SPOILER WARNING: This article contains major spoilers for "Dark Side of the Moon," the latest episode of Supergirl.
Krypton isn't as dead as Supergirl believed. In "Dark Side of the Moon," the latest episode, Kara and Mon-El headed into space in search of the Rock of Yuda Kal, a mythical rock that could stop Reign. In the process, they discovered Argo City, a Kryptonian city that survived the planet's destruction.
When Kara and Mon-El touched down on Argo City, they discovered a slice of Kryptonian life barely touched by the catastrophic disaster. There, they ran into Kara's mother Allura, who -- as it turns out -- hadn't died all those years ago. Indeed, she and her husband Zor-El even helped create a council that oversaw the Kryptonian way of life (or, at least, what was left of it).
Following the reunion, Allura explained how Argo City survived the death of the planet. She revealed Zor-El had reversed the city's defensive shield in an attempt to preserve it. However, they weren't sure if this would work, and so sent Kara ahead of them to Earth just in case. Allura even watched Kara's pod enter the Phantom Zone from Argo City and assumed her daughter was dead.
Since Zor-El's technology was only theoretical, Argo City underwent some unforeseen complications. The explosion that rocked the planet released some toxic gas into the dome, which sickened the oldest and youngest members of the community. Fortunately, Zor-El soon discovered the properties of the Rock of Yuda Kal, which allowed him to filter the toxins out of their atmosphere. Zor-El's discovery saved many lives, but cost him his own.
So long as the Rock of Yuda Kal exists in Argo City, its people can enjoy a relatively normal life. Once it's used up, however, they will suffer the same problems as before. This makes the Rock a precious resource for the Kryptonian refugees.
Because of the dome, the citizens of Argo City cannot communicate with the rest of the universe. This prevented Kara from discovering Argo City up until this point. It also stopped her from sending a message back to the DEO on Earth to let them know what she and Mon-El had found. However, it seems the Worldkiller priestess Selina has found a way to do so, which bodes ill for the people of Earth.
Airing Mondays at 8 p.m. on The CW, Supergirl stars Melissa Benoist as the Girl of Steel, David Harewood as Martian Manhunter, Mehcad Brooks as Jimmy Olsen, Chyler Leigh as Alex Danvers and Jeremy Jordan as Winn Schott, with appearances by Calista Flockhart as Cat Grant and Tyler Hoechlin as Superman.