Located somewhere in the Arctic, Superman maintains a secret hideout called the “Fortress of Solitude”, acts as a place of solace for him in times of loneliness and despair. This is where he also keeps a collection of mementos and a laboratory for science experiments.
In Action Comics #241 (1958), the Fortress of Solitude is a cave in a mountain, sealed with a very heavy door that is opened with a gigantic key too heavy for anyone but Superman to use. In the 1978 movie, the Fortress of Solitude is a structure made out of ice.
The movie Man of Steel (2013) portrays the Fortress as a Kryptonian exploratory craft buried deep beneath rock and ice.