For centuries, people traveled to Delphi in southern Greece hoping for a glimpse of their future. There, at the temple of the god Apollo, a priestess was said to enter a trance and issue prophecies in the voice of Apollo himself. Everyday people, kings, even Alexander the Great traveled for miles to hear the priestess’s input on important decisions, from personal finance to matters of state.

Known as the Pythia or the Oracle of Delphi, the priestess wasn’t believed to be a psychic. Ancient writers like Plutarch, who served as a priest at Delphi in the first and second centuries, described her as a vessel for a power that came from the Earth. 

According to Plutarch’s account, the temple of Delphi was constructed around a natural spring, where the water and fissures in the rock produced a sweet-smelling gas called pneuma. On designated days a few times per year, the chosen priestess sat amidst the pneuma on a tripod stool and inhaled enough to enter her trance. This was an exhausting ordeal for the woman. She might cry out, become hysterical, or collapse.

Plutarch claimed that there was less pneuma in his time than there had once been, leading to a decline in the temple’s popularity. After the temple closed down in 393 AD, the pneuma remained an enduring scientific puzzle. Was the trance-inducing vapor real? And if so, what exactly was it, and where had it come from?

Decoding ancient sources for clues

The first modern excavations at Delphi, conducted between 1892 and 1950, failed to find a large fissure in the rock, which they had pictured as the source of the gas. At the time, experts believed that gases could only rise from the Earth in connection with volcanoes, which Delphi doesn’t have. This led scholars to dismiss the ancient accounts as hearsay. However, subsequent investigations came to a very different conclusion, spurred on by the words of the ancient authors. 

“When I’ve got written sources from the ancient world, my first effort is, ‘What can I learn from them?’” archaeologist John Hale tells Popular Science. In the 1990s, Hale and a multidisciplinary team of researchers finally uncovered scientific evidence that corroborated the ancient descriptions of Delphi. 

Shifting tectonic plates can cause gases to rise from the Earth

Hale explains that his colleague, Dutch-American geologist Jelle Zeilinga de Boer, had noticed a fault line passing under the temple of Delphi during a 1980s surveying project. Fault lines are places where two of the Earth’s tectonic plates bump against one another. The plates’ movement can cause earthquakes and other forms of geological activity, including the emission of gases. 

De Boer wondered if the ancient pneuma at Delphi was “a light hydrocarbon gas” that rose from the permeable limestone under the temple, says Hale. 

Hydrocarbons are compounds made entirely of carbon and hydrogen. A fundamental component of living things, they also occur in fossil fuels like petroleum. Such chemicals “are found in a lot of geological formations all over the planet,” says Hale. “They’re part of the mix of the Earth’s crust.” 

When two tectonic plates rub against each other along a fault line, they produce friction, which can generate enough heat to convert those solid hydrocarbons in the Earth’s crust into gas. And if there’s enough holes or channels in the Earth, that gas can rise to the surface, similar to what ancient authors described at Delphi. 

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When tectonic plates shift, hydrocarbons, such as methane and ethane, can rise to the Earth’s surface. Video: Insane Natural Gas Discovery in the Wild! / @CrafterDUCK

Testing Delphi’s bedrock for prophetic fumes

Early excavations at Delphi discovered a porous limestone bedrock far below the temple. That stone could provide the necessary, near-invisible channels for the flow of gases to reach ground level and, in turn, a waiting priestess’s lungs. 

But there was no evidence of a hydrocarbon deposit at the site. Together, Hale and De Boer decided to see if Delphi’s limestone really did contain these compounds. If found, they might represent the final piece of the puzzle.

In 1996, after gaining permission from the Greek government, Hale and De Boer made their first expedition to Delphi. They took samples of the bedrock and sent them to a lab for analysis. As they suspected, the porous limestone was rich in hydrocarbons, such as ethane, methane, and ethylene.

What exactly was the Oracle of Delphi inhaling?

Ethylene is a hydrocarbon and one of the world’s most widely-produced organic compounds. In industry, it’s a building block for plastics. In agriculture, it’s used to induce ripening in fruit. (Have you ever put a green banana in a paper bag to make it ripen faster? Fruit releases ethylene to encourage 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 causes unconsciousness. 

But what happens if someone inhales a lower, though still highly-concentrated, dose? To find out, Hale and De Boer turned to toxicologist Henry Spiller, due to his research on “huffing,” the inhalation of hydrocarbons and other toxic gases for recreation. 

What does inhaling ethylene do to a person?

Spiller found many parallels between the altered state of mind produced by ethylene inhalation and ancient accounts of the Pythia’s trance. People under the influence of ethylene remain lucid and responsive, but may speak or behave strangely. They may become agitated, scream, or convulse, and may be unable to remember what happened after the gas wears off. Hale calls ethylene “a perfect match” for the ancient pneuma. Ethylene even smells sweet, just as Plutarch described. 

Repeated inhalation of gases like ethylene carries serious health risks. Plutarch noted that inhaling the gas shortened the priestess’s lives and could even kill them on rare occasions. At the temple’s height, multiple women shared the office of oracle because of how physically demanding it was to enter the trance state. Being Pythia was considered a great honor, but it was also a burden. 

Why ethylene comes to the surface

Today, we know that shifting tectonic plates can produce gases even when a volcano is not present. And, if there are channels up to ground level, those gases only have one direction to go: up.

“Ethylene is one of those lighter-than-air gases that comes straight to the surface if it’s being emitted,” Hale explains, rising through openings like those in the porous limestone at Delphi. And after that, Hale adds, the gas “can be huffed by anybody who’s on top and put them in an altered state.” 

Early excavations at Delphi were looking for one big chasm in the rocks. The most recent evidence suggests that gas actually seeped through many small openings, following the paths made by spring water. Hydrocarbons have also been found in the water at Delphi itself, and some still rises from groundwater as gas today; 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 was not the only temple where an oracle claimed to foretell the future, but “it’s the only one that ever mentioned a sweet-smelling gas as part of the sacred experience,” he says. 

When compared with 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 vapor-inhaling oracles, but many were also positioned over sites of high geological activity, such as the temple at 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 which carries gases to the surface also deposits minerals. This may gradually clog the channels in the stone, so that less gas reaches the surface over time. Earthquakes, which occurred at Delphi even in ancient times, might also lead to changes in the pathways for the gas. An earthquake might close previously open channels for ethylene or release a large buildup of it at once. So while we can’t know for certain, there may be a geological explanation for Plutarch’s assertion that the pneuma declined over time.

Today, Delphi’s unique geology is far from inactive. Gases can still rise from the porous limestone beneath the temple ruins, serving as a very real connection between us and our ancient ancestors. 

In That Time When, Popular Science tells the weirdest, surprising, and little-known stories that shaped science, engineering, and innovation. 

 

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