An ancient city is home to a creepy 2,000-year-old structure used in antiquity for animal sacrifices to the god of the underworld, and known for its deadly secret.

The site, known as Pluto's Gate (or Ploutonion in Greek), was discovered by Italian archaeologists a decade ago. It was dug out from amid the ruins of the ancient Phrygian city of Hierapolis in southwestern Turkey.

Located within the modern town of Pamukkale, the opening is in one wall of a rectangular arena with a temple and stone seating around it. People could watch the rites from the temple, but they were forbidden from entering the gate's entrance.

Historical accounts tell us that the shrine was once thought to be a "Gate to Hell," a powerful portal from which the toxic breath of Cerberus, a three-headed dog said to dwell in the underworld, flowed, as per BBC Travel.

People believed those who inhaled it would be felled on behalf of the hound's master, Pluto, the ruler of the Greek underworld.

The sacrificial process was described by contemporary writers, explaining that a priest would lead an animal through the arches, and the creature would then mysteriously drop dead, apparently by a divine hand, while the priest would emerge alive.

In 63 BC, the ancient Greek geographer Strabo wrote of his amazement at the site in Book 13 of his encyclopaedia Geography, saying: “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", he added.

When the site was excavated in 2013, archaeologists reported witnessing the same thing, with several birds dropping dead after flying too close to the mouth of the passage.

Scientists subsequently discovered that these ancient accounts were somewhat true, given that the Ploutonion was filled with volcanic carbon dioxide, as per the Sun.

Hardy Pfanz, a volcano biologist from the University of Duisburg-Essen in Germany, tested the air around the vent using a portable gas analyser and discovered that CO2 concentration around the shrine was as high as 80%, explaining: "Just a few minutes exposure to 10% carbon dioxide can kill you, so the levels here are really deadly," as per BBC Travel.

Hierapolis was built on an active tectonic fault zone with cracks from which mineral-rich water and deadly gasses seeped out, with one reportedly running under the city and into the site.

Gas coated the bottom of the cave, which may explain why it was more deadly to animals like the bulls or sheep that may have been sacrificed, as they breathed with their noses closer to the ground.

Additionally, priests who entered would also "hold their breath as much as they [could]", according to Strabo's account.

Today, only ruins remain of the ancient Hellenistic city, which UNESCO has designated a World Heritage Site.