Millennia ago, the Ancients had left their home in the Milky Way to settle in the Pegasus Galaxy, where they had established the city of Atlantis. The Lantian civilization flourished in the Pegasus Galaxy, but there they encountered the Wraith, an alien species that feeds on humans by drawing the life force from them. Besieged by the Wraith, the Lantian Ancients abandoned Atlantis and left the Pegasus Galaxy more than 10,000 years ago to return to the Milky Way, and Atlantis became the "Lost City."
When Daniel calculated the location of the Lost City of Atlantis in the Pegasus Galaxy, an expedition, headed by Dr. Elizabeth Weir, was sent through the stargate to set up a base of operations in what was expected to be a one-way mission. It was nearly a year before contact with the Pegasus Galaxy was established and travel between Earth and Atlantis was possible. Two years after the Atlantis expedition first departed, members of SG-1 finally had the opportunity to travel to the Pegasus Galaxy. As Daniel and Vala conducted research in Atlantis's Ancient library database, McKay joined Carter and Mitchell aboard the Odyssey in an attempt to block the Ori supergate. A wormhole was established from a black hole in the Pegasus Galaxy to the Ori supergate, effectively preventing additional Ori ships from entering the Milky Way.
Episode Reference: The Pegasus Project