The Fortress of Solitude was Superman's private hideaway and occasional headquarters. It was located in the ocean beneath the Arctic tundra.
Superman discovered it by accident but opted to use it as his regular base of operations thereafter. In the beginning, the Fortress just contained the memory orb recovered from Brainiac's collection. The orb gave the Man of Steel information concerning Krypton. Superman later expanded the Fortress to include a control room, a hall of weapons, a zoo housing animals from alien worlds, and various trophies from Superman's various adventures and triumphs over his oponents. Apart from this, it contained the necessary facilities and equipment to build android doubles of Superman when he went missing. There were also giant ice sculptures of Superman's biological mother and father.