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The science behind the Roman 'gate to hell'

Updated 2114 GMT (0514 HKT) March 9, 2018
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Scientists believe they have figured out the mystery behind a Greco-Roman temple where animals were reported to drop dead as part of a ritual sacrifice. Known as the "Plutonium," the temple consisted of a stone doorway that led to a cave-like grotto, established around 190 BC. A team of Italian archaeologists rediscovered the cave in 2013 -- this is their 3D virtual reconstruction of the site. Courtesy of Italian Archaeological Mission in Hierapolis Archive
These are the present-day remains of the Plutonium, in Hierapolis. In ancient times it was believed to be the gateway to the underworld as animals, led into the cave by Roman priests, would die of asphyxiation. The phenomenom continues today, as small birds and beetles have been found dead at the same site. Courtesy of Carole Raddato/Flickr
During excavation, archaeologists found evidence of benches surrounding the Plutonium -- as this 3D virtual reconstruction shows. Visitors could watch the animal sacrifices, at a safe distance from the cave. New research shows that a fissure in the earth's surface, deep beneath the cave, emits carbon dioxide at such high levels that it can be deadly. Courtesy of Italian Archaeological Mission in Hierapolis Archive
An inscription discovered by archaeologists shows that the Ancient Romans dedicated the site to Pluto, god of the underworld and his wife Kore (Persephone). As the myth goes, Kore was abducted by Pluto and taken to the underworld. Jupiter, king of the gods, ordered her release, but because she had eaten a single pomegranate seed in the underworld she could not be completely freed, and spent one-third of each year with Pluto. Courtesy of Italian Archaeological Mission in Hierapolis Archive
The Plutonium was situated beside the Temple of Apollo, in the ancient city of Hierapolis (modern-day Pamukkale in southwestern Turkey). Courtesy of Carole Raddato/Flickr
The ruins of Hierapolis -- now a UNESCO World Heritage Site -- attract thousands of tourists every year. Courtesy of Carole Raddato/Flickr
The site includes the remains of a Roman amphitheater, built in the 2nd century AD under Emperor Hadrian. It was later renovated under Septimius Severus. Courtesy of Carole Raddato/Flickr
Hierapolis is also famous for its hot springs and travertine terraces, formed by deposits of calcium carbonate. At the end of the 2nd century BC, the dynasty of the Attalids, the kings of Pergamon, established the city as a thermal spa. Courtesy of Carole Raddato/Flickr
This use continues today, as the calcium-rich water is reputed to have healing qualities for eye and skin diseases. MUSTAFA OZER/AFP/Getty Images
The Plutonium is not the only example of an ancient site centered around noxious fumes. Ancient literature suggests that at the Palace of Apollo in Delphi, Greece -- where the oracle once prophesied the future -- people would enter a trance after inhaling noxious fumes. Modern research puts this down to a mixture of carbon dioxide and methane coming from an earth fissure. Sean Gallup/Getty Images