charnel house vs undercroft

charnel house

noun
  • a vault or other building in which the bones of the dead are stored 

undercroft

noun
  • A cellar or vaulted storage room. 

  • A ground-level car park that occupies the base of a building. 

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