cave-in vs plunging

cave-in

plunging

adj
  • That descends steeply. 

  • Aimed from higher ground, as fire upon an enemy. 

  • Very low-cut. 

noun
  • A headlong violent motion like that of a horse trying to throw its rider. 

  • An occurrence of putting or sinking under water or other fluid. 

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