The Oracle at Delphi is referenced throughout Greek myths and history. Supposedly she was rendered psychic by Apollo. Practically, she was off her skull on gas that seeped out of the fissures of the temple in which she lived. Here is the scientific explanation for what caused this woman to utter her confused prophecies.
Even during the Oracle at Delphi's time, it was widely known that the Oracle's visions had a practical cause. Gas seeped out of the cracks in the cave where she sat, causing her to talk nonsense. This nonsense would then be interpreted by priests around her. Some of the predictions were surprisingly accurate - according to legend. Croesus, the richest man of his time, performed a kind of scientific test on oracles, when he had messengers go out to all of them and ask what he would be doing on a certain date. Delphi got the only correct answer - cooking a tortoise in a pot. (Bold choice. I wouldn't think of the richest guy in the world doing his own cooking.)
Modern archaeologists weren't convinced - not about the tortoise and not about the gas. They inspected the geology of the area for volcanic activity that might vent gas, and found nothing, not even in the distant past. Later, however, they took another look, and found two fault lines converging just under the temple of Delphi. Perhaps the mystic "vapors" that the Oracle breathed in had seeped through these. In interdisciplinary team found that dissolving limestone along those lines gave off gas, and spiked the local water.
The main components found in the water were ethane, methane, and ethylene. Most think that it's ethylene that is the key component. Ethylene was one of the early painkillers used during medical procedures. It packed more of a punch than nitrous oxide. It smells a bit like flowers, and renders the sniffer euphoric, but extremely out-of-it. It's not especially bad for people (although various compounds of ethylene can be very harmful), and its main danger is that of suffocation. It displaces air and makes people too high to notice. The combination of extreme anesthesia, euphoria, and lack of oxygen, can make a person say all kinds of things.
Other scientists argue that ethylene wasn't the chemical that made the Oracle talk, though. Methane, which decidedly doesn't smell like flowers, can also cause hallucinations if inhaled in high enough doses. The methane theory isn't without its faults. Methane can kill people a lot faster than ethylene, particularly if it's being burned, or otherwise decomposing, to produce carbon dioxide. A by-product of the conversion is carbon monoxide, which attaches itself to red blood cells more readily than oxygen does. Too much can smother a person, but not before they often experiences confusion and hallucinations. Carbon monoxide is a regular by-product of burning fuels, though, and so there's no reason why the ancient Greeks, who surely had mastered fire, would consider the gas and its effects so special.
It would be considered irresponsible, even by the most hardcore archaeologists, to dose a grad student with spiked water, or shut them up in a cave, to either be poisoned or suffocated. So there's no way we'll be sure whether the Oracles of Delphi were high or were suffocating or were faking it. All we can be sure of is she craved tortoise meat.
Top Image: Frank Fleschner