Of course Plato just mentioned it in passing, so there was a lot of myth around it even then. Who knows, it may have originated with a Mycenean village of 200 being submerged by a wave in 1900 BC ;-P
There certainly was, and Plato was writing about an event that occurred long before his time. His account can hardly be taken as fact.
If the speed of inundation is an issue, the theory in question involves a huge ice-dammed lake in Scandinavia that suddenly voided its reservoir due to "global warming." When those things finally flooded (and there were clearly many of them), all hell broke loose. Vast amounts of low-lying territory could have been flooded very quickly.
For comparison purposes look into a similar event caused by Pleistocene Lake Missoula that created the Scablands in Idaho-Washington. Or the violent flooding that occurred when former Lake Bonneville (now a small remnant called the Great Salt Lake) poured at an estimated 15 million cubic feet of water per second through the Portneuf Narrows LAKE BONNEVILLE
Two of Plato’s dialogues are in large part devoted to the discussion of Atlantis, so it wasn’t really just mentioned in passing. He’s literally the only ancient source of the tale; there’s a reference I’ve seen to a later Greek tourist, Creon, who claimed to have seen the written version of the story orally transmitted in the dialogues, carved on a wall of some grand building in Egypt, but as there is no such written version on any known Egyptian structure, Creon was probably getting buggered by a tour guide, who were no more reliable then than they are now.