manhole vs overburden

manhole

noun
  • A hole in the ground used to access the sewers or other underground vaults and installations. 

  • A man's anus, in a sexual context. 

  • A hole providing access to the inside of a boiler, tank etc. 

overburden

noun
  • The rock and subsoil that lies above a mineral deposit such as a coal seam. 

  • A sterile stratum that lies above the stratum being investigated 

verb
  • To overload or overtax. 

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