From Herodotus and Homer to the warriors of Ancient Greece the mystic utterances from the Oracle of Delphi were regarded as sacrosanct. But now the hugely influential pronouncements of the oracle are said by Greek and Italian archaeologists to have been the result of oxygen deficiencies in the priestesses’ brains.

Delphi, which draws tourists by the thousand each year, lies on the almost sheer side of Mount Parnassus in central Greece. Great fissures in the cliff overlooking the site mask deep geological faults through which toxic gases seep to the surface, reducing oxygen in the cave — the Navel of the Earth — where the priestess de- livered her often obscure political oracles.

The priestess, known to the ancients as Pythia, would thus be in a state of mild anoxia — a partial lack of oxygen in the brain — inducing the ecstatic trance that classical writers said brought forth the oracles. They, however, claimed that Pythia entered her trance by chewing laurel leaves while sniffing the vapours of hallucinogenic herbs.

Two years ago a team headed by George Papatheodorou, Emeritus Professor of Geology at Patras University, and Giorgio Etiope, of Italy’s National Institute of Geophysics and Vulcanology, detected traces of methane, ethanol and carbon dioxide in the narrow cave where the Pythia is believed to have sat on a tripod while uttering her messages, often in high-pitched shrieks.

“There is a close relationship between the site of the Delphic Oracle and its geology,” Dr Papatheodorou told Kathimerini newspaper. “The site lies on a fault where gases leak out. These gases cause an oxygen reduction that induces a mild hypnotic state that could well produce hallucinations.”

The gases were detected in the summers of 2004 and 2005 by a sensor placed on the floor of the cave where the Pythia reputedly sat. “We have formulated a scientific hypothesis that we believe is a credible scenario,” Dr Papatheodorou said.

The historian Plutarch, who himself served as a priest at Delphi, wrote that a sweetish odour inundated the premises while the Pythia was in her trance. This, according to Dr Papatheodorou, could have been ethylene gas, though no trace of it was found during the recent search. “Nothing can be ruled out, as geological changes could have taken place since ancient times,” he said.

To the ancient Greeks the Delphic Oracle was the supreme divine word. But its often ambiguous pronouncements were shamelessly reinterpreted to suit particular policies and interests. Some modern writers speculate that the Pythian trance was an elaborate fraud, and that the priestesses were highly alert and well informed about Greek affairs thanks to a network of agents.