prison vs underground cell

prison

noun
  • Confinement in prison. 

  • A place or institution where people are held against their will, especially for long-term confinement of those awaiting trial or convicted of serious crimes or otherwise considered undesirable by the government. 

  • Any restrictive environment, such as a harsh academy or home. 

verb
  • To imprison. 

underground cell

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