tumulus vs undercroft

tumulus

noun
  • A mound of earth, especially one placed over a prehistoric tomb; a barrow. 

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 tumulus and undercroft occurred in a corpus of books? (source: Google Ngram Viewer )