black hole vs underground cell

black hole

verb
  • To redirect (network traffic, etc.) nowhere; to discard (incoming traffic). 

noun
  • A place of punitive confinement; a lockup or cell; a military guardroom. 

  • A void into which things disappear, or from which nothing emerges; an impenetrable area or subject; an area impervious to communication. 

  • A dangerous optical illusion that can occur on a nighttime approach with dark, featureless terrain between the aircraft and a brightly-lit runway, where the aircraft appears to the pilots to be higher up than it actually is, potentially triggering a premature or overly-steep descent and a crash short of the runway. 

  • A place where incoming traffic is silently discarded. 

  • A gravitationally domineering celestial body with an event horizon from which even light cannot escape; the most dense material in the universe, condensed into a singularity, usually formed by a collapsing massive star. 

  • A bit bucket; a place of permanent oblivion for data. 

underground cell

How often have the words black hole and underground cell occurred in a corpus of books? (source: Google Ngram Viewer )