The phrase “deep blue sea” is so common in English usage that all three words are individually synonymous for “ocean.” Poems, song titles, and movies, have used these words to conjure up images that few people have observed firsthand. The real “deep blue sea” can typically be seen only hundreds of miles offshore. The areas of the ocean that most people are able to observe are coastal waters, which are rarely “deep,” and only in the cleanest, clearest regions of the world, such as along the coast of Australia, do these waters appear blue. Soil runoff from rivers, algal blooms, and suspended sediments make coastal waters appear to be black, brown, red, blue, or green.

Coble, P., C. Hu, R.W. Gould, Jr., G. Chang, and A.M. Wood. 2004. Colored dissolved organic matter in the coastal ocean: An optical tool for coastal zone environmental assessment and management. Oceanography 17(2):50–59, https://doi.org/10.5670/oceanog.2004.47.

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