrines in places that most evoked the world that they believed lay beneath their feet." "But such proximity to the forces of nature came at a price: an active fault zone also causes earthquakes, which levelled the city in 17 AD, 60 AD, and again in the 17th and 14th Centuries. Eventually, Hierapolis was abandoned."