in earlier times, a lot of the freshwater was locked into glaciers, which have since melted and caused the water level to rise. There is evidence around the world of former civilizations that are now submerged.

about 400 foot lower sea level. When the glaciers melted the water rose FAST. Fast enough to cause the universal flood myths and destroy any civilizations that were on the waters edge.
Today 80 or 90 percent of the global population live near coasts or rivers...I doubt it was any different then.

Perhaps instead of AN Atlantis it was a generic term for many lost civilizations.

Hmmm...I wonder how High Tech they might have been?