The Oracle of Delphi

Hon. John Collier : Priestess of Delphi (1891)

In the painting, “Priestess of Delphi” by The Honorable John Collier, a priestess - the Pythia - is depicted in a trance state, seated over a fissure in the rock through which vapors rise from the underground stream. In her left hand is a sprig of laurel - in Greek mythology, Apollo’s sacred tree - and in the other hand a bowl meant to hold some of the water from the stream containing the gases.

Image via marinni and Uncertain Times.  More information from Wikipedia:

Apollo spoke through his oracle: the sibyl or priestess of the oracle at Delphi was known as the Pythia; she had to be an older woman of blameless life chosen from among the peasants of the area. She sat on a tripod seat over an opening in the earth. When Apollo slew Python, its body fell into this fissure, according to legend, and fumes arose from its decomposing body. Intoxicated by the vapors, the sibyl would fall into a trance, allowing Apollo to possess her spirit. In this state she prophesied. It has been postulated that a gas high in ethylene, known to produce violent trances, came out of this opening, though this theory remains debatable.  While in a trance the Pythia "raved" – probably a form of ecstatic speech – and her ravings were "translated" by the priests of the temple into elegant hexameters. People consulted the Delphic oracle on everything from important matters of public policy to personal affairs. The oracle could not be consulted during the winter months, for this was traditionally the time when Apollo would live among the Hyperboreans. Dionysus would inhabit the temple during his absence.