Ancient Romans believed it was the breath of Pluto that killed animals led into the cave, but science has since offered an explanation.

An aerial view of the Pluto's Gate, a religious site dedicated to the god Pluto, in the ancient city of Hierapolis in modern-day Turkey, 2018. Mahmut Serdar Alakus/Getty Images

Hierapolis, a ruined town in modern-day Turkey, was once a destination for people eager to witness an otherworldly and frightening ritual. A bull or other animal would be led into the cavern of its Ploutonion, a temple to Pluto, the God of Death. Sounds of the animal’s rising panic would emanate from within—followed by silence and then the thud as it hit the ground. Priests would pull on ropes to extract the limp body, supposedly killed by the breath of Pluto himself.

For centuries, the priests claimed these animals were killed by the power of the gods. And no one knew how it was actually done—until recently.

Hierapolis was a Roman city built on a plateau over the modern site of Pamukkale, a natural site whose name means “cotton castle.” The landscape here is distinctive: water bubbles from hot springs full of calcium, creating the calcareous landforms that give the place its name.

An aerial view of people at the thermal pools of Pamukkale in ancient city of Hierapolis in Denizli, Turkey, 2018. Thermal water have been running at Pamukkale for 2500 years. Mahmut Serdar Alakus/Getty Images

The city was founded as a thermal spa early in the 2nd century BC. Ancient people put great store in the power of thermal springs to heal ailments, and so the town rose to prominence, soon attracting people from Greece and Rome to bathe. Originally part of the kingdom of Pergamon, Hierapolis became a Roman possession when King Attalus III bequeathed his entire kingdom to Rome on his death. As part of the Roman province of Asia, Hierapolis flourished, despite the numerous earthquakes that shook the geologically active area. In 60 AD, the Emperor Hadrian built a vast theater for the town, able to seat over 15,000 people. A grand temple to the sun god, Apollo, was also built, with marble staircases and grand Doric columns.

But as well as the healing waters and its grand temple of Apollo, Hierapolis had its darker side. This was expressed in its Ploutonion, a center for the worship of Pluto. Pluto was the ruler of the underworld in Roman mythology, and a cognate with the Greek god Hades. While Zeus ruled the earth, and Poseidon the sea, Pluto governed the underworld and was solely responsible for who lived and died. And nowhere was this power more evident than in Hierapolis’ Ploutonion.

The Hierapolis Ancient Theater, a prominent feature at the UNESCO World Heritage Site of Pamukkale in Denizli, Turkey, 2017. Cem Oksuz/Getty Images

The Greek scholar Strabo visited the temple in the first century BC, and wrote about its frightening properties in his Geographica: “the Ploutonion… is an opening large enough to admit a man, but it reaches a considerable depth… This space is full of a vapor so misty and dense that one can scarcely see the ground. Now to those who approach the handrail anywhere round the enclosure the air is harmless… but any animal that passes inside meets instant death."

Until recently, it was thought that this frightening account was a product of classical imagination, or some trick performed by the priests of the temple. But recent excavations have shown that the temple’s deadly effects were very real. In February 2018, archaeologists reported they had traced seams of natural gas pouring out of the hot springs, and discovered that they led directly to the ruins of the Ploutonion.

The archaeologists found that the same volcanic activity in the area that causes the hot springs also causes noxious gases to pour out of the earth, particularly carbon dioxide. While CO₂ is not normally toxic, in high enough concentrations it can starve the body of oxygen. Normal atmospheric levels of CO₂ are around 0.039 percent, and 10 percent is enough to kill a person through asphyxiation in 30 minutes.Scientists testing the area of the ruins found CO₂ making up as much as 53 percent of the air at the mouth of the cave, and reaching as high as 91 percent inside.

“We went there with 90 percent confidence that we would find CO₂ beneath the temples,” says Hardy Pfanz, the archaeologist in charge of the expedition. “It was a great feeling to see that the gas we are studying for more than a decade also influenced humankind in antiquity.”

A view of the Ploutonion. Mahmut Serdar Alakus/Getty Images

Since CO₂ is 1.5x heavier than oxygen, the asphyxiating gas hung close to the floor, killing animals that entered the Ploutonion’s floor, but leaving the humans outside unharmed. When early settlers observed animals dying near this fissure, they must have concluded that the god of death himself was breathing from the earth, or his hell hound, Cerberus. Over time, Hierapolis became a draw for ancient tourists, who wanted to see the macabre spectacle themselves. As well as sacrificing bulls in the deadly cave mouth, the eunuch priests of the temple also made a spectacle of entering the Ploutonion themselves, although it is clear they understood its dangers. “They hold their breath as much as they can," Strabo tells us. “I could see in their countenances a kind of suffocating attack.”

The priests even sold caged birds to visitors, so they could try the cave’s powers for themselves. The temple’s lethal potential is still very much in action. While archaeologists uncovered the site, they observed birds flying near to the cave mouth and dropping dead from the vapors.

There's even some suggestion that the priests and perhaps temple visitors experienced hallucinations, ecstasies and visions due to the early stages of carbon dioxide poisoning. Several studies in the past have linked heightened levels of carbon dioxide in the blood caused by high altitude or cardiac arrest to experiences of visual hallucinations.

Hierapolis is not the only place where geological activity fed into the mythology of the ancients. Greece lies at the intersection of three separate tectonic plates—the Eurasian, Arabian and African. Geological turbulence defined a great deal of the experience of ancient people, and its echoes can be found throughout their mythology. The cataclysmic eruption of the Thera volcano, for instance, is speculated to have influenced the myth of Atlantis.

Geological activity also had subtler effects. At Delphi, the high priestess at the Temple of Apollo was known as the Pythia, now commonly known as the Oracle at Delphi. She was the most prestigious and respected oracle in the ancient world, and whoever held the title was probably the most powerful woman alive. The priestess would experience visions of the future that were held in great authority. In the temple’s adyton, the innermost sanctuary where entrance was forbidden, the priestess would descend and breathe in a kind of sweet vapor that was supposed to emanate from cracks in the rock. The Greek writer Plutarch, who served for some time as a priest of the temple, wrote that the temple filled with a sweet smell when the rituals were underway: “the room in which they seat those who would consult the god is filled… with a delightful fragrance coming on a current of air which bears it towards the worshippers, as if its source were in the holy of holies; and it is like the odor which the most exquisite and costly perfumes send forth.”

Pluto and Persephone (left), King and Queen of the Underworld, viewing their kingdom. Ann Ronan Pictures/Getty Images

It is not known exactly how these visions and ecstasies were produced, but some have suggested that the visions were caused by ethylene, a hydrocarbon gas sometimes given off by geothermal activity. Ethylene is known to have a sweet smell such as the one described by Plutarch, and it has been shown to cause hallucinations and near psychedelic experiences if present in sufficient quantities. However, others have disputed this explanation.

While the true source of the Delphic Oracle’s power remains a mystery, in Hierapolis scientists seem to have found the explanation for a historical mystery.

“The old Greeks needed the hellhound Kerberos and its deadly breath to explain what they saw,” says Pfanz. “We now know that it was the effect of carbon dioxide escaping from deep sources. Every period explained phenomena with the background available. Our explanations prove that many of the ancient stories were rather correct and not pure fairy tales.”

Hierapolis’ temple to Pluto and its “gate to hell” didn’t survive long after the arrival of Christianity to the region. When the Christians arrived, the deathly rituals of the Plutonian cult were curbed, the temple was vandalized, and memories of the rituals that had once occurred there fell into the realm of legend. In the 6th century CE, the Ploutonion’s cave was bricked up permanently. Further earthquakes in the centuries to follow took care of the rest, and the temple fell into ruin.

Today, the ruin of the Ploutonion at Hierapolis reminds us of a time when gods and demons used to breathe out of the cracks in the earth—and of another world that used to exist just out of sight beneath our own.