Atlantis on Lantea.

Lantea is the original homeworld of the Lanteans and the location where the Earth Exploratory Expedition found the city 10,000 after the Lantean/Wraith war.

Mostly a waterworld, Lantea does possess one large continent, uninhabited. However, the oceans of the planet do contain a species of 'whale' that possesses moderate levels of intelligence...enough to communicate with Atlantis through a form of telepathy.

After moving away from Lantea, the Alterra returned the city to her second home just before the second war against the Wraith began.