Ghouls are victims of a massive radiation poisoning which has turned them into decrepit, ragged, almost rotting, zombie-like mutants. Most known ghouls were created from vault dwellers living in Vault 12 under the city of Bakersfield (better known as Necropolis after the Great War). As part of the vast Vault Experiment Program, the Vault 12 door was designed not to close properly. Thus, massive amounts of radiation attacked those within the Vault, most of whom were turned into the current ghoul population. Generally, in the Fallout universe, massive exposure to radiation might cause humans to transform into ghouls.