Argo City

Argo City is the last surviving piece of the dead planet Krypton.

History
When Krypton exploded, Zor-El built a fail-safe for Argo City, fearing it wouldn't work, Zor-El and Alura Zor-El sent their daughter Kara away. The Fail-Safe ended up working, saving Argo from Krypton's destruction, and making it the last remaining city of Krypton.

Supergirl's return to Argo
When Lena Luthor needed more Harun-El to make an antidote for Samantha Arias, Winn Schott used the Harun-El's radiation signature to try to find more of it, and discovered that there was more on a meteor 5 light years away from Earth. Kara and Mon-El borrowed J'onn J'onzz's spaceship and followed the Harun-El's radiation signature to the meteor, which was revealed to be a domed city. During their search for the Harun-El, Kara recognized the city as Argo City; her hometown on Krypton. Just before they could locate the Harun-El, Kara encountered a familiar face: her mother Alura. They spent time together before Kara ultimately convinced the High Council to give her the Harun-El.

After Kara seemingly defeated Reign on Earth, she returned to Argo City accompanied by Mon-El. Kara was overwhelmed to reunite with her childhood friend Thara, who had become one of Argo's leading security officers. The next day, a building began to crumble onto Kara. Although Alura and Thara hailed Kara's uneasiness of her safety as her being on guard during her time on Earth, Mon-El understood Kara's concerns and gave her his Legion ring so she can fly. Later, Kara and Mon-El were attacked in a garden and flew up to see that the person who attacked her was also present at the time that the building had nearly collapsed on her. She and Thara captured the suspect who revealed that she is a dark priestess and Selena is their high priestess. Alura then sent a message to Thara, who brought Kara and Mon-El to her, only to find that J'onn's ship was stolen by Selena and two other dark Kryptonians; leaving Kara and Mon-El stranded on Argo.

The four of them investigated Selena's house and through a hologram, discovered that Selena and the dark Kryptonians were intending to revive Reign and use her to terraform Earth into a New Krypton. The house then exploded, leaving Thara injured. Kara then realized that Selena could not have known that Kara would return again after giving her the Harun-El, and said that she had to have had another plan of getting to Earth. Alura then led her and Mon-El to a transmatter portal in Zor-El's lab. Kara used Alura's hologram on Earth to contact Winn to get him to help link the D.E.O's portal to theirs so they could return to Earth. Before Winn could activate his portal, the dark Kryptonians attacked the D.E.O., but Kara used Alura's hologram to trick a Kryptonian witch into activating the portal. Kara and Mon-El then returned to Earth to join the fight, accompanied by Alura.

After the final defeat of Reign, Kara decided to stay on Earth, and Alura returned to Argo with the Kryptonian witches to serve them justice. Before leaving Earth, Lena gave Alura the remaining piece of Harun-El and provided her with the recipe to make more of it. Alura then stated that the Harun-El would continue to help Argo survive.

Superman's visit to Argo City
Sometime after it's discovery, Clark Kent and Lois Lane took a sabbatical period on Argo City, and conceived their child there. Lois's pregnancy resulted in them moving there, as they feared that the pregnancy would strain Lois's health the longer they stayed on Earth.

In an attempt to kill Superman once and for all, Lex Luthor planned to destroy Argo. However, this attempt was stopped by Supergirl.

Current residents

 * Alura Zor-El
 * Jul-Us
 * Val
 * Val's mother
 * Thara Ak-Var
 * Lir-Al
 * Thara and Lir-Al's two children
 * Ol-Irus
 * Laura
 * Felra's husband
 * Kal-El
 * Lois Lane

Incarcerated residents

 * ​​​​​Selena
 * Felra
 * Vita
 * Ayala

Former residents

 * Mon-El
 * Kara Zor-El

Supergirl
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Season 4
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Behind the scenes

 * In the comics, Argo City was Kara Zor-El's original birthplace. She was born and raised on Argo after Krypton's explosion and was sent to Earth as the city was struck by a meteor shower and destroyed. Subsequent reboots in the DC Comics continuity then established that Kara and Kal-El were sent from Krypton simultaneously, as depicted on.