The Pythia, commonly known as the Oracle of Delphi, was the priestess at the Temple of Apollo at Delphi, located on the slopes of Mount Parnassus, beneath the Castalian Spring. The Pythia was widely credited for her prophecies inspired by Apollo. The Delphic oracle was established in the 8th century BC, although it may have been present in some form in Late Mycenaean times, from 1,400 BC and was abandoned, and there is evidence that Apollo took over the shrine from an earlier dedication to Gaia. The last recorded response was given during 393 A.D., when the emperor Theodosius I ordered pagan temples to cease operation.
During this period the Delphic Oracle was the most prestigious and authoritative oracle among the Greeks. The oracle is one of the best-documented religious institutions of the classical Greeks. Authors who mention the oracle include Aeschylus, Aristotle, Clement of Alexandria, Diodorus, Diogenes, Euripides, Herodotus, Julian, Justin, Livy, Lucan, Ovid, Pausanias, Pindar, Plato, Plutarch, Sophocles, Strabo, Thucydides, and Xenophon.
The name 'Pythia' derived from Pytho, which in myth was the original name of Delphi. The Greeks derived this place name from the verb, pythein (πύθειν, "to rot"), which refers to the decomposition of the body of the monstrous Python after she was slain by Apollo. The usual theory has been that the Pythia delivered oracles in a frenzied state induced by vapors rising from a chasm in the rock, and that she spoke gibberish which priests interpreted as the enigmatic prophecies preserved in Greek literature.
Recent geological investigations have shown that gas emissions from a geologic chasm in the earth could have inspired the Delphic Oracle to "connect with the divine." Some researchers suggest the possibility that ethylene gas caused the Pythia's state of inspiration. However, Lehoux argues that ethylene is "impossible" and benzene is "crucially underdetermined." Others argue instead that methane might have been the gas emitted from the chasm, or CO2 and H2S, arguing that the chasm itself might have been a seismic ground rupture, although this idea has been challenged by scholars such as Joseph Fontenrose and Lisa Maurizio, who argue that the ancient sources uniformly represent the Pythia speaking intelligibly, and giving prophecies in her own voice.
There are also many later stories of the origins of the Delphic Oracle. One late explanation, which is first related by the 1st century BC writer, Diodorus Siculus, tells of a goat herder named Coretas, who noticed one day that one of his goats, who fell into a crack in the earth, was behaving strangely. On entering the chasm, he found himself filled with a divine presence and could see outside of the present into the past and the future. Excited by his discovery he shared it with nearby villagers. Many started visiting the site to experience the convulsions and inspirational trances, though some were said to disappear into the cleft due to their frenzied state. A shrine was erected at the site, where people began worshiping in the late Bronze Age, by 1600 BC. The villagers chose a single young woman as the liaison for the divine inspirations. Eventually she spoke on behalf of gods. (read more) (the oracle)