Where was Superman's Fortress of Solitude located?

The Fortress of Solitude is an iconic part of Superman's lore. Made of massive crystals, the hidden fortress is where Superman goes to be alone. It is also where he keeps all sorts of weird and wonderful loot, from his primary robotic assistant, Kelex, to the entire Kryptonian city of Kandor, which was shrunk down and stuck in a bottle by the villain Brainiac. But the Fortress of Solitude wasn't always Superman's home base.

Before the Man of Steel shacked up there, a hideaway called the Fortress of Solitude appeared in the adventures of comic book pulp hero Doc Savage in the 1930s. Superman had his own hideout in comic books' "Golden Age," but it was in the mountains near Metropolis and was called the "Secret Citadel." It wasn't until the comic book "Silver Age" of the 1950s and 60s that Superman got his now-familiar Fortress. Since its introduction, it's been located in the same place for almost the entirety of Superman's run: An undisclosed secret spot in the Arctic Circle.