High Quality Content by WIKIPEDIA articles! In the science fiction television series Stargate Atlantis, the spin-off of Stargate SG-1, Atlantis is a city-sized starship originally located in the Pegasus galaxy. Atlantis serves as the base of operations for the main characters, from which they explore other planets through the Stargate. Based on the ancient Greek accounts of the Lost City of Atlantis, according to the mythology of the show, the city was built by an advanced race known as the Ancients.