Earth-One
Argo City was one of the largest cities on the planet Krypton (the others being Kandor and Kryptonopolis.) Warned of the impending destruction of the planet by his brother Jor-El, scientist Zor-El convinces the Argo City leaders to construct a dome force around the city's perimeter. The dome force allows the city to survive the destruction of Krypton, and provides oxygen and light for the inhabitants (in earlier stories, the air bubble is kept in place by unspecified means). The same chain reaction that turned the rest of Krypton into Kryptonite begins to affect the surface of Argo City; the surviving Kryptonians manage to line the surface with lead, protecting everyone from the deadly rays. Zor-El and his wife eventually have a daughter named Kara, the future Supergirl.
About thirty years after the destruction of Krypton a large meteor shower pierces the lead lining of Argo City, dooming its inhabitants to a slow death. In desperation, Zor-El sends Kara in a spaceship to Earth, where her cousin Kal-El (Superman) helps her become Supergirl. Zor-El and Alura manage to survive in the "Survival Zone", a similar parallel dimension to the Phantom Zone, but the other inhabitants of Argo City perish.
Read more about this topic: Argo City, Pre-Crisis