obscure vs unable to be solved

obscure

verb
  • To hide, put out of sight etc. 

  • To render obscure; to darken; to make dim; to keep in the dark; to hide; to make less visible, intelligible, legible, glorious, beautiful, or illustrious. 

adj
  • Dark, faint or indistinct. 

  • Difficult to understand. 

  • Not well-known. 

  • Hidden, out of sight or inconspicuous. 

  • Unknown or uncertain; unclear. 

unable to be solved

How often have the words obscure and unable to be solved occurred in a corpus of books? (source: Google Ngram Viewer )