iceberg vs iceberg lettuce

iceberg

noun
  • A huge mass of ocean-floating ice which has broken off a glacier or ice shelf 

  • An aloof person. 

  • An impending disastrous event whose adverse effects are only beginning to show, in reference to one-tenth of the volume of an iceberg being visible above water. 

iceberg lettuce

noun
  • A lettuce cultivar (Lactuca sativa var. capitata) noted for its crunchiness; the most familiar of all lettuces sold in the United States. 

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