Atlantis, also called the Lost City of the Ancients and the City of the Ancestors, Atlantus in Ancient, is a City-ship built by the Ancients several million years ago, which has approximately the same internal space as found in Manhattan. The Ancients left the Milky Way galaxy several million years ago for the Pegasus galaxy, taking Atlantis with them and settling on a planet they named Lantea, which became their new capital.
Millions of years later, after the Lanteans—the name the Ancients came to be known by—submerged the city to protect it from their enemies, the Wraith, and returned to Earth through the Stargate. Their story, over time, inspired the Tau'ri myth of Atlantis.As of 2009, Atlantis has returned to Earth. It is currently hardbound in the San Francisco bay with its cloak active to hide it from the public. The area is placed under naval quarantine to prevent ships from passing through the cloaking field.
Atlantis has a great deal of advanced technology in it, ranging from the Stargate and Puddle Jumpers, to Drone weapon, the Stardrive, the all-important shield, transporters (from Duyi'zu), Hyperdrive (from Filo)