Consulting the Delphic oracle was a scheduled procedure with a fee, a queue and a widely discussed possible chemical cause.
What a visitor actually did
The Pythia, a priestess chosen from local women, gave oracles on fixed days, traditionally the seventh of the month, for roughly nine months of the year. Petitioners paid a fee, sacrificed a goat, and had their question put to her by attending priests while she sat on a tripod inside the temple's inner chamber, the adyton. Her answers, often ambiguous, were then rendered into verse or prose by the priests for the petitioner to carry away and interpret.
The vapours question
Ancient writers, including Plutarch, who served as a priest at Delphi, describe a pneuma, a vapour rising from a fissure beneath the chamber, as the source of her trance. Geologists later found the temple sits on intersecting faults, and a 2001 study led by Jelle de Boer identified traces of ethylene and light hydrocarbons in the local spring water, lending some support to an old claim that scholars had spent much of the twentieth century dismissing as legend.
What is not settled
No one has directly measured gas in the actual adyton, so the mechanism remains an informed hypothesis rather than a proven one, sitting alongside psychological and ritual explanations for the Pythia's state.