Flash Gordon's secret base.
Plato definitively located Atlantis as outside the pillars of Hercules. And he ought to know, since he made it up.
As in other of his writings, Plato made overt and exlicit use of myths and stories as instructional aids, especially for communicating moral lessons to children, which is the context in which the Atlantis story is put in the mouth of Timaeus.
It's a lot of fun to imagine a great ancient city lost beneath the waves, but that's all it is. The fact remains, there was no record of the Atlantis myth anywhere, ever, in any language, before Plato's stories.
It is a better thing to cultivate wonder at the mysteries of the real world, rather than to besot the imagination with idle fantasies...