The Oracle of Delphi was the most famous oracle of Ancient Greece and the known world. It is located in Delphi.

It is considered the Omphalos (navel) of the world because, according to tradition, two eagles met at Delphi when Zeus left them, one in the east and one in the west.

It was dedicated to the god Apollo. Pythia was the vessel through which the god communicated, and gave the oracles which were recorded.

The Pythian feast was held every four years, in honor of Apollo, which Angelos Sikelianos tried to revive.

Visitors who asked the Pythia for an oracle were called Theopropes, and during the oracle ritual they had first to purify themselves with water from the Castalia spring and pay a certain tax, called the Pelanon, which was not the same for everyone, while in the temple of Apollo they had to offer a sacrifice, usually a goat. The animal was not allowed to have any defect, while a necessary omen for Pythia to be able to give her oracles was that the animal began to tremble after it had first been soaked in cold water.

Among the Theopropes, there was a hierarchy as to who preceded the consecration. The first to qualify for the oracle were the inhabitants of Delphi. They were followed by those who had been honored by Delphi with the privilege of divination and then the rest. Within the last two groups, the turn was decided by lot.

Oracles in ancient times were known for their ambiguous meaning. There were many times when the oracle did not take an impartial stance but openly sided with one city, such as in the Peloponnesian War, which sided with the Lacedaemonians. However, its services were granted to all, regardless of the political situation that prevailed in their territories.

In 1892, French archeologists began excavations in the area of the sanctuary of Delphi to find something that would prove that there was indeed a gap that allowed the gasses to emerge and create the channel of "communication" between Pythia and Apollo.

But they could not find anything to prove it, and so the legend remained in the realm of myth. It was not until the late 1990s that a team of an archeologist, a toxicologist, a chemist and a geologist argued otherwise. According to their theory, the navel rock where Pythia sat and told her oracles among the vapors was just above a fissure created, providing a passage for the vapors of gasses like ethane and methane, to rise to the surface and be inhaled by Pythia.

Pliny says that the gas produced a divinatory state of inspiration. Of special importance is the testimony of Plutarch, who was a priest at Delphi for more than 20 years. In two of his works, he speaks of the Delphic problems. The gap and the gas are mentioned many times. But he never said that the gap was in the depths of the temple.

He also says that the vapors were not constant, but were sometimes weak and sometimes strong. He also quotes witnesses that the emanating spirit was full of fragrance at times. Significantly, he confirms that the house was filled with this fragrance when people waited for the oracles and came out of the sanctuary.