Once an ancient tourist destination, the Plutonium cave, also known by the nickname “Gate to Hell,” harbors a secret that modern scientists have only been able to explain after 2,000 years.
Two thousand years ago, ancient travelers visited a Greco-Roman temple in Hierapolis (now in Turkey), located above a cave believed to be the entrance to the underworld. Their purpose was simple: to observe various animals, from birds to cattle, collapsing right at the cave’s entrance. This cave, named “Plutonium”—after Pluto, the god of the underworld in Greek mythology—was thought to exhale “the breath of death,” claiming the lives of any creature that dared to approach, except for the high priests. These priests were thought to be immune to “the breath of death” and were tasked with bringing animals into the cave for sacrifice.
The natural historian Pliny the Elder described this phenomenon as “the gate of Charon,” the ferryman who transported the souls of the deceased across the rivers Styx and Acheron to the realm of the dead in Greek mythology.
Toxic Gas
Entrance to the Plutonium cave.
However, today, scientists have lifted the veil of mythology surrounding this seemingly supernatural phenomenon. According to a study published in the journal Archaeology and Anthropology last February, a fissure in the earth’s surface deep inside the cave produces CO2 gas at a “deadly” concentration. Specifically, using a portable gas analyzer, Hardy Pfanz and his team of volcanologists found CO2 concentrations ranging from 4-53% at the cave’s mouth, peaking at 91% within. This concentration is more than enough to kill any living creature.
“Mammals (including humans) can start having problems at levels below 5% CO2,” Pfanz told American broadcaster CNN. “At 7%, the longer you stay, symptoms such as sweating, dizziness, and increased heart rate become progressively severe. Higher concentrations lead to suffocation due to lack of oxygen, as well as blood and brain cells being acidified.”
Thus, any animals that ventured into the cave quickly collapsed. Pfanz disclosed that during their research, the team found the remains of several birds, mice, and over 70 beetles.
During the day, this gas is dispersed by sunlight. The cave becomes most dangerous at dawn, with volcanic gas accumulation overnight sufficient to kill any nearby creature.
At dawn, CO2 concentrations reach 50% at the bottom of the cave lake, rising to 35% at a height of 10 cm but rapidly decreasing above 40 cm. This height-related accumulation also explains the eerie events recorded, where Roman priests led a bull close to this “Gate to Hell,” causing the bull to die while the humans remained unharmed: simply because humans stood much taller than the animal, thus avoiding the toxic gas.
Tourist Destination
Like in the past, today, Plutonium still attracts thousands of tourists each year. Italian archaeologist Francesco D’Andria, who rediscovered the cave in 2013, noted that he and his colleagues also found evidence of a grandstand built around Plutonium for ancient visitors.
The ancient Greek geographer Strabo once described: “Any animal that enters will face instant death. The bulls led inside all collapse and are dragged out. I threw in some sparrows, and they immediately fell down,” he wrote.
Graphic simulation of the buildings surrounding Plutonium, including seating for spectators.
Strabo recognized this response was related to the gas—“a space filled with vapor, so dense that the ground could not be seen clearly”—but did not understand why the gas affected animals but left the priests unharmed. He speculated that perhaps they were protected by the gods or simply held their breath when entering the cave.
However, Pfanz’s research has also unraveled the mystery behind this: the priests and the animals were of different heights! It is known that CO2 is heavier than oxygen and will sink, forming a “pool of toxic gas” on the ground. Thus, the animals would “take the brunt,” while the priests remained safe.
“The noses of the animals are right at the level of the toxic gas pool,” Pfanz observed.
Additionally, Pfanz suggested that the priests were likely aware of the source of “the breath of death” and the changes in gas concentration throughout the day. Research results also indicated that CO2 levels were relatively high at dawn and dusk due to the gas being dispersed by sunlight. However, archaeologist D’Andria disagreed with this viewpoint. He argued that based on the discovery of many oil lamps at the cave’s entrance, “it is highly likely that many religious activities were conducted at night.”
Thus, the “Gate to Hell” is essentially a hiding place for a rather dangerous scientific phenomenon. Nevertheless, the ancient city of Hierapolis continues to be one of the attractions drawing thousands of tourists not only because of the Plutonium cave but also due to its hot springs. If you have the chance to visit this site, remember to pay attention to your safety!
