The colorful and dramatic accounts of the Oracle of Delphi in Greece spanning many centuries appear to be strongly related to the geological setting and seismic activity of the locale. The cultural and geologic history is long and complex. Over time, people have speculated on the explanation as being supernatural, psychic, geological, or a hoax. It took a multidisciplinary effort to reach the most probable proposed explanation we have so far, but there are still many details we’ll never know.

Delphi as Geomythology

Delphi is the most famous place on earth where the gods were said to make their presence known and speak to humans through the high priestess designated as the Oracle. For 1200 years, a prophetess was enlisted to enter the most sacred space in the temple (adyton) and breathe the magical vapor (pneuma) in order to commune with Apollo and relay messages to mortals. Several ancient historians including Homer, Aristotle, Diogenes, Euripides, Ovid, Strabo, and Plutarch were consistent in their descriptions of Delphi. When explorers and scientists went looking for confirmation of the story, however, they failed to find it. The Oracle of Delphi was disregarded as a myth until the turn of our modern century when geologists, archaeologists, and toxicology experts worked together in a multidisciplinary effort using modern testing techniques to show that the ancient myth was almost certainly true to some extent. The story of the Delphic oracle is long, convoluted, disputed, and a stellar example of geomythology – legends and myths related to geologic processes.

The story of the Delphic Oracle is chock-full of weirdness and spooky themes. It begins with the legends of possessed goats and a crack in the earth. Supernatural gods interacted with a giant dragon-snake whose dead body produced divine vapors. Magical waters flowed from the mountain springs. A chosen woman was assigned to sit at the navel of the earth and channel the revelations from the gods. The story does not end even as scientists discover more evidence and fit together conclusions that support the ancient stories. The final explanation for what made Delphi a great, wealthy, and magical place until the 4th century BC may never be completely resolved. But it’s a cracking good story.

Shaped by geology

Delphi is located near Mount Parnassus. The secluded and rugged site is shaped by its geology in the Korinth rift zone, a seismically active area where the crust is spreading. Strong, destructive earthquakes occur periodically, though the pattern of rupture has not been established. Landslides are also common. The land is dotted with springs that were considered sacred to the ancient Greeks in this warm, dry climate. Archaeological evidence shows that the Delphi area was a place of worship in 1600 BCE.

Mount Parnassus

The topography and geology were always part of the story of the Delphic Oracle. Delphi was a religious center before 1200 BCE but the oracle was documented in the 8th century BCE. Before Apollo’s temple was here, the Greeks worshiped Gaia or Ge (the earth) or Poseidon (earthshaker). Though the details are lost to history, the initial discovery of the special nature of Delphi is described as originating from a cleft in the side of Mount Parnassus around which the grazing goats would act frenzied. The people noticed the strange air emitting from the chasm and considered it a gift from the gods. They built a shrine.

The monster, Python

One traditional origin story of the gases is monstrous. In the days of the gods, the chasm was said to have been guarded by an enormous dragon-like snake, called Python. The birth and death of the snake mentioned in ancient writings evoked ideas of earthquakes and destruction. Apollo killed the snake and the body was left to rot. The sickly sweet or sulfurous vapors supposedly came from her corpse.

Apollo kills Python

The Pythia

Apollo’s high priestess was called the “Pythia” in association with the monstrous snake. She was an ordinary woman who acquired her extraordinary powers of prophecy as Apollo’s mouthpiece only when she entered the sacred space and was possessed by the god. Then she was the Oracle. The power of the Oracle went unchallenged. Her voice held more impact than other civil or religious leaders. She was supremely influential. The great leaders and rich men paid to hear from the Oracle. Money flowed into Delphi. Beginning in the 7th century BCE, the secluded village of Delphi became wealthy. The Pythia was periodically replaced by other women chosen from the local minor priestesses.

The Pythia underwent purification rituals before descending into the inner sanctum below the floor level of the temple called the adyton. This she did nine times a year during the warm months. Apollo was said to leave during the winter. Springs emerged near or in the temple and the Pythia was said to drink from them as part of her ritual. The divine pneuma arose from an opening in the floor over which the Pythia sat on a tripod seat to deliver the words of Apollo. Greeks thought of Delphi as the center of the world. A conical stone with a hole through it, called the omphalos (navel or center), was placed in the temple, probably over the pneuma vent. This helped focus the “breath of Apollo” to the Pythia.

The Pythia is enveloped by the pneuma in the essential version of the Oracle we imagine.

The Oracle continued to perform duties until the 4th century AD. Plutarch (at the end of the 1st century AD) speculated that the vapors were variable in strength and that they were weakening overall at this time. Plutarch also had the inkling that the vapors were influenced by rainfall, earthquakes, and temperature.

The temple is destroyed

Earthquakes eventually destroyed the temple and the city was pillaged and abandoned. The sloping terraces were prone to landslides and mudslides. Eventually, Delphi was forgotten. The village of Kastri arose on the site but the new residents had no idea of the history underneath.

Fast forward to 1892 when French archaeologists were the lucky group to obtain permission to excavate at the Delphi temple. The remains were jumbled in the area they believed to be the temple and the holy adyton. They were not confident they had reached the right area as it did not correspond to the descriptions in the historical accounts. In addition, they found no recessed area, and no fissure or chasm. They found clay and encountered groundwater seepage but no pneuma.

This failure to expose the historical place of the Oracle was highlighted several years later in an influential text that concluded the idea of the Oracle as described, with a mystic pneuma enveloping the Pythia in the adyton, was all a myth – that it was not possible for the natural geology to host such a spectacle. And so this was the view that remained for decades.

New ideas about Delphi

During the 1980s, geologist Jelle Zeilinga de Boer of Wesleyan University was working in Greece when he visited the Delphi area for the first of many times. Specializing in the study of plate tectonics and related seismic and volcanic consequences, de Boer also held expertise in limestone areas of the Mediterranean region where the Cretaceous-aged Tethys bedrock also held hydrocarbon deposits. The crumpled remains of the Tethys sea had been squeezed into what became the Greek peninsula. De Boer had read Plutarch’s accounts of the Delphic Oracle. Plutarch had supposed that the phenomenon of the Oracle had some natural basis and that possibly the great earthquake of 373 BCE destroyed the temple. When exploring the flanks of Mount Parnassus, de Boer discovered an exposed fault scarp that continued in the direction towards Delphi.

The fault traces identified by de Boer and Hale.

William Broad’s book The Oracle: Ancient Delphi and the Science Behind Its Lost Secrets details the personal journey of Dr. de Boer who returned to Greece with students in the summers. The fault, he guessed, was obviously well known to the previous geologists who had studied the Delphi area. A few years later, de Boer made the acquaintance of John Hale, an American archaeologist. They shared a common interest in Greece and the idea that great quakes played a significant role in historical events. When de Boer mentioned his explorations in Delphi, Hale informed de Boer that according to the French dig in the late 19th century, there was no fault at Delphi. But de Boer knew better. The two men, after playing ideas off each other, realized that they may be on to something very new and important. Hale looked back at the French records and found that the archaeologists did document fissures in the rock at the area of the temple, and that flowing water had been present. But the fact that the dramatic chasm as they envisioned was not found overshadowed these observations. The contradiction of fractures or no fractures was frustrating. The archaeologists had not the geologically-trained eye to see the clues that were there. The artificial terracing of the land and erosion hid the obvious fault. An older geologic map showed the probable fault trace that de Boer had found that appears to pass right through the temple area. When the men visited the temple site together, they found travertine on the walls indicating that flowing water had deposited the mineral. Old photos from the excavation showed fissures in the rock walls.

De Boer also had noticed how several springs, including the still active Kerna spring, lined up, suggesting a common origin, typically a fault trace. Another team had previously noticed the same. In 1998, de Boer confirmed to his satisfaction through field checking that there indeed was a second faulted zone trending north-south. They named it the Kerna fault. It cross-cut the larger fault he established earlier trending east-west right beneath the temple of Apollo.

De Boer and Hale had the location, and evidence of faults, and springs as noted in the historical accounts. The underlying bedrock provided the next piece of the puzzle. The limestone below the surface shale held small concentrations of petrochemicals – tar, oil, and gas. De Boer surmised that the frictional heating from the fault movements not only volatilized the tar-like bitumen, releasing the lighter gases, but the fault movement opened additional fractures that allow these gases to migrate upwards. Scientists today still do not entirely accept that these kinds of volatilized gases can be released in non-volcanic regimes, but there appears to be clear evidence that they can.

In subsequent efforts, their team was allowed to take samples of the travertine from the temple location and other spring areas to test for gas concentrations that were trapped during the formation of the mineral deposit. The travertine gas results also provided evidence that the intersecting faults were an area where hydrocarbon gases were preferably liberated from underground. The gradient of gas concentrations increased closer to the temple. The remaining questions were: did the gas influence the Pythia and how could they identify it?

The Omphalos stone

Jeffrey Chanton, a geochemist, had measured the hydrocarbon gas samples in the study. He found ethane and methane. These were not the gases that would fit the descriptions in historic accounts. The best match for the intoxicating gas was ethylene – a sweet-smelling gas used for anesthesia that produced a mild euphoria. They hypothesized that ethylene was a component of the gases released into the adyton via fissures and that was also dissolved in the bubbling spring water around the temple. The low ceiling and walls of the adyton trapped the released gas, possibly aided by the use of the omphalos stone as a vent. But ethylene in this free form does not last long. It dissipated quickly or reacted with the other gases and no longer remained in the travertine samples. Ethylene was later found emitting in small quantities from other springs, including Kerna. Even though the gas is no longer prevalent, it’s feasible that it was released in higher concentrations in the past, enough to produce the effect on the Oracle.

Even inhaling a low concentration of ethylene can result in a calm trance, with the still conscious person experiencing strange sensations and remembering little afterward. If the gas concentrations were too high or impure, a bad reaction could occur and the Pythia could die. There are some accounts where the Oracle readings went wrong. From the 1920s to the 1970s, ethylene was widely used as a medical anesthetic because it worked fast and patient recovery was quick when the gas was removed.

By bringing all the data together and forming several lines of evidence, the team of American scientists concluded that the most likely explanation for the historical accounts of the Delphic Oracle phenomenon was the result of intersecting faults that emitted ethylene gas that was concentrated in the adyton structure. The behavior of the Pythia, the timing of the waxing and waning of her powers seasonally and over time, and the connection to the seismic events (20-100 years apart) that opened and eventually closed off the pneuma, all matched up well.

De Boer and Hale published their findings in 2000 in a special publication for the Geological Society of London. This is available and well worth reading for a fuller account. In 2001, with Chanton, their work was in the journal Geology. They further defended the gaseous vent theory with Henry Spiller, a toxicologist, in Clinical Toxicology in 2002.

The Pythia appears on her tripod.

Divergent views

But not everyone agreed with their theory about the Oracle. Piccardi, et al (2008) held onto the idea that the presence of ethylene was not possible in the non-volcanic area and suggests benzene was the gas involved. This paper also disputes Plutarch as a reliable source and rejects the identified Kerna fault as valid. Their conclusion instead was that the fault rupture during seismic events released hydrogen sulfide-rich gas (in accordance with the stench of the corpse of the dragon-snake) but only for a limited time (until another quake reopened the plumbing). The travertine from the spring areas did contain sulfur. Earthquakes and stress mechanisms are known to create fractures that release gases (methane, carbon dioxide and radon, for example). The underground movements can also alter and discolor springs. The 1870 earthquake near Kastri turned the spring water reddish and was compared to the blood of the dead dragon of the ancient tales.

Some still suggest the gas or smoke of the pneuma was deliberately created. Harissis in 2014 suggested that oleander was deliberately burned under the floor of the adyton to create the intoxicating fumes. Was it also possible that the high priests of Apollo “helped” the Pythia along in times when nature didn’t cooperate? It’s not impossible, they were creative.

Jelle Zeilinga de Boer passed away in 2016.

Because earthquakes are infrequent, they destroy villages then the people move on, the true origin of the Oracle and important details of history have been lost. We may never know for sure if the Pythia breathed natural hydrocarbon emissions from the fissures and drank from the bubbling springs as the geology and archaeology suggest. We can only make the best guess. While some geological components do add up, other important evidence has, literally, evaporated or collapsed. The historians had a consistent description of a temple, a fissure, a spring, and a sweet-smelling, mystical gas that the American team of scientists matched to reasonable geological conditions. Further data collection may clarify the characteristics of the Kerna faulting that intersects with the Delphic fault under the temple. In other areas around the world, particularly in volcanic areas, local gas emissions are also endowed with magical provenance, some are deadly.

After the publications by de Boer, et al, the popular press picked up the story. The headlines condensed the phenomenally impressive and influential 1200-year stretch of the Oracle of Delphi into the oversimplification that the priestess just “got high”. In Broad’s book, de Boer expressed frustration in the reductive, materialistic conclusions that seemingly demystified the mystical. Science had destroyed the magic, some said. In truth, there was far more to the story than geology and a stoned woman spouting profundities. There remained much that was intangible and special, even mysterious, in this incredibly potent confluence of nature and culture. The Delphic Oracle was legendary — the story is renowned millennia later. Her pronouncements changed history and helped define Greek society. Even though her power was derived of this earth, it had all the spectacular appearance of a spiritual union with the gods. The intersection of geology and supernatural belief generated an enduring legend that was far greater than the sum of its scientific parts.


Broad, William J. (2007) “The Oracle: Ancient Delphi and the Science Behind Its Lost Secrets.” Penguin Group.
De Boer, J.Z. and Hale, J.R. (2000) “The Geological Origins of the Oracles at Delphi, Greece.” In The Archaeology of Geological Catastrophes, Geol Soc London Special Publ 171, 399-412.
De Boer, J.Z., Hale, J.R. and Chanton, J. (2001) “New evidence for the geological origins of the ancient Delphic oracle (Greece).” Geology, 29:8, 707-710.
Piccardi, L., Monti, C., Vaselli, O., Tassi, F., Gaki-Papanastassiou, K., Papanastassiou, D. (2008) “Scent of a Myth: tectonics, geochemistry and geomythology at Delphi (Greece).” J of the Geol Soc, London 165, 5-18.
Spiller, H., Hale, J.R., de Boer, J.Z. (2002) “The Delphic Oracle: A Multidisciplinary Defense of the Gaseous Vent Theory.” Clinical Toxicology. 40:2, 189-196.

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