burial mound vs undercroft

burial mound

noun
  • A mound of earth and stones raised over a grave or graves. 

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