The Elusinian Mysteries were a widespread Greek cult practice which had exceptional continuity, focused on Demeter & Persephone, thought to continue a Minoan cult (so pre-Greek), and which continued well into the Roman era, and only ended with full Christianisation of the Roman Empire around 400AD.
What exactly the Mysteries were, is the subject of much speculation, particularly because they were intentionally secret & kept mysterious, on pain of death to initiates revealing them - it seems they gained some of their power from new people not knowing what to expect, as for instance Masonic rituals do today. It may be totally irrelevant, but it is interesting to note the step well at Quinta le Regaleira in Sintra, which would have involved a precession down into the ground, where at the base of the stairs it meets paths along an underground stream with niches for events.
It is notable that Persephone was a Cthonic deity, which means sacrifices would have been made in a pit or underground in a bothros probably by burying, as opposed to an Olympic deity like Demeter, who would have recieved sacrifices on an altar typically by burning (see passage tombs, for a comparable example to a bothros, considered gates to the underworld). We have learned substantially more about these practices of Greek religion since the decipherment of Linear B, the written language of the Mycenae (the related Linear A is the even earlier written language of the Minoans, still undeciphered).
Here is what has been deciphered from inscriptions:
"At Eleusis inscriptions refer to "the Goddesses" accompanied by the agricultural god Triptolemos (probably son of Ge and Oceanus), and "the God and the Goddess" (Persephone and Plouton) accompanied by Eubuleus who probably led the way back from the underworld. The myth was represented in a cycle with three phases: the "descent", the "search", and the "ascent" (Greek "anodos") with contrasted emotions from sorrow to joy which roused the mystae to exultation. The main theme was the ascent of Persephone and the reunion with her mother Demeter. At the beginning of the feast, the priests filled two special vessels and poured them out, the one towards the west, and the other towards the east. The people looking both to the sky and the earth shouted in a magical rhyme "rain and conceive". In a ritual, a child was initiated from the hearth (the divine fire). The name pais (child) appears in the Mycenean inscriptions. It was the ritual of the "divine child" who originally was Ploutos. In the Homeric hymn the ritual is connected with the myth of the agricultural god Triptolemos. The goddess of nature survived in the mysteries where the following words were uttered: "Mighty Potnia bore a great son".3 Potnia (Linear B po-ti-ni-ja : lady or mistress), is a Mycenaean title applied to goddesses, and probably the translation of a similar title of Pre-Greek origin. The high point of the celebration was "an ear of grain cut in silence", which represented the force of the new life. The idea of immortality didn't exist in the mysteries at the beginning, but the initiated believed that they would have a better fate in the underworld. Death remained a reality, but at the same time a new beginning like the plant which grows from the buried seed. A depiction from the old palace of Phaistos is very close to the image of the "anodos" of Persephone. An armless and legless deity grows out of the ground, and her head turns to a large flower." - from Wikipedia, where these are all supported by references
"Upon reaching Eleusis, there was an all-night vigil (pannychis) according to Mylonas and Kerenyi. perhaps commemorating Demeter's search for Persephone. At some point, initiates had a special drink (kykeon), of barley and pennyroyal, which has led to speculation about its chemicals perhaps having psychotropic effects from the Ergot fungi.
"Discovery of fragments of Ergot (fungi containing LSD-like psychedelic alkaloids) in a temple dedicated to the two Eleusinian Goddesses excavated at the Mas Castellar site (Girona, Spain) provided legitimacy for this theory. Ergot fragments were found inside a vase and within the dental calculus 25-year-old man, providing evidence of Ergot being consumed (Juan-Stresserras, 2002). This finding seems to support the hypothesis of ergot as an ingredient of the Eleusinian kykeon." - Greater Mysteries, on Wikipedia
So, it seems there was a powerful, ancient, widespread and accessible cult practice, of going into a cave, having a psychaedelic substance, and coming out with mysterious knowledge, and this would definitely have been known to Plato, whether or not he was an initiate. It's hard not to think, Plato may have been specifically satirising this as a source of knowledge with his Cave metaphor.
Edited to add: Plato was definitely an initate, and so was Socrates (presumably before he was a soldier, because anyone who had killed someone could not participate).
"self-restraint and justice and courage and wisdom itself are a kind of purification. And I fancy that those men who established the mysteries were not unenlightened, but in reality had a hidden meaning when they said long ago that whoever goes uninitiated and unsanctified to the other world will lie in the mire, but he who arrives there initiated and purified will dwell with the gods. For as they say in the mysteries, 'the thyrsus-bearers are many, but the mystics few'; and these mystics are, I believe, those who have been true philosophers. And I in my life have, so far as I could, left nothing undone, and have striven in every way to make myself one of them." - spoken by Socrates in The Phaedo
There follows a discussion of the immortality of the soul, and true knowledge as remembering or recollection rather than as discovery or as a creative process, and goes on to give the account of the soldier Err of the afterlife which is conjectured by scholars to align with the Elusinian account. It is notable that these stories being allegorical is an interpretation rejected by modern scholars, though it has had a powerful sway in the past on Neoplatonists, and influencing Abrahamic faiths.
The plot thickens! I am inclined to think even while respecting the Mysteries, in the Cave story Plato is still elevating reason over initiate story-based sources of knowledge.