The Toxic Secret of Pluto’s Gate, the Ancient ‘Gate to Hell’

When the Roman Republic turned western Anatolia into their new province of Asia in 129 B.C., the ancient city of Hierapolis became a booming spa destination. Visitors from across the Roman Empire reportedly traveled to Hierapolis to bathe and heal in its warm thermal waters and baths. 

One particularly mysterious site became known as the Ploutonion, or Pluto’s Gate—a place of worship dedicated to the Roman god of the underworld. Romans believed this small cave had a direct connection to hell, substantiated by a disturbing phenomenon. Animals that ventured near the cave’s opening would suddenly drop dead while priests emerged from its depths unharmed.

What really transpired at Hierapolis’ “Gate to Hell”?

Although Eumenes II, the king of Pergamon, is credited with founding Hierapolis in the early second century B.C., it’s likely Indigenous people settled there during the Archaic Period, says Gil Renberg, an ancient historian and lecturer at Howard University. “Other people living in Turkey would almost certainly have been using it as a holy site destination before Greeks colonized the west coast of Turkey and into the Black Sea,” he says.

Hierapolis is home to thermal springs because it resides on the 21-mile-long Pamukkale fault. This allows mineral- and carbon dioxide-rich thermal waters to bubble up. The city was likely founded because of its thermal waters, Sarah Yeomans, an archaeologist at the University of Southern California, told the BBC.

Thousands of years ago, Greco-Romans interpreted the unusually hot water and animal deaths around these waters as messages from the gods, says Renberg. “Hierapolis means sacred city in Ancient Greek," he explains. "Not being schooled in geology, the ancient mind saw the noxious fumes coming from the ground, grottos and caves, and associated it with the underworld.”

Talk of visitors experiencing visions and hallucinations while bathing in the nearby waters only reinforced Hierapolis’ connection to the spiritual world.

Ancient priests took small birds, and sometimes even a sheep or bull, into the passage as part of dramatic rituals. The animals would quickly collapse and die during sacrifices to Pluto.

Several ancient writers—including Pliny the Elder and geographer Strabo—wrote about the ceremonies at Hierapolis. They described priests throwing small animals inside and noted that priests held their breath or turned their heads away. They also described a theater that was built outside of the cave where people could watch the sacrifices. 

According to legend, the Gate to Hell emitted the toxic breath of the three-headed hellhound Cerberus, who claimed victims for Pluto, volcano biologist Hardy Pfanz told the BBC.

When archaeologist Francesco D’Andria led an excavation of Hierapolis in 2013, he discovered a marble statue of Cerberus guarding the entrance to Pluto’s Gate. Further discoveries included ionic columns with inscriptions dedicated to Pluto and Kore, queen of the underworld, as well as a pool and steps above the cave.

When Pfanz visited Hierapolis in 2013, he saw a variety of dead creatures—including mice, sparrows, blackbirds and insects—around the entrance. Testing the atmosphere, he discovered that carbon dioxide made up as much as 80 percent of the air around the Gate to Hell—compared to 0.04 percent in normal air. “Just a few minutes exposure to 10 percent carbon dioxide can kill you,” Pfanz told the BBC, “so the levels here are really deadly.”

The carbon dioxide leaks into the air from the Pamukkale fault. Pfanz’s additional research revealed the carbon dioxide level rapidly dissipates during the day, especially when it’s hot and sunny. “But because carbon dioxide is heavier than air, at night when it’s cooler it pools in the arena, creating a lethal lake of gas at ground level,” he added.

Renberg believes priests were able to survive the carbon dioxide because their heads were out of the cave and the dangerous gas didn’t rise to their level.

Hierapolis wasn’t the only site across the Mediterranean to have such unusual geographical features. According to Renberg, at least half a dozen other towns or cities in the region had the same conditions as Hierapolis, and there “was a definite belief that certain caves led down to the underworld.”

The Pamukkale fault caused numerous earthquakes to level Hierapolis, including reports of disasters in A.D. 17 and A.D. 60. It’s believed Christians sealed the cave by the sixth century as they erected their own places of worship. 

In 1957, the Italian Archaeological Mission began excavations around Hierapolis, which eventually led to the site’s rediscovery.