The Oracle at Delphi in central Greece was a major religious center for more than 1000 years. Citizens and rulers alike made pilgrimages there to get advice on everything from mistresses to military conquests. The officiant at the oracle was always a woman (the Pythia) who perched on a tripod above a chasm in the bowels of the Temple of Apollo and inhaled fumes from the earth that would induce a prophetic, often crazed, trance during which she would relay the wisdom of the gods.
The story was dismissed as a myth during the first half of the 20th century. But in the late 1990s a geologist found traces of ethylene, a central-nervous stimulant that can produce euphoria, in a local spring and concluded it was the likely source of the oracle's frenzied trances (image below).
Giuseppe Etiope of the National Institute of Geophysics and Volcanology in Rome asserts that the marine limestones underlying the temple couldn't have contained ethylene in high enough concentrations to account for the trances. The team brought a portable laser sensor to Delphi and discovered no traces of ethylene. Other lab work turned up significant amounts of carbon dioxide and methane seeping from the ground suggesting these gases caused oxygen deprivation in the enclosed temple chamber that was the source of the Pythias' inspiration.
Link: The geological links of the ancient Delphic Oracle (Greece): A reappraisal of natural gas occurrence and origin.
In the time of Oracles....: