What, you say it's an island because the only guy in antiquity who wrote about it as if he had any intimate knowledge of the subject said so?

You fool!

https://en.wikipedia.org/wiki/Atlantis#Plato's_dialogues

Atlantis, at least the Atlantis Plato wrote about, was a similar topsy-turvy version of Athens as Wonderland was a topsy-turvy version of Victorian England, but with an added moral lesson. The whole idea was- "wouldn't it be nutty if Athens was like this? Well we're lucky it isn't because look what happened." It starts with Socrates asking for a description of a perfect society, and then Critias says, "okay, here's the perfect society and here's what went down and here's why we don't want that." And basically Atlantis believers say it was real because Critias said so. Because Plato said he did. In order to make a point that had nothing to do with history.

But sure, a global space-level civilization, we'll go with that.