The Pythia (or Oracle of Delphi) was the priestess who held court at Pytho, the sanctuary of the Delphinians, a sanctuary dedicated to the Greek god Apollo. Pythia were highly regarded, for it was believed that she channeled prophecies from Apollo himself, while steeped in a dreamlike trance.

It is a Hellenic tradition that the Pythia entered her trance through the influence of volcanic fumes or hallucinogenic gases emerging from a crevice in the floor of the Castalian Spring. Although the sanctuary lies directly above two geological fault lines and the spring near the sanctuary contains ethylene, which can provoke hallucinations, there has been no conclusive determination on whether the trance states of the Pythiai were induced through exposure or was self-induced.