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.
It describes in detail how Apollo chose his first priests, whom he selected in their "swift ship"; they were "Cretans from Minos' city of Knossos" who were voyaging to sandy Pylos.
Diodorus also explained how, initially, the Pythia was an appropriately clad young virgin, for great emphasis was placed on the Oracle's chastity and purity to be reserved for union with the god Apollo.
Echecrates the Thessalian, having arrived at the shrine and beheld the virgin who uttered the oracle, became enamoured of her because of her beauty, carried her away and violated her; and that the Delphians because of this deplorable occurrence passed a law that in the future a virgin should no longer prophesy but that an elderly woman of fifty would declare the Oracles and that she would be dressed in the costume of a virgin, as a sort of reminder of the prophetess of olden times.
Authors who mention the oracle include Aeschylus, Aristotle, Clement of Alexandria, Diodorus, Diogenes, Euripides, Herodotus, Julian, Justin, Livy, Lucan, Nepos, Ovid, Pausanias, Pindar, Plato, Plutarch, Sophocles, Strabo, Thucydides and Xenophon.
Nevertheless, details of how the Pythia operated are missing as authors from the classical period (6th to 4th centuries BC) treat the process as common knowledge with no need to explain.