The Temple of Apollo: hot air at fault for the oracle.

The Greeks and Romans took their prophesies from a woman who was high on the fumes of natural gas, say US geologists1. Geological surveys of the site of the Greek Temple of Apollo in Delphi reveal that the temple ruins lie over a fault cross that emits intoxicating vapours.

The oracle at Delphi made the site a major religious centre for 2,000 years. Greek and Roman rulers flocked there, seeking advice on private and political affairs. The oracle was originally sacred to the Earth goddess Gea; later, a temple was dedicated to the Greek god Apollo. The oracle was finally forbidden in AD 392 by the Christian emperor of Rome.

The Greek writer Plutarch, who, in the first century AD, served as a high priest of the temple, left clear records of how the oracle worked. It was spoken by a local woman - the Pythia - who entered a trance inside a small chamber, called the adyton. These trances occasionally deepened into delirium, even death.

In the adyton, Plutarch says, the Pythia inhaled vapours from a fissure or spring. He describes the fumes as sweet-smelling, like perfume. Despite his priestly role, Plutarch was canny about the origin of the gases, speculating that they issued from the rocks below and might be affected by nearby earthquakes.

But when the temple was excavated in the nineteenth century, archaeologists found no fissure or vapour emissions, leading some to wonder whether the legendary intoxicating fumes may have been inspired by other nearby geological features.

Last year, geologist Luigi Piccardi in Florence, Italy, suggested that the idea for the myhthical chasm might have been prompted by a rupture opened up by a massive earthquake in the region, similar to the one in 373 BC that destroyed nearby cities on the Gulf of Corinth.2.

Now Jelle de Boer of Wesleyan University in Connecticut, USA, and co-workers have discovered a previously unknown geological fault passing straight through the Sanctuary and Temple of Apollo. The fault is punctuated by active and dried-up springs. Indeed, there was an ancient spring house in the sanctuary right on the fault line.

The new-found fault crosses the long-known Delphi fault, apparently right below the temple. This crossing makes the bitumen-rich limestone there more permeable to gases and groundwater.

Seismic activity on the faults could have heated up these deposits, releasing light hydrocarbon gases, the researchers speculate. Indeed, water from a spring northwest of the temple contains methane, they report - and, even more intriguingly, traces of ethylene.

Ethylene, a sweet-smelling gas, stimulates the central nervous system - it was once used as an anaesthetic. Although fatal in large quantities, small doses produce a floating sensation and euphoria. In other words, just what an oracle needs to start having visions.