Atlantis was first discussed in Plato's dialogues between Timaeus and Critias.  The accounts of Atlantis are viewed by many as a complete fabrication created in the mind of the philosopher.  Whereas others believe Plato was serious when he dubbed Atlantis as the lost civilization.

Some believe a catastrophic event took place and Plato was merely describing that event.  This event, according to many, destroyed the Minoan civilization on Crete and Thera.  

Plato stated Atlantis was an advanced civilization that lay "beyond the pillars of Hercules."  There is much debate over where the pillars of Hercules actually stand.  But in a single day Atlantis sank nearly 9000 years before Plato's life.

There is much debate over whether or not the story is true or if Plato simply used the story as symbolism for his philosophical lesson.  That lesson being that if one becomes so sure of one's self that they actually have more power than the Supreme Being, they will destroy themselves as a result.  This philosophy would explain why Atlantis was always considered to be a very advanced civilization.

Keeping in mind that what Plato described was the ultimate utopia - a society that lived without violence, bigotry, anger, etc. But because of its behavior and beliefs managed to destroy the entire civilization in a single day.

Edgar Cayce, famous American psychic, predicted that the lost continent would appear in the Bahamas in 1968 or 1969.  In 1968 a pilot photographed what appeared to be buildings or walls and a road under the water off the coast of Bimini island.  The Bimini Road is believed by many to be the road to Atlantis.

One must make up their own mind as to the truth of Plato's story and the predication of Mr. Cayce.  Beyond these statements there is no absolute evidence that Atlantis ever existed.

Hope this answers your questions!