The "Stargate Atlantis" series follows the adventures of the "Atlantis expedition", a combination of military forces and civilian scientists that travel to the Pegasus Galaxy in search of the Lost City of Atlantis, left behind by the most powerful race known to ever have lived, referred to as the Ancients, also known as Lanteans and Alterans. The finding of the city had been a plot arc for most of "SG-1"'s Season 7, and the Ancients themselves had been a long-running facet of the "SG-1" setting. Arriving at the city, the expedition discovers Atlantis rests at the bottom of an ocean, protected by shields keeping the city habitable. As the Atlantis expedition team explores the myriad corridors and equipment, they quickly drain the already nearly depleted power source (known as a ZPM) allowing the shields to hold and Atlantis to remain watertight. Through an emergency default program, Atlantis detects this sudden power drop and activates a failsafe, causing the city to surface and appear as an island in the middle of an ocean.