attic vs understructure

attic

noun
  • A person's head or brain. 

  • The space, often unfinished and with sloped walls, directly below the roof in the uppermost part of a house or other building, generally used for storage or habitation. 

understructure

noun
  • The supporting structure underneath something, such as a vehicle or bridge. 

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