Atlantis had been granted to Poseidon

http://www.skepticssa.org.au/html/atlantis.html

The Atlantean story that Plato told was concerned with the visit of Solon, the great Athenian lawgiver, (circa 630–560 BCE), to Egypt. According to Plato’s story, Solon was criticized by an Egyptian priest of the city of Saïs (in the West Nile delta), about his lack of knowledge concerning the ancient history of Athens.

He was told by the priest that some 9000 years before, Athens had fought a great war with Atlantis, a great island located in the Atlantic Ocean that was larger than both Libya and Asia.

The priest explained how, after the Creation the Greek gods divided Earth amongst themselves, and Atlantis had been granted to Poseidon, who blessed and protected the island.

What about the claims that Plato based his stories upon earlier factual historical material? The fact is that there were no references to Atlantis before Plato. Some claim that Herodotus mentioned Atlantis in his Histories, 4.184.1 (circa 484 BCE) but this is incorrect. 

Herodotus’ references were not to an island but to Mount Atlas, which he located at the westernmost part of Africa, a place he believed to be the western edge of the world. Mount Atlas, he claimed, was “a peak so high that its top was never seen”, inhabited by the “Atlantes” a race who “eat no living creature, and see no dreams in their sleep.”

In many ways Herodotus was notoriously unreliable as a reporter; he tended to include the most fantastic traveller’s tales as being factual events.

Nevertheless he was on more substantial ground as a historian, and in this respect he did mention certain information that was in his time, common knowledge, and some of these tend to cast serious doubt on the claims made by Plato concerning Atlantis. For instance, Herodotus mentioned that:

• although Plato described Atlantis as a huge island, Herodotus stated that Sardinia was the largest known island in the world
• the greatest danger Athens had ever faced was from the Persians at the Battle of Marathon in 480 BCE
• King Minos of Knossos had created the earliest sea power known in the Mediterranean.