land mine vs landmine

land mine

noun
  • A piece of exercise device consisting of a pivoting tube attached to the ground to which a barbell can be fitted to lift it off the ground. 

  • A critical subject or situation with concealed or unpredictable implications. 

  • A mine that is placed on land and designed to explode when stepped upon or touched. 

landmine

verb
  • To sow (an area) with land mines. 

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