Fortress of Solitude
Superman maintains a secret hideout called the "Fortress of Solitude", which is located somewhere in the Arctic. Here, Superman keeps a collection of mementos and a laboratory for science experiments. In "Action Comics" #241, 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" portrays the Fortress as a Kryptonian exploratory craft buried deep beneath rock and ice.