(The). The Symplegades at the entrance of the Euxine Sea. Said to close together when a vessel attempted to sail between them, and thus crush it to pieces. Cyanean means dark, and Symplegades means dashers together.
“Here are those hard rocks of trap, of a greenish-blue,coloured with copper, and hence called the Cyanean.” —Olivier
Source: Dictionary of Phrase and Fable, E. Cobham Brewer, 1894