Ancient Greece’s most famous oracle was just high on gas fumes
For centuries, people have traveled to Delphi in southern Greece, hoping to get a glimpse of their future. There, in the temple of the god Apollo, a priestess was said to enter a trance and issue prophecies with the voice of Apollo himself. Ordinary people, kings, and even Alexander the Great would travel miles to hear the priestess’s advice on important decisions, from personal finances to affairs of state.
Known as Pythia or the Oracle of Delphi, the priestess was not considered a medium. Ancient writers like Plutarch, who was priest at Delphi in the first and second centuries, described it as the vessel of a power from Earth.
According to Plutarch’s account, the temple of Delphi was built around a natural spring, whose water and cracks in the rock produced an odorous gas called pneuma. On designated days, several times a year, the chosen priestess would sit in the middle of the pneuma on a tripod stool and inhale sufficiently to enter a trance. It was an exhausting ordeal for the woman. She might scream, become hysterical, or collapse.
Plutarch claimed that there were fewer pneuma in his time than there had once been, leading to a decline in the temple’s popularity. After the temple closed in 393 AD, pneuma remained a persistent scientific enigma. Was the trance-inducing steam real? And if so, what exactly was it and where did it come from?
Decoding ancient sources in search of clues
The first modern excavations at Delphi, carried out between 1892 and 1950, failed to find a large crack in the rock, which they had imagined was the source of the gas. At the time, experts believed that gases could only rise from Earth in connection with volcanoes, which is not the case at Delphi. This has led scholars to dismiss ancient accounts as hearsay. However, later research came to a very different conclusion, inspired by the words of ancient authors.
“When I have written sources from the ancient world, my first effort is: ‘What can I learn from this?’ “, explains archaeologist John Hale. Popular science. In the 1990s, Hale and a multidisciplinary team of researchers finally discovered scientific evidence supporting ancient descriptions of Delphi.
Shifting tectonic plates can cause gas to rise from Earth
Hale explains that his colleague, Dutch-American geologist Jelle Zeilinga de Boer, had noticed a fault line running beneath the temple of Delphi during a surveying project in the 1980s. Fault lines are places where two of Earth’s tectonic plates collide. Plate movement can cause earthquakes and other forms of geological activity, including the release of gases.
De Boer wondered whether the ancient pneuma at Delphi was “a light hydrocarbon gas” that rose from the permeable limestone beneath the temple, Hale says.
Hydrocarbons are compounds made entirely of carbon and hydrogen. A fundamental component of living beings, they are also present in fossil fuels such as oil. Such chemicals “are found in many geological formations all over the planet,” says Hale. “They are part of the mixture of the Earth’s crust.”
When two tectonic plates rub together along a fault line, they produce friction that can generate enough heat to convert solid hydrocarbons in the Earth’s crust into gas. And if there are enough holes or channels in the Earth, this gas can rise to the surface, as ancient authors described at Delphi.
Insane discovery of natural gas in nature!
When tectonic plates shift, hydrocarbons, such as methane and ethane, can rise to the Earth’s surface. Video: Insane discovery of natural gas in nature! / @CrafterDUCK
Testing the foundation of Delphi to detect prophetic fumes
Early excavations at Delphi discovered porous bedrock well below the temple. This stone could provide the necessary, almost invisible channels for the flow of gas to reach ground level and, therefore, the lungs of a waiting priestess.
But there was no evidence of a hydrocarbon deposit at the site. Together, Hale and De Boer decided to see if Delphi limestone actually contained these compounds. If found, they could represent the final piece of the puzzle.
In 1996, after obtaining permission from the Greek government, Hale and De Boer undertook their first expedition to Delphi. They took samples of the bedrock and sent them to a laboratory for analysis. As they suspected, the porous limestone was rich in hydrocarbons, such as ethane, methane and ethylene.
What exactly did the Oracle of Delphi inhale?
Ethylene is a hydrocarbon and one of the most produced organic compounds in the world. In industry, it is a constituent element of plastics. In agriculture it is used to induce fruit ripening. (Have you ever put a green banana in a paper bag to make it ripen faster? The fruit releases ethylene to aid its own ripening, which builds up inside the bag). In the past, ethylene gas was even used as a surgical anesthetic, because inhaling it at a concentration of 20 percent caused loss of consciousness.
But what happens if someone inhales a smaller dose, although still very concentrated? To find out, Hale and De Boer turned to toxicologist Henry Spiller, because of his research on “huffing,” the inhalation of hydrocarbons and other toxic gases for recreational purposes.
What does inhaling ethylene do to a person?
Spiller found many parallels between the altered state of mind produced by inhaling ethylene and the ancient trance stories of the Pythia. People under the influence of ethylene remain lucid and responsive, but may speak or behave strangely. They may become agitated, scream or convulse and be unable to remember what happened after the gas wears off. Hale calls ethylene a “perfect match” to ancient pneuma. Ethylene even smells good, as Plutarch described it.
Repeatedly inhaling gases like ethylene poses serious health risks. Plutarch noted that inhaling gas reduced the lives of priestesses and could even kill them on rare occasions. At the height of the temple, several women shared the function of oracle due to the physical requirement of entering a trance state. Being a Pythian was considered a great honor, but it was also a burden.
Why does ethylene rise to the surface?
Today we know that the movement of tectonic plates can produce gases even in the absence of a volcano. And if there are channels down to ground level, these gases have only one direction to go: up.
“Ethylene is one of those lighter-than-air gases that rises straight to the surface if emitted,” says Hale, rising through openings like those in porous Delphi limestone. And after that, Hale adds, the gas “can be blown by whoever is above it and put them into an altered state.”
The first excavations at Delphi looked for a large chasm in the rocks. The most recent evidence suggests that the gas actually seeped through numerous small openings, following the paths made by the spring water. Hydrocarbons were also found in the water of Delphi itself, and some still comes from groundwater today in the form of gas; enough to occasionally kill birds that come too close.
What makes Delphi unique
Hale notes that the physical site of Delphi was recognized as unique in the ancient world. It wasn’t the only temple where an oracle claimed to predict the future, but “it’s the only one to mention an odorous gas as part of the sacred experience,” he said.
Compared to other Greek temples, Delphi was likely designed to enclose the spring, allowing gas to accumulate in the inner chamber where the Pythia sat. Other Greek temples may not have had steam-inhaling oracles, but many were also located at sites of high geological activity, such as the temple in the ancient city of Hierapolis (modern Pamukkale, Turkey). There, carbon dioxide rather than ethylene rises from the Earth, which was also used in ancient religious rites to kill sacrificial animals.
We know from Hierapolis and similar sites that the water that carries gases to the surface also deposits minerals. This can gradually block the channels in the stone, so less gas reaches the surface over time. Earthquakes, which have occurred in Delphi since ancient times, could also cause changes in the gas’s path. An earthquake could close previously open ethylene channels or release a large amount at once. So, although we cannot know for sure, there may be a geological explanation for Plutarch’s assertion that the pneuma diminished over time.
Today, Delphi’s unique geology is far from dormant. Gases can still rise from the porous limestone beneath the temple ruins, providing a very real connection between us and our ancient ancestors.
In This time whenPopular Science tells the strangest, surprising, and little-known stories that have shaped science, engineering, and innovation.
Other “At That Time” Stories
The best new PopSci 2025 releases
