It seems like the oracle might have been breathing in some kind of intoxicant, or maybe was just experiencing hypoxia through replacing air. Do we have any clues from history (maybe combined with geology!) about what the actual stuff is that was being inhaled?
In short, yes, no and maybe. =|
In 1990s, a geologist, a geochemist and archaeologist (forget their names, I'll include a citation to a summation of this investigation below) got together to try to figure out the sources of these reports in literature. They found trace amount of methane and ethane and ethylene in the water - essentially hydrocarbon gases. Exposure to these gases could lead to... let's say, vivid experiences.
So for awhile, I believe this hypothesis was sufficient for most academics. Then in 2007, Foster and Lehoux questioned it. They argue that there was not enough exposure to these gases to actually cause these vivid experiences. So it's for up for debate. The main issue right now is whether there was enough exposure of these gases to participants during the periods at which these reports of intoxication were written.
De Boer, Jelle Zeilinga. "The Oracle at Delphi: The Pythia and the Pneuma, Intoxicating Gas Finds, and Hypotheses." In History of Toxicology and Environmental Health: Toxicology in Antiquity Volume 1, edited by Philip Wexler, 83-91. Cambridge: Academic Press, 2014.
Foster, J., and D. Lehoux. "The Delphic Oracle and the ethylene-intoxication hypothesis." Clinical Toxicology 45, no. 1 (2007): 85-89.