She advised generals about invasions; told citizens about the fates of their investments; and even warned Oedipus about the dangers of murdering his father and marrying his mother.

Yet the oracle at Delphi was not blessed with prophetic vision, scientists have discovered. In fact, she was high on alcoholic vapours.

This is the conclusion of scientists - writing in this month's Scientific American - who have found that the oracle chamber was built over a geological fault from which seeped ethane and ethylene gases. As a result, the oracle, the temple maiden who uttered Delphi's prophecies, was probably in a permanent narcotic state.

In other words, the oracle's utterings, upon which so much of ancient Greek life depended, were not the words of Apollo, the god of prophecy, but the babblings of a drunk or glue-sniffer.

'The petrochemical-rich layers in the limestone formations most likely produced ethylene, a gas that induces a trancelike state, that could have risen through fissures,' state the team led by Prof John Hale of Louisville University.

The oracle was typically depicted in Greek art as sitting on a tall three-legged stool, with a laurel sprig in one hand, and a cup of water filled from the spring that bubbled into her chamber. In a trance, she answered the questions of supplicants.

Plutarch noted that the oracle sat in a chamber deep in the Delphic temple's bowels, from which a sweet-smelling gas emanated. He linked this gas with her trances, though scholars later concluded that her behaviour was probably a sham.

But recent research by geologists has shown that the chamber rested above two intersecting fault lines.

'This intersection made the rock more permeable and provided pathways along which both groundwater and gases were able to rise,' state the team. 'These vapours then moved through fissures into the small chamber where the oracle sat.'

The team analysed water from springs around the temple and discovered high levels of methane, ethane and ethylene. 'Because ethylene has a sweet odour, the presence of this gas supports Plutarch's description of a gas that smelled like expensive perfume.'

Ethylene fumes produce an effect in test patients in which they experience trances, euphoria, out-of-body sensations and amnesia. Occasionally, they suffer spasms and can even die. Ancient Greek texts record that such fates were also suffered by the oracle who, on one or two occasions, died after convulsions.

'God though he was, Apollo had to speak through the voices of mortals,' state the team, 'and he had to inspire them with stimuli that were part of the natural world.'