I think that Plato making up Atlantis is the most convincing argument, since if the first reference to it is fiction or a metaphor, it would not make sense to believe it was real.

Interestingly, there is evidence to suggest that the city of Helike, Greece was submerged in 373 BC when an earthquake lowered the delta it sat on, and it was covered by a tsunami. The remains of what are believed to be Helike are actually on land, because they were already near the coast, and sediment covered them up. Some people believe that Helike gave Plato the idea for Atlantis, though this is not certain.