Who illustrated the novel in which the fictional location "The Underland" mainly appears?
Underland (Narnia):  Described by Lewis as lying beneath the land of Narnia, Underland appears mainly in The Silver Chair, where Eustace Scrubb and Jill Pole travel under the ground to reach it in their search for Prince Rilian. The Silver Chair:  Like the others, it was illustrated by Pauline Baynes and her work has been retained in many later editions.
Pauline Baynes