From about 1400 BC to 400 AD, the Oracle of Delphi was considered one of the most sacred sites in all of ancient Greece. It is located on Mount Parnassus in Phocis some 200 kilometers northwest from Athens.
People from all walks of life made pilgrimages there to seek advice from the God Apollo, which was relayed to them by Pythia (Πῡθίᾱ), the High Priestess. Her often cryptic ramblings were highly regarded and affected everything from the outcome of wars to when farmers should plant their crops. No kingdom, city or private person could afford to make critical decisions without consulting the Pythia. Thanks to her prestige, Delphi also became the richest Hellenic sanctuary. The Greeks called it the omphalos, or 'navel of the world'.
The ancient sources describe two distinct types of prophetic trance experienced by the Pythia. First, and more normally, she would lapse into benign semi-consciousness, during which she remained seated on the tripod, responding to questions—though in a strangely altered voice. According to Plutarch, once the Pythia recovered from this trance, she was in a composed and relaxed state, like a runner after a race. A second kind of trance involved a frenzied delirium characterized by wild movements of the limbs, harsh groaning and inarticulate cries. When the Pythia experienced this delirium, Plutarch reports, she died after only a few days—and a new Pythia took her place.
The Pythia entered her trance by inhaling sweet-smelling noxious fumes coming from deep fissures underneath the temple, according to the ancient historian Plutarch.
At first, a lack of evidence led modern archaeologists to dismiss Plutarch’s observations, but it appears the ancients were right after all. Tests showed that the waters of a nearby spring showed the presence of methane and ethane, which can be intoxicating, as well as ethylene.
. In small doses, ethylene stimulates the central nervous system, causing hallucinations and emits a sweet odor. However, it was not particularly successful as an anesthetic, because high concentrations were needed to achieve unconsciousness and it was dangerously explosive.
 De Boer et al: New evidence of the geological origins of the ancient Delphic oracle (Greece) in Geology – 2001
 Spiller et al: The Delphic oracle: a multidisciplinary defense of the gaseous vent theory in Journal of Toxicology – 2002