Synonyms:
Acheronian, Acherontic, Stygian
Meaning: dark and dismal as of the rivers Acheron and Styx in Hades; "in the depths of an Acheronian forest"; "upon those roseate lips a Stygian hue"-Wordsworth
I saw a wide expanse of water, black as ink--a Stygian pool; but no rocks were visible, and it seemed as though I had been carried into a subterranean sea.