Atlantis has been lost for thousands of years. Tidal forces could sufficiently erode any identifying marks on buildings. Anything above ground could be weathered beyond recognition.

How would we ever know for certain that we had found Atlantis?

Indeed, who says we have not found it, or part of it, already?

it may be a long while. people dont live on the bottom of the ocean as much as we live on the land, and we still havent bagged a bigfoot so who knows?

There is also that possibility that we may never know because it isn't there, no matter how much I would like it to be.