Calufrax Major

 was an ice planet where the First Gardener built the Galactic Seed Vault. It was not far from Tellus IV. It was one of the coldest places in the universe. Its sky was of indigo colour.

The Galactic Seed Vault was in a polar region of the planet, where jagged mountain peaks grew began where a featureless white plateau ended. (PROSE: The Secret in Vault 13)

Behind the scenes
The name of the planet seems to be an homage to the planet Calufrax from the Fourth Doctor television story The Pirate Planet, which was also involved in a quest for several keys.