For questions about the TV series "Stargate Atlantis" that ran from 2004 to 2009. Always use in conjunction with the [stargate] tag.
The top-secret US military organization Stargate Command (SGC) launches an international expedition of explorers and scientists backup by the US military to another galaxy as the location of the lost Ancient city of Atlantis is discovered by Dr. Daniel Jackson of SG-1.
Stargate Atlantis is set in the Pegasus galaxy where the Ancients fled after a plague ravaged our own Milky Way galaxy.
There they seeded hundreds of worlds with life until they met a powerful race known as the Wraith who defeated them after a 100-year war. Fleeing back to Earth, they left Atlantis submerged on the bottom of the ocean where it remained until the Atlantis expedition gated there (using an eight-chevron address) and re-awakened it.