catacomb vs necropolis

catacomb

noun
  • An underground system of tunnels and chambers with recesses for graves, used (in former times) as a cemetery; a tunnel system used for burying the dead, as in Paris or Ancient Rome. 

necropolis

noun
  • An ancient site used for burying the dead, particularly if consisting of elaborate grave monuments. 

  • A cemetery; especially a large one in or near a city. 

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