gazebo vs grotto

gazebo

noun
  • A belvedere, either a type of summer-house or a roofed, detached porch-like structure, usually in a yard, park or lawn. 

grotto

noun
  • A small cave. 

  • A Marian shrine, usually built in a cavern-like structure. 

  • An artificial cavern-like retreat. 

  • A local organization of cavers that typically organizes trips to caves and provides information and training for caving; a caving club. 

  • A secretive name for a local group of underground Satanists. 

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