What is the name of the underground world in The Silver Chair?

The underground world in The Silver Chair is Underland. Eustace Scrubb, Jill Pole, and Puddleglum reach it while searching for Prince Rilian, the missing son of King Caspian X.

Underland lies beneath Narnia and is ruled by the Lady of the Green Kirtle. Its landscape includes the Sunless Sea and a dark subterranean city, where Rilian has been held under an enchantment. The travellers initially mistake his desperate cries for madness, but realize that he is signaling that he has been imprisoned by magic.

Underland is sometimes confused with Bism, but the two are separate places. Bism lies even deeper beneath Underland and is the true home of the gnomes, or Earthmen. After the Lady’s defeat, the freed gnomes hurry back toward Bism while Rilian and his companions escape upward to Narnia. The distinction matters because Underland is the setting of the rescue quest, while Bism is the deeper world glimpsed during the escape.