Natural Wonders: The Breath of Cerberus

An ancient Greek shrine contains a chamber where carbon dioxide leaking through subterranean cracks killed sacrificial animals 2,000 years ago.

The ruins of Hierapolis, an ancient Greek city in modern Turkey, lie in a geologically active area. Pergamum king Eumenides II founded the city in the 2nd century BC to access its natural hotsprings. It became a hub for physicians and their patients, who used the baths to treat various illnesses. While this sounds inviting, Hierapolis also hid a deadly chamber.

Ploutonion, a shrine to Pluto. Pluto’s Gate is on the right. Photo: Daily Sabah


Within an enclosed section of the shrine to Pluto, god of the underworld, sits sinister Pluto’s Gate. Eunuch priests led sacrificial animals through the doorway, where they died instantly. It was said that the breath of Cerberus, guardian hound of the underworld, killed them. Miraculously, the priests were unharmed. Citizens revered the priests, believing that Cybele, the goddess of fertility, protected them.

A reconstruction of the Ploutonion. Photo: Francesco D’Andria


An archeological team found an arched doorway that led to a cave emitting particularly high concentrations of carbon dioxide. They found dead insects, birds, and mice around the doorway. 

Hot springs in the area create enchanting pools. Pamukkale, Turkey. Photo: Shutterstock


Far beneath Hierapolis lies not the god of the underworld but colliding tectonic plates. The Anatolian plate interacts with the Eurasian, Arabian, and African plates. The city lies directly over the 35km-long Pamukkale fault line. Here, CO2 escapes to the surface through cracks in the rock.

The CO2 concentration rises mornings and evenings

Through Pluto’s Gate, carbon dioxide levels can fluctuate from 4 percent to 90 percent, depending on the time of day. Animals die instantly because they are low to the ground, inhaling fumes in high concentrations. The priests, being taller, rarely felt any effects in the brief time they were there. Carbon dioxide is heavier than air and settles close to the ground.

Tests show that above 40cm, the CO2 concentration drops dramatically. The priests conducted sacrifices in the mornings and evenings when CO2 concentrations were highest. It is likely that they took note of the fluctuations and capitalized on the mystery by selling offerings to pilgrims.

The doorway was blocked up in the 4th century AD, probably due to the spread of Christianity. Today, green water fills the ruins of the enclosed shrine, but visitors can see Pluto’s Gate on the right-hand side. You are not allowed inside the enclosure for your own safety. 

This is one of many “gates of hell” on Earth. And there are other volcanic areas that emit carbon dioxide and kill animals, including one in Russia’s Kamchatka called the Valley of Death.