From The Stargate Omnipedia

Ancient name for the great city of Atlantis -- at least while it was on Earth (Terra). When the city departed the planet to travel to the Pegasus Galaxy it left behind the Atlantus outpost, which over the eons would come to be buried beneath the ice of present-day Antarctica.

See also: Atlantus outpost


Lost City, Part 2 - The library of the Ancients directs Jack O'Neill to "Terra Atlantus," which they find to be only an outpost beneath the surface of Antarctica.
Rising, Part 1 - Millions of years ago Atlantis departed the surface of the Earth, leaving behind only a small outpost and a lone Ancient witness.