inland sea vs millpond

inland sea

noun
  • A large but relatively shallow expanse of water within a landmass, sometimes connected to the ocean. 

millpond

noun
  • A pond or reservoir produced by damming a river or stream in order to provide a steady source of water for a millrace. 

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