Pindar (522-443 BC) was one of the nine ancient Greek lyric poets. He refers to the ‘Pillars of Heracles’ in three of his victory odes, Olympian 3.43-45, Nemean 3.19-23 and Isthmian 4.11-13, doing so in a manner which indicates that the term was used as a metaphor for the limit of human (Greek) experience. As such, the location of the Pillars would have changed as the colonial and commercial expansion of the Greeks developed beyond the Aegean.
>One commentator has interpreted Pindar’s Olympian Ode 3 as referring to the Pillars of Herakles being placed at the ‘Springs of Ister’ on the Danube near Belgrade, noting that “this is the very edge of Pindar’s Celtic world and a point beyond which a Greek probably did not want to venture.“(b)<
Oliver D.Smith has drawn attention to the Nemean Ode 4, 65-70 as having echoes of Plato’s description in it(a).
(a) https://atlantisresearch.wordpress.com/ (now offline)