dungeon vs underground prison

dungeon

noun
  • A room dedicated to sadomasochistic sexual activity. 

  • An area inhabited by enemies, containing story objectives, treasure and bosses. 

  • An underground prison or vault, typically built underneath a castle. 

verb
  • To imprison in a dungeon. 

underground prison

How often have the words dungeon and underground prison occurred in a corpus of books? (source: Google Ngram Viewer )