What is the name of the sunless sea in The Silver Chair?

The sunless sea in The Silver Chair is called the Sunless Sea.

The Sunless Sea is a vast subterranean body of water in Underland, the underground realm beneath Narnia. After Eustace, Jill, and Puddleglum fall into the deep country below the ruined giant city, earthmen find them and carry them by boat across this lightless sea.

Their voyage leads to the underground city ruled by the Lady of the Green Kirtle. The setting helps make Underland feel like a complete hidden world rather than merely a cave system: it has waterways, cities, enslaved inhabitants, and its own political threat to Narnia. The travellers’ journey across the sea is one stage in their attempt to rescue the missing Prince Rilian.

The name is sometimes confused with the Dead Sea because both are real or fictional geographic names associated with water and darkness. In Lewis’s novel, however, “Sunless Sea” is the specific proper name. The Great River is a different Narnian waterway, while “Silver Sea” is not the name used for this underground body of water.