From The Stargate Omnipedia
The home world of Atlantis
for many generations.
Ancients slumbered here for many thousands of years following the Ancients' devastating war with the Wraith. The war led to a Wraith assault on the planet -- which was the center of Ancient activity in the Pegasus Galaxy.
Not long after the arrival of the Atlantis expedition from Earth, the city's power reached critical lows and an automated mechanism rose the city to the surface to prevent its destruction -- though it left the city vulnerable to attack from above.
A large portion of the planet's surface is water, and the raised city now sits as an island amidst a great ocean. Near the city -- only 25 minutes away by Puddle Jumper -- is a large continent. After it was discovered, the Athosian refugees in Atlantis decided to resettle there, in order to start a new life in relative safety from the Wraith. They are still able to communicate with the city, where the planet's Stargate is located.
Lieutenant Aiden Ford suggested the name "Atlantica" for the planet.
When the Asurans assaulted Atlantis from orbit Dr. Elizabeth Weir made the decision to leave Lantea in search of a new home world.
HOME TO - Ancients, Atlantis expedition
FIRST APPEARED - Rising, Part 1
Rising, Part 1 - The Earth team travels to Atlantis and discovers that the city is on a planet with no signs of land.
Rising, Part 2 - When the city runs out of power, the city automatically rises to the surface of the planet -- where it sits unprotected.
Suspicion - Sheppard and Ford explore the planet with a Puddle Jumper, and discover a land mass near Atlantis -- to which the Athosians resettle.
Grace Under Pressure - The planet's great ocean is explored when a Puddle Jumper accidently crashes and sinks deep into the sea.
First Strike - The Replicators launch a devastating attack on Atlantis, forcing the city to relocate to another world.