TRADITION ATTRIBUTED the prophetic inspiration of the powerful oracle to geologic phenomena: a chasm in the earth, a vapor that rose from it, and a spring. Roughly a century ago scholars rejected this explanation when archaeologists digging at the site could find no chasm and detect no gases. The ancient testimony, however, is widespread, and it comes from a variety of sources: historians such as Pliny and Diodorus, philosophers such as Plato, the poets Aeschylus and Cicero, the geographer Strabo, the travel writer Pausanias, and even a priest of Apollo who served at Delphi, the famous essayist and biographer Plutarch.

Strabo (64 BC–AD 25) wrote:“They say that the seat of the oracle is a cavern hollowed deep down in the earth, with a rather narrow mouth, from which rises a pneuma [gas, vapor, breath; hence our words “pneumatic” and “pneumonia”] that produces divine possession. A tripod is set above this cleft, mounting which, the Pythia inhales the vapor and prophesies.” Plutarch (AD 46–120) left an extended eyewitness account of the workings of the oracle. He described the relationships among god, woman and gas by likening Apollo to a musician, the woman to his instrument and the pneuma to the plectrum with which he touched her to make her speak. But Plutarch emphasized that the pneuma was only a trigger. It was really the preconditioning and purification (certainly including sexual abstinence, possibly including fasting) of the chosen woman that made her capable of responding to exposure to the pneuma. An ordinary person could detect the smell of the gas without passing into an oracular trance.