Such very shallow waters and mud shoals do exist on the north side of Europe, following the flooding of Doggerland (around 6000 BC). Those waters are "beyond" t

Such very shallow waters and mud shoals do exist on the north side of Europe, following the flooding of Doggerland (around 6000 BC). Those waters are "beyond" the Pillars of Heracles if you circumnavigate the entire Iberian coastline. It's likely that tales of this region could have made it to Greece, but I don't think many Greeks had seen it themselves.