Atlantis, a city in the Pegasus Galaxy, was once the home of the Ancients. Millennia ago, the Ancients had left their home in the Milky Way to settle in the Pegasus Galaxy, and there they had established the highly advanced 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. Atlantis became the "Lost City," for which SG-1 searched, hoping to find Ancient technology that would help them in their battle against the Goa'uld.
SG-1's search for the Lost City of the Ancients brought them to Praclarush Taonas where O'Neill drew upon the knowledge of the Ancients and spoke the words, "Terra Atlantus, subo glaciuse," indicating that the Lost City they sought was Atlantis, and the key to finding it lay under the ice of Antarctica. On Earth, SG-1 found a small Ancient outpost beneath the Antarctic ice, and although they determined that the city they were seeking was not on Earth, research at the outpost did eventually lead to the discovery of the Lost City of Atlantis in the Pegasus Galaxy.
The SGC was able to use a zero point module to make a single connection to the distant galaxy, and when MALP telemetry indicated an enclosed structure with viable life support, an expedition was sent through the stargate to set up a base of operations in Atlantis in what was expected to be a one-way mission. The expedition was headed by Dr. Elizabeth Weir, and the team of many dozens included Major John Sheppard and Major Lorne from the military as well as Dr. Rodney McKay and Dr. Radek Zelenka, both civilian scientists. The ZPM did not have sufficient power to dial a destination in the Pegasus Galaxy a second time, and there had been no word from the expedition team for nearly a year following their departure.
Once the Prometheus had been fitted with an Asgard hyperdrive, it was capable of making the journey between galaxies. Prometheus was launched under the command of General Hammond as a follow-up mission to the Pegasus Galaxy, however the ship was hijacked and had to return to Earth for repairs before making another attempt at intergalactic travel. The Daedalus, the next evolution in the series of ships that would replace the Prometheus, was designed with more advanced alien technologies, including an Asgard hyperdrive, with the intention of using the ship for the next follow-up mission to Atlantis. Once the SGC acquired a ZPM power source from Egypt, it also became possible to reach Atlantis by stargate, and a year after the discovery of the Lost City, contact between Earth and Atlantis became possible.
Once communications with Atlantis had been reestablished, a ZPM was used to power the Atlantis stargate to allow one-way travel between Atlantis and the SGC, but the return trip from Earth to Atlantis had to be made on a BC-304 ship such as the Daedalus or the Odyssey, a journey that would take three weeks. Travel between galaxies became more frequent, and the International Oversight Advisory became increasingly interested in the Atlantis expedition. The IOA believed that their efforts and full financial backing should be focused on the Atlantis mission because of the prevalence of Ancient technology available there, and they considered the SGC as serving little more than a support role in the effort to defend the planet, although their decision may have had more to do with the fact that Atlantis is a civilian rather than a military operation and Dr. Weir had been appointed by the IOA, giving them more control in how the program is run. However, when the threat of an Ori invasion was fully comprehended, the IOA agreed to continue the financial support of both programs.
Two years after the Atlantis expedition first departed, members of SG-1 finally had the opportunity to visit the Lost City they had sought. As Daniel and Vala used Atlantis's holo room to access the Ancient library database to search for a reference to Castiana and Sahal in the hope of finding Merlin's weapon to defeat the Ori, McKay joined Carter and Mitchell aboard the Odyssey in an attempt to block the Ori supergate by establishing a wormhole from the Pegasus Galaxy which would prevent additional Ori ships from entering the Milky Way.
Cross Reference: Ancients, Ancient Outpost, Ancient Spacecraft, Antarctica, Daedalus, Holo Room, International Oversight Advisory, Major Lorne, Lost City, Rodney McKay, Merlin, Morgan Le Fay, Odyssey, Pegasus Galaxy, Praclarush Taonas, Prometheus, John Sheppard, Supergate, Elizabeth Weir, Wraith, Radek Zelenka, Zero Point Module