According to a 19th century book, Atlantis broke up and sank piecemeal over thousands of years. The last Isle was called Poseid and sank beneath the Atlantic ocean around 10,000 BC. It had colonies in South America, and it's total population was evidently just shy of 300 million when it was destroyed. Some were forewarned and left the continent for Egypt and the South American colonies just before the end. During it's last few thousand years the nation degenerated from an ultra high tech democracy to a low tech theocracy which included human and animal sacrifice. That was the living end.
Here's a summary of the fall of Atlantis from the 1886 book "A Dweller on 2 Planets" by Phylos the Thibetan:
http://www.sacred-texts.com/atl/dtp/index.htm This is the index page. There's two chapters in "Book the Third" on how Atlantis perished.