A portal to the underworld: The ancient Roman mystery that was solved by scientists

Hierapolis hid a secret for a long time in its mysterious door to hell. But modern science has finally discovered the truth behind the Roman myths. In Pamukkale, in western Turkey, a huge white rock formation rises above the surrounding plain. The mountain falls in the form of petrified waterfalls to the bottom of the valley, filling it with stalactites and pools of sparkling turquoise water.

These rock formations are called travertines, limestone cliffs slowly created over 400,000 years by the bubbling of mineral springs. In its formation process, the water degasses as it flows down the slope, leaving a large deposit of bright white calcium carbonate almost 3 kilometers long and 160 meters high.

This is not the only place on the planet where travertines are found. There are more in Huanglong, China, and Mammoth Hot Springs in Yellowstone National Park, USA. But those of Pamukkale are the largest and possibly the most magnificent in the world. They are one of the most popular tourist attractions in the country and are so spectacular that their name in Turkish means cotton castle.

Before the pandemic, more than 2.5 million people a year traveled to Pamukkale from Izmir or Istanbul, exiting tour buses atop the dazzling plateau and swarming the landscape like ants on a gigantic mound of sugar before heading to the beaches of Bodrum or the historic ruins of Ephesus.

But visitors who simply dip their toes in the vivid mineral pools and take a selfie in front of the natural columns miss out on a detail. Because perched high on the white cliffs of Pamukkale is an even more fascinating attraction: the ruins of the beautiful ancient city of Hierapolis.

A city with a chilling show

Hierapolis was founded by the Athalid kings of Pergamum at the end of the 2nd century BC. Before being taken by the Romans in 133 d. C. Under Roman rule, the place became a thriving spa town. During the third century, visitors from all over the Empire came to admire the landscape and bathe in the supposedly healing waters.

The city's success is still visible in its impressive arched front door, colonnaded main street, and beautifully restored amphitheater, all built from the same local travertine stone that glows golden in the warm Turkish sun.

Dr. Sarah Yeomans, an archaeologist at the University of Southern California who specializes in the Roman Empire said, the hot springs are probably one of the main reasons for the founding of the city. In the middle of the second century, Hierapolis was a beautiful and bustling spa town with what I imagine was a dynamic and diverse population given the popularity of these places with visitors.

But Hierapolis was also known throughout the Roman world for another, more sinister reason. It was said to be the location of a "gate to hell", a portal to the underworld where the toxic breath of the three-headed dog Cerberus flowed from the ground, claiming unsuspecting victims on behalf of its master, the god Pluto.

The city included a sanctuary, Plutonio, to which pilgrims came from different parties and paid to the priests that they should sacrifice on behalf of Pluto.

Writers of the time, including Pliny the Elder and the Greek geographer Strabo, described these sacrifices as a chilling sight.

The priests brought animals to the sanctuary, such as sheep or bulls. By "the hand of the god", the animal fell dead instantly while the priest came out alive.

I threw sparrows, and they immediately breathed their last and fell," Strabo wrote in Book 13 of his Geography encyclopedia, clearly astonished by what he had just witnessed.

If you visit Plutonion today, it's hard to imagine those dramatic scenes ever being real.

Now excavated and restored, a sanctuary is a quiet place that includes a rectangular enclosure filled with crystal clear water and a small arched doorway on one side. At the top, there are tiered seats for spectators and a replica of the Pluto statue.

How could the priests survive while the animals died?

Carbon dioxide at ultra-high levels

Hardy Pfanz, a biologist at the University of Duisburg-Essen in Germany who studies geogenic gases, was intrigued.

He said, when I read the descriptions of the ancient writers, I began to wonder if there could be a scientific explanation. I was wondering, could this door to hell be a volcanic vent?

Eager to test his theory, Pfanz traveled to Hierapolis in 2013. We weren't sure what we would find. It could have been made up, it could have been nothing. We certainly weren't expecting to get a response that fast.

But he got an answer, almost immediately. We saw dozens of dead creatures around the entrance: mice, sparrows, blackbirds, lots of beetles, wasps, and other insects. So we knew immediately that the stories were true.

When Pfanz tested the air around the vent with a portable gas analyzer, he discovered the reason: toxic levels of carbon dioxide. Normal air contains only 0.04% CO2, but Pfanz was surprised to find that the concentration around the sanctuary reached a staggering 80%.

A few minutes of exposure to 10% carbon dioxide would kill. So that the levels here are really fatal.

These ultra-high levels of carbon dioxide are caused by the same geological system that created the hot springs and spectacular travertine terraces in the area.

Hierapolis is built on the Pamukkale fault, a 35 km long active tectonic fault where cracks in the earth's crust allow mineral-rich water and deadly gases to escape to the surface. One of them passes directly under the city center and enters Plutonian.

Yeomans said, the choice of Plutonium location was almost certainly directly related to the seismic gas vents that exist here, Since the underworld and the deities and myths associated with it were an important part of their religious spirit, it makes sense that they built temples and shrines in places that most evoked the world they believed was under their feet.

But such proximity to the forces of nature came at a price: Several earthquakes ripped through the city in AD 17. C., 60 d. C., and again in the seventeenth and fourteenth centuries.

Finally, Hierapolis was abandoned.

Myth or trick?

But Pfanz was still puzzled by one thing: if this area was so deadly, why didn't the priests on Plutonium die as well?

He returned to Hierapolis the following year and this time studied gas concentrations at different times of the day.

We noticed that during the day, when it's hot and sunny, carbon dioxide dissipates quickly. But because carbon dioxide is heavier than air, at night when it's colder, it builds up in the sand, creating a deadly lake of gas at ground level.

Their conclusion: the animals, with their noses pressed to the ground, quickly suffocated in this toxic cloud, but the taller priests breathed much lower levels of CO2 and were able to survive.

Was this show a massive money-making trust trick, or did the priests really believe they were communicating with the gods?

Yeomans said, there is no doubt that Plutonium in Hierapolis was big business, but it is difficult to be sure whether the priests really understood what was going on. Some may have attributed its survival to the favor of the divine, while others may have viewed it as a natural, albeit enigmatic, phenomenon that could be observed and, at least to some extent, predicted. 

Today, the Temple of Plutonium is bricked up and a walkway has recently been built around it to give visitors a chance to see the site without getting too close to the source of the deadly gas.

But even with these modern adornments, it is exciting to be able to follow in the footsteps of the Greek and Roman pilgrims and see where mythology and reality meet; where the ancient gods reached out and touched people's lives.

Pfanz said, when I first recognized that the legendary Cerberus breath is actually carbon dioxide, I was standing right in front of the arch. At that moment, I realized that we had solved this ancient mystery; it was a really fantastic feeling.

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