Archeological Odyssey recently published a detailed account of the research, reprinted in Bible History Daily. Here’s an excerpt:
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.
According to toxicologist Henry Spiller, both of these symptoms are associated with the inhalation of hydrocarbon gases. Spiller studies the effects of such inhalants on young people, known as “huffers,” who breathe in fumes from gas, glue, paint thinner and other substances because of their intoxicating properties. Perhaps the Pythia too was high on one of these hydrocarbon gases.
It may even be possible to identify the kind of gas. Plutarch—who, we recall, was a priest of Apollo at the Delphic sanctuary—noted that the intoxicating pneuma had a sweet smell, like expensive perfume. Of the hydrocarbon gases, only ethylene has a sweet smell—so ethylene was probably a component in the gaseous emission inhaled by the Pythia.
Most researchers agree that the Pythia was chosen for her clairvoyant abilities as a trance medium, and that the fumes likely played an auxiliary role in her pronouncements.
Painting of the oracle is by the Hon. John Collier, from 1891.