A fabled city of the Ancients, believed to hold a cache of advanced weapons and technology. SG-1 discovered a stone tablet written in a dialogue of the Ancients that spoke of the city. It is also likely that Anubis (who coveted the Ancients' technology) discovered a map to the city, which he was finally unable to decipher.
While investigating the newly-discovered Ancient outpost of Atlantus in Antarctica, Daniel Jackson and a research team under the command of Elizabeth Weir discovered that the lost city is, in fact, the mythical city of Atlantis. Once on the continent we now know as Antarctica, Atlantis left the planet and moved to the Pegasus Galaxy between five and 10 million years ago. It was discovered by Weir's Atlantis expedition after Jackson uncovered an eight-symbol gate address to the city's new home planet.
Full Circle - Daniel, Sam and Jonas discover a tablet hidden on Abydos, which tells of the Lost City of the Ancients. SG-1 makes it its mission to find the city, in order to defeat the powerful Goa'uld Anubis.
[Fallen - Jonas incorrectly translates part of the Ancient tablet, leading him to conclude that the planet Vis Uban may contain the lost city.
[Chimera - Osiris uses advanced technology to invade Daniel's dreams, hoping he can translate an Ancient map to the lost city -- but he can't.
[Lost City, Part 2 - The Ancients' knowledge leads O'Neill to what SG-1 hopes is the lost city, but they find only an outpost left by the Ancients in Antarctica.
[Rising, Part 1 - Jackson and the research team discover that the lost city is Atlantis itself, and Weir leads and expedition to find the city in a distant galaxy.