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Vapor around the tripod feet. Geologists found two faults and traces of ethylene under the temple in the 1990s, which is roughly what Plutarch said.

For something like a thousand years, states did not go to war, found colonies, or change their laws without asking a woman sitting on a three-legged stool at Delphi. The Pythia was not an aristocrat or a trained priestess in the modern sense; she was chosen from the local women of the town, sometimes a farmer's wife, and once installed she gave up her old life and her own name. On the seventh day of the month, nine months a year, she washed at the Castalian spring, went down into the inner chamber of Apollo's temple, and sat over a fissure in the floor, holding laurel and a bowl of spring water, and answered. The answers were famously slippery, and the most quoted ones are the ones that ruined the people who received them. Croesus of Lydia was told that if he crossed the river Halys he would destroy a great empire, which he did: his own. Athens, facing the Persian invasion, was told to trust in a wooden wall, and spent an anxious week arguing about whether that meant a fence or a fleet. The temple carried two mottoes cut into the forecourt, know yourself and nothing in excess, which read less like mysticism than like advice from someone who has watched a lot of powerful men make the same mistake. Ancient writers said a sweet-smelling vapor rose from the chasm and put the Pythia into her state, and for most of the twentieth century scholars dismissed that as fiction, since French excavators had found no chasm. Then in the 1990s a geologist and an archaeologist working together identified two intersecting faults directly beneath the temple, bituminous limestone below, and traces of ethylene and methane in the spring water at the site: a gas with a sweet smell and a mild narcotic effect, exactly where Plutarch, who was a priest at Delphi and had every reason to know, said it came out of the ground. Plutarch also recorded the ugly day. A Pythia was pushed to prophesy on an unfavorable date to suit important visitors, went in unwilling, and came out screaming in a voice the priests described as harsh; she flung herself toward the door, the delegation fled, and she died within days. Collier painted her in 1891 with her eyes half shut, the vapor coming up around the tripod's gilded feet, and one bare foot resting on the frame of the stool.